JavaScript for impatient programmers (ES2021 edition)

Dr. Axel Rauschmayer

2021

JavaScript for impatient programmers (ES2021 edition)

1 Before you buy the book

1.1 About the content

1.1.1 What’s in this book?

1.1.2 What is not covered by this book?

1.1.3 Isn’t this book too long for impatient people?

1.2 Previewing and buying this book

1.2.1 How can I preview the book, the exercises, and the quizzes?

1.2.2 How can I buy a digital version of this book?

1.2.3 How can I buy the print version of this book?

1.3 About the author

1.4 Acknowledgements

1.1 About the content

1.1.1 What’s in this book?

This book makes JavaScript less challenging to learn for newcomers by offering a modern view that is as consistent as possible.

Highlights:

Get started quickly by initially focusing on modern features.

Test-driven exercises and quizzes available for most chapters.

Covers all essential features of JavaScript, up to and including ES2021.

Optional advanced sections let you dig deeper.

No prior knowledge of JavaScript is required, but you should know how to program.

1.1.2 What is not covered by this book?

Some advanced language features are not explained, but references to appropriate material are provided – for example, to my other JavaScript books at ExploringJS.com, which are free to read online.

This book deliberately focuses on the language. Browser-only features, etc. are not described.

1.1.3 Isn’t this book too long for impatient people?

There are several ways in which you can read this book. One of them involves skipping much of the content in order to get started quickly. For details, see §2.1.1 “In which order should I read the content in this book?”.

1.2 Previewing and buying this book

1.2.1 How can I preview the book, the exercises, and the quizzes?

Go to the homepage of this book:

All essential chapters of this book are free to read online.

The first half of the test-driven exercises can be downloaded.

The first half of the quizzes can be tried online.

1.2.2 How can I buy a digital version of this book?

There are two digital versions of JavaScript for impatient programmers:

Ebooks: PDF, EPUB, MOBI, HTML (all without DRM)

Ebooks plus exercises and quizzes

The home page of this book describes how you can buy them.

1.2.3 How can I buy the print version of this book?

The print version of JavaScript for impatient programmers is available on Amazon.

1.3 About the author

Dr. Axel Rauschmayer specializes in JavaScript and web development. He has been developing web applications since 1995. In 1999, he was technical manager at a German internet startup that later expanded internationally. In 2006, he held his first talk on Ajax. In 2010, he received a PhD in Informatics from the University of Munich.

Since 2011, he has been blogging about web development at 2ality.com and has written several books on JavaScript. He has held trainings and talks for companies such as eBay, Bank of America, and O’Reilly Media.

He lives in Munich, Germany.

1.4 Acknowledgements

Cover by Fran Caye

Parts of this book were edited by Adaobi Obi Tulton.

Thanks for answering questions, discussing language topics, etc.:

Allen Wirfs-Brock (@awbjs)

Benedikt Meurer (@bmeurer)

Brian Terlson (@bterlson)

Daniel Ehrenberg (@littledan)

Jordan Harband (@ljharb)

Maggie Johnson-Pint (@maggiepint)

Mathias Bynens (@mathias)

Myles Borins (@MylesBorins)

Rob Palmer (@robpalmer2)

Šime Vidas (@simevidas)

And many others

Thanks for reviewing:

Johannes Weber (@jowe)

[Generated: 2021-06-23 10:54]

2 FAQ: book and supplementary material

2.1 How to read this book

2.1.1 In which order should I read the content in this book?

2.1.2 Why are some chapters and sections marked with “(advanced)”?

2.1.3 Why are some chapters marked with “(bonus)”?

2.2 I own a digital version

2.2.1 How do I submit feedback and corrections?

2.2.2 How do I get updates for the downloads I bought at Payhip?

2.2.3 Can I upgrade from package “Ebooks” to package “Ebooks + exercises + quizzes”?

2.3 I own the print version

2.3.1 Can I get a discount for a digital version?

2.3.2 Can I submit an error or see submitted errors?

2.3.3 Is there an online list with the URLs in this book?

2.4 Notations and conventions

2.4.1 What is a type signature? Why am I seeing static types in this book?

2.4.2 What do the notes with icons mean?

This chapter answers questions you may have and gives tips for reading this book.

2.1 How to read this book

2.1.1 In which order should I read the content in this book?

This book is three books in one:

You can use it to get started with JavaScript as quickly as possible. This “mode” is for impatient people:

Start reading with §6 “Using JavaScript: the big picture”.

Skip all chapters and sections marked as “advanced”, and all quick references.

It gives you a comprehensive look at current JavaScript. In this “mode”, you read everything and don’t skip advanced content and quick references.

It serves as a reference. If there is a topic that you are interested in, you can find information on it via the table of contents or via the index. Due to basic and advanced content being mixed, everything you need is usually in a single location.

The quizzes and exercises play an important part in helping you practice and retain what you have learned.

2.1.2 Why are some chapters and sections marked with “(advanced)”?

Several chapters and sections are marked with “(advanced)”. The idea is that you can initially skip them. That is, you can get a quick working knowledge of JavaScript by only reading the basic (non-advanced) content.

As your knowledge evolves, you can later come back to some or all of the advanced content.

2.1.3 Why are some chapters marked with “(bonus)”?

The bonus chapters are only available in the paid versions of this book (print and ebook). They are listed in the full table of contents.

2.2 I own a digital version

2.2.1 How do I submit feedback and corrections?

The HTML version of this book (online, or ad-free archive in the paid version) has a link at the end of each chapter that enables you to give feedback.

2.2.2 How do I get updates for the downloads I bought at Payhip?

The receipt email for the purchase includes a link. You’ll always be able to download the latest version of the files at that location.

If you opted into emails while buying, you’ll get an email whenever there is new content. To opt in later, you must contact Payhip (see bottom of payhip.com).

2.2.3 Can I upgrade from package “Ebooks” to package “Ebooks + exercises + quizzes”?

Yes. The instructions for doing so are on the homepage of this book.

2.3 I own the print version

2.3.1 Can I get a discount for a digital version?

If you bought the print version, you can get a discount for a digital version. The homepage of the print version explains how.

Alas, the reverse is not possible: you cannot get a discount for the print version if you bought a digital version.

2.3.2 Can I submit an error or see submitted errors?

On the homepage of the print version, you can submit errors and see submitted errors.

2.3.3 Is there an online list with the URLs in this book?

The homepage of the print version has a list with all the URLs that you see in the footnotes of the print version.

2.4 Notations and conventions

2.4.1 What is a type signature? Why am I seeing static types in this book?

For example, you may see:

Number.isFinite(num: number): boolean

That is called the type signature of Number.isFinite(). This notation, especially the static types number of num and boolean of the result, are not real JavaScript. The notation is borrowed from the compile-to-JavaScript language TypeScript (which is mostly just JavaScript plus static typing).

Why is this notation being used? It helps give you a quick idea of how a function works. The notation is explained in detail in “Tackling TypeScript”, but is usually relatively intuitive.

2.4.2 What do the notes with icons mean?

  Reading instructions

Explains how to best read the content.

  External content

Points to additional, external, content.

  Tip

Gives a tip related to the current content.

  Question

Asks and answers a question pertinent to the current content (think FAQ).

  Warning

Warns about pitfalls, etc.

  Details

Provides additional details, complementing the current content. It is similar to a footnote.

  Exercise

Mentions the path of a test-driven exercise that you can do at that point.

  Quiz

Indicates that there is a quiz for the current (part of a) chapter.

3 History and evolution of JavaScript

3.1 How JavaScript was created

3.2 Standardizing JavaScript

3.3 Timeline of ECMAScript versions

3.4 Ecma Technical Committee 39 (TC39)

3.5 The TC39 process

3.5.1 Tip: Think in individual features and stages, not ECMAScript versions

3.6 FAQ: TC39 process

3.6.1 How is [my favorite proposed feature] doing?

3.6.2 Is there an official list of ECMAScript features?

3.7 Evolving JavaScript: Don’t break the web

3.1 How JavaScript was created

JavaScript was created in May 1995 in 10 days, by Brendan Eich. Eich worked at Netscape and implemented JavaScript for their web browser, Netscape Navigator.

The idea was that major interactive parts of the client-side web were to be implemented in Java. JavaScript was supposed to be a glue language for those parts and to also make HTML slightly more interactive. Given its role of assisting Java, JavaScript had to look like Java. That ruled out existing solutions such as Perl, Python, TCL, and others.

Initially, JavaScript’s name changed several times:

Its code name was Mocha.

In the Netscape Navigator 2.0 betas (September 1995), it was called LiveScript.

In Netscape Navigator 2.0 beta 3 (December 1995), it got its final name, JavaScript.

3.2 Standardizing JavaScript

There are two standards for JavaScript:

ECMA-262 is hosted by Ecma International. It is the primary standard.

ISO/IEC 16262 is hosted by the International Organization for Standardization (ISO) and the International Electrotechnical Commission (IEC). This is a secondary standard.

The language described by these standards is called ECMAScript, not JavaScript. A different name was chosen because Sun (now Oracle) had a trademark for the latter name. The “ECMA” in “ECMAScript” comes from the organization that hosts the primary standard.

The original name of that organization was ECMA, an acronym for European Computer Manufacturers Association. It was later changed to Ecma International (with “Ecma” being a proper name, not an acronym) because the organization’s activities had expanded beyond Europe. The initial all-caps acronym explains the spelling of ECMAScript.

In principle, JavaScript and ECMAScript mean the same thing. Sometimes the following distinction is made:

The term JavaScript refers to the language and its implementations.

The term ECMAScript refers to the language standard and language versions.

Therefore, ECMAScript 6 is a version of the language (its 6th edition).

3.3 Timeline of ECMAScript versions

This is a brief timeline of ECMAScript versions:

ECMAScript 1 (June 1997): First version of the standard.

ECMAScript 2 (June 1998): Small update to keep ECMA-262 in sync with the ISO standard.

ECMAScript 3 (December 1999): Adds many core features – “[…] regular expressions, better string handling, new control statements [do-while, switch], try/catch exception handling, […]”

ECMAScript 4 (abandoned in July 2008): Would have been a massive upgrade (with static typing, modules, namespaces, and more), but ended up being too ambitious and dividing the language’s stewards.

ECMAScript 5 (December 2009): Brought minor improvements – a few standard library features and strict mode.

ECMAScript 5.1 (June 2011): Another small update to keep Ecma and ISO standards in sync.

ECMAScript 6 (June 2015): A large update that fulfilled many of the promises of ECMAScript 4. This version is the first one whose official name – ECMAScript 2015 – is based on the year of publication.

ECMAScript 2016 (June 2016): First yearly release. The shorter release life cycle resulted in fewer new features compared to the large ES6.

ECMAScript 2017 (June 2017). Second yearly release.

Subsequent ECMAScript versions (ES2018, etc.) are always ratified in June.

3.4 Ecma Technical Committee 39 (TC39)

TC39 is the committee that evolves JavaScript. Its member are, strictly speaking, companies: Adobe, Apple, Facebook, Google, Microsoft, Mozilla, Opera, Twitter, and others. That is, companies that are usually fierce competitors are working together for the good of the language.

Every two months, TC39 has meetings that member-appointed delegates and invited experts attend. The minutes of those meetings are public in a GitHub repository.

3.5 The TC39 process

With ECMAScript 6, two issues with the release process used at that time became obvious:

If too much time passes between releases then features that are ready early, have to wait a long time until they can be released. And features that are ready late, risk being rushed to make the deadline.

Features were often designed long before they were implemented and used. Design deficiencies related to implementation and use were therefore discovered too late.

In response to these issues, TC39 instituted the new TC39 process:

ECMAScript features are designed independently and go through stages, starting at 0 (“strawman”), ending at 4 (“finished”).

Especially the later stages require prototype implementations and real-world testing, leading to feedback loops between designs and implementations.

ECMAScript versions are released once per year and include all features that have reached stage 4 prior to a release deadline.

The result: smaller, incremental releases, whose features have already been field-tested. Fig. 1 illustrates the TC39 process.

Figure 1: Each ECMAScript feature proposal goes through stages that are numbered from 0 to 4. Champions are TC39 members that support the authors of a feature. Test 262 is a suite of tests that checks JavaScript engines for compliance with the language specification.

ES2016 was the first ECMAScript version that was designed according to the TC39 process.

3.5.1 Tip: Think in individual features and stages, not ECMAScript versions

Up to and including ES6, it was most common to think about JavaScript in terms of ECMAScript versions – for example, “Does this browser support ES6 yet?”

Starting with ES2016, it’s better to think in individual features: once a feature reaches stage 4, you can safely use it (if it’s supported by the JavaScript engines you are targeting). You don’t have to wait until the next ECMAScript release.

3.6 FAQ: TC39 process

3.6.1 How is [my favorite proposed feature] doing?

If you are wondering what stages various proposed features are in, consult the GitHub repository proposals.

3.6.2 Is there an official list of ECMAScript features?

Yes, the TC39 repo lists finished proposals and mentions in which ECMAScript versions they were introduced.

3.7 Evolving JavaScript: Don’t break the web

One idea that occasionally comes up is to clean up JavaScript by removing old features and quirks. While the appeal of that idea is obvious, it has significant downsides.

Let’s assume we create a new version of JavaScript that is not backward compatible and fix all of its flaws. As a result, we’d encounter the following problems:

JavaScript engines become bloated: they need to support both the old and the new version. The same is true for tools such as IDEs and build tools.

Programmers need to know, and be continually conscious of, the differences between the versions.

You can either migrate all of an existing code base to the new version (which can be a lot of work). Or you can mix versions and refactoring becomes harder because you can’t move code between versions without changing it.

You somehow have to specify per piece of code – be it a file or code embedded in a web page – what version it is written in. Every conceivable solution has pros and cons. For example, strict mode is a slightly cleaner version of ES5. One of the reasons why it wasn’t as popular as it should have been: it was a hassle to opt in via a directive at the beginning of a file or a function.

So what is the solution? Can we have our cake and eat it? The approach that was chosen for ES6 is called “One JavaScript”:

New versions are always completely backward compatible (but there may occasionally be minor, hardly noticeable clean-ups).

Old features aren’t removed or fixed. Instead, better versions of them are introduced. One example is declaring variables via let – which is an improved version of var.

If aspects of the language are changed, it is done inside new syntactic constructs. That is, you opt in implicitly. For example, yield is only a keyword inside generators (which were introduced in ES6). And all code inside modules and classes (both introduced in ES6) is implicitly in strict mode.

  Quiz

See quiz app.

4 New JavaScript features

4.1 New in ECMAScript 2021

4.2 New in ECMAScript 2020

4.3 New in ECMAScript 2019

4.4 New in ECMAScript 2018

4.5 New in ECMAScript 2017

4.6 New in ECMAScript 2016

4.7 Source of this chapter

ECMAScript 6 (ES2015) was a massive release with too many new features to list here. What’s new in later releases is described below, in reverse chronological order.

4.1 New in ECMAScript 2021

ES2021 officially became a standard on 22 June 2021. New features:

String.prototype.replaceAll()

Promise.any()

WeakRefs (beyond the scope of this book, see proposal)

Logical assignment operators

Underscores (_) as separators in number literals and bigint literals

4.2 New in ECMAScript 2020

The following features were added in ECMAScript 2020:

String.prototype.matchAll()

Dynamic imports via import()

Bigints – arbitrary-precision integers

Promise.allSettled()

globalThis

for-in mechanics (beyond the scope of this book, see proposal)

Optional chaining for property accesses and method calls

Nullish coalescing operator (??)

import.meta

Namespace re-exporting: export * as ns from "mod";

4.3 New in ECMAScript 2019

The following features were added in ECMAScript 2019:

Optional catch binding

Symbol.prototype.description

Object.fromEntries()

String methods: .trimStart() and .trimEnd()

Array methods: .flatMap() and .flat()

These new ES2019 features are beyond the scope of this book:

JSON superset (see 2ality blog post)

Function.prototype.toString() revision (see 2ality blog post)

Well-formed JSON.stringify() (see 2ality blog post)

4.4 New in ECMAScript 2018

The following features were added in ECMAScript 2018:

Template literal revision

s (dotAll) flag for regular expressions

RegExp named capture groups

Rest properties (destructuring), spreading into object literals

RegExp lookbehind assertions

RegExp Unicode property escapes

Promise.prototype.finally()

Asynchronous iteration

4.5 New in ECMAScript 2017

The following features were added in ECMAScript 2017:

Object.values(), Object.entries()

String padding: .padStart() and .padEnd()

Object.getOwnPropertyDescriptors() (beyond the scope of this book, see “Deep JavaScript”)

Trailing commas in function parameter lists and calls

Async functions

Shared memory and atomics (beyond the scope of this book, see proposal)

4.6 New in ECMAScript 2016

The following features were added in ECMAScript 2016:

Array.prototype.includes()

Exponentiation operator (**)

4.7 Source of this chapter

ECMAScript feature lists were taken from the TC39 page on finished proposals.

5 FAQ: JavaScript

5.1 What are good references for JavaScript?

5.2 How do I find out what JavaScript features are supported where?

5.3 Where can I look up what features are planned for JavaScript?

5.4 Why does JavaScript fail silently so often?

5.5 Why can’t we clean up JavaScript, by removing quirks and outdated features?

5.6 How can I quickly try out a piece of JavaScript code?

5.1 What are good references for JavaScript?

Please consult §6.3 “JavaScript references”.

5.2 How do I find out what JavaScript features are supported where?

This book usually mentions if a feature is part of ECMAScript 5 (as required by older browsers) or a newer version. For more detailed information (including pre-ES5 versions), there are several good compatibility tables available online:

ECMAScript compatibility tables for various engines (by kangax, webbedspace, zloirock)

Node.js compatibility tables (by William Kapke)

Mozilla’s MDN web docs have tables for each feature that describe relevant ECMAScript versions and browser support.

“Can I use…” documents what features (including JavaScript language features) are supported by web browsers.

5.3 Where can I look up what features are planned for JavaScript?

Please consult the following sources:

§3.5 “The TC39 process” describes how upcoming features are planned.

§3.6 “FAQ: TC39 process” answers various questions regarding upcoming features.

5.4 Why does JavaScript fail silently so often?

JavaScript often fails silently. Let’s look at two examples.

First example: If the operands of an operator don’t have the appropriate types, they are converted as necessary.

> '3' * '5'
15

Second example: If an arithmetic computation fails, you get an error value, not an exception.

> 1 / 0
Infinity

The reason for the silent failures is historical: JavaScript did not have exceptions until ECMAScript 3. Since then, its designers have tried to avoid silent failures.

5.5 Why can’t we clean up JavaScript, by removing quirks and outdated features?

This question is answered in §3.7 “Evolving JavaScript: Don’t break the web”.

5.6 How can I quickly try out a piece of JavaScript code?

§8.1 “Trying out JavaScript code” explains how to do that.

6 Using JavaScript: the big picture

6.1 What are you learning in this book?

6.2 The structure of browsers and Node.js

6.3 JavaScript references

6.4 Further reading

In this chapter, I’d like to paint the big picture: what are you learning in this book, and how does it fit into the overall landscape of web development?

6.1 What are you learning in this book?

This book teaches the JavaScript language. It focuses on just the language, but offers occasional glimpses at two platforms where JavaScript can be used:

Web browser

Node.js

Node.js is important for web development in three ways:

You can use it to write server-side software in JavaScript.

You can also use it to write software for the command line (think Unix shell, Windows PowerShell, etc.). Many JavaScript-related tools are based on (and executed via) Node.js.

Node’s software registry, npm, has become the dominant way of installing tools (such as compilers and build tools) and libraries – even for client-side development.

6.2 The structure of browsers and Node.js

Figure 2: The structure of the two JavaScript platforms web browser and Node.js. The APIs “standard library” and “platform API” are hosted on top of a foundational layer with a JavaScript engine and a platform-specific “core”.

The structures of the two JavaScript platforms web browser and Node.js are similar (fig. 2):

The foundational layer consists of the JavaScript engine and platform-specific “core” functionality.

Two APIs are hosted on top of this foundation:

The JavaScript standard library is part of JavaScript proper and runs on top of the engine.

The platform API are also available from JavaScript – it provides access to platform-specific functionality. For example:

In browsers, you need to use the platform-specific API if you want to do anything related to the user interface: react to mouse clicks, play sounds, etc.

In Node.js, the platform-specific API lets you read and write files, download data via HTTP, etc.

6.3 JavaScript references

When you have a question about a JavaScript, a web search usually helps. I can recommend the following online sources:

MDN web docs: cover various web technologies such as CSS, HTML, JavaScript, and more. An excellent reference.

Node.js Docs: document the Node.js API.

ExploringJS.com: My other books on JavaScript go into greater detail than this book and are free to read online. You can look up features by ECMAScript version:

ES1–ES5: Speaking JavaScript

ES6: Exploring ES6

ES2016–ES2017: Exploring ES2016 and ES2017

Etc.

6.4 Further reading

A bonus chapter provides a more comprehensive look at web development.

7 Syntax

7.1 An overview of JavaScript’s syntax

7.1.1 Basic constructs

7.1.2 Modules

7.1.3 Classes

7.1.4 Exception handling

7.1.5 Legal variable and property names

7.1.6 Casing styles

7.1.7 Capitalization of names

7.1.8 More naming conventions

7.1.9 Where to put semicolons?

7.2 (Advanced)

7.3 Identifiers

7.3.1 Valid identifiers (variable names, etc.)

7.3.2 Reserved words

7.4 Statement vs. expression

7.4.1 Statements

7.4.2 Expressions

7.4.3 What is allowed where?

7.5 Ambiguous syntax

7.5.1 Same syntax: function declaration and function expression

7.5.2 Same syntax: object literal and block

7.5.3 Disambiguation

7.6 Semicolons

7.6.1 Rule of thumb for semicolons

7.6.2 Semicolons: control statements

7.7 Automatic semicolon insertion (ASI)

7.7.1 ASI triggered unexpectedly

7.7.2 ASI unexpectedly not triggered

7.8 Semicolons: best practices

7.9 Strict mode vs. sloppy mode

7.9.1 Switching on strict mode

7.9.2 Improvements in strict mode

7.1 An overview of JavaScript’s syntax

This is a very first look at JavaScript’s syntax. Don’t worry if some things don’t make sense, yet. They will all be explained in more detail later in this book.

This overview is not exhaustive, either. It focuses on the essentials.

7.1.1 Basic constructs

7.1.1.1 Comments

// single-line comment

/*
Comment with
multiple lines
*/

7.1.1.2 Primitive (atomic) values

Booleans:

true
false

Numbers:

1.141
-123

The basic number type is used for both floating point numbers (doubles) and integers.

Bigints:

17n
-49n

The basic number type can only properly represent integers within a range of 53 bits plus sign. Bigints can grow arbitrarily large in size.

Strings:

'abc'
"abc"
`String with interpolated values: ${256} and ${true}`

JavaScript has no extra type for characters. It uses strings to represent them.

7.1.1.3 Assertions

An assertion describes what the result of a computation is expected to look like and throws an exception if those expectations aren’t correct. For example, the following assertion states that the result of the computation 7 plus 1 must be 8:

assert.equal(7 + 1, 8);

assert.equal() is a method call (the object is assert, the method is .equal()) with two arguments: the actual result and the expected result. It is part of a Node.js assertion API that is explained later in this book.

There is also assert.deepEqual() that compares objects deeply.

7.1.1.4 Logging to the console

Logging to the console of a browser or Node.js:

// Printing a value to standard out (another method call)
console.log('Hello!');

// Printing error information to standard error
console.error('Something went wrong!');

7.1.1.5 Operators

// Operators for booleans
assert.equal(true && false, false); // And
assert.equal(true || false, true); // Or

// Operators for numbers
assert.equal(3 + 4, 7);
assert.equal(5 - 1, 4);
assert.equal(3 * 4, 12);
assert.equal(10 / 4, 2.5);

// Operators for bigints
assert.equal(3n + 4n, 7n);
assert.equal(5n - 1n, 4n);
assert.equal(3n * 4n, 12n);
assert.equal(10n / 4n, 2n);

// Operators for strings
assert.equal('a' + 'b', 'ab');
assert.equal('I see ' + 3 + ' monkeys', 'I see 3 monkeys');

// Comparison operators
assert.equal(3 < 4, true);
assert.equal(3 <= 4, true);
assert.equal('abc' === 'abc', true);
assert.equal('abc' !== 'def', true);

JavaScript also has a == comparison operator. I recommend to avoid it – why is explained in §13.4.3 “Recommendation: always use strict equality”.

7.1.1.6 Declaring variables

const creates immutable variable bindings: Each variable must be initialized immediately and we can’t assign a different value later. However, the value itself may be mutable and we may be able to change its contents. In other words: const does not make values immutable.

// Declaring and initializing x (immutable binding):
const x = 8;

// Would cause a TypeError:
// x = 9;

let creates mutable variable bindings:

// Declaring y (mutable binding):
let y;

// We can assign a different value to y:
y = 3 * 5;

// Declaring and initializing z:
let z = 3 * 5;

7.1.1.7 Ordinary function declarations

// add1() has the parameters a and b
function add1(a, b) {
  return a + b;
}
// Calling function add1()
assert.equal(add1(5, 2), 7);

7.1.1.8 Arrow function expressions

Arrow function expressions are used especially as arguments of function calls and method calls:

const add2 = (a, b) => { return a + b };
// Calling function add2()
assert.equal(add2(5, 2), 7);

// Equivalent to add2:
const add3 = (a, b) => a + b;

The previous code contains the following two arrow functions (the terms expression and statement are explained later in this chapter):

// An arrow function whose body is a code block
(a, b) => { return a + b }

// An arrow function whose body is an expression
(a, b) => a + b

7.1.1.9 Plain objects

// Creating a plain object via an object literal
const obj = {
  first: 'Jane', // property
  last: 'Doe', // property
  getFullName() { // property (method)
    return this.first + ' ' + this.last;
  },
};

// Getting a property value
assert.equal(obj.first, 'Jane');
// Setting a property value
obj.first = 'Janey';

// Calling the method
assert.equal(obj.getFullName(), 'Janey Doe');

7.1.1.10 Arrays

// Creating an Array via an Array literal
const arr = ['a', 'b', 'c'];
assert.equal(arr.length, 3);

// Getting an Array element
assert.equal(arr[1], 'b');
// Setting an Array element
arr[1] = 'β';

// Adding an element to an Array:
arr.push('d');

assert.deepEqual(
  arr, ['a', 'β', 'c', 'd']);

7.1.1.11 Control flow statements

Conditional statement:

if (x < 0) {
  x = -x;
}

for-of loop:

const arr = ['a', 'b'];
for (const element of arr) {
  console.log(element);
}
// Output:
// 'a'
// 'b'

7.1.2 Modules

Each module is a single file. Consider, for example, the following two files with modules in them:

file-tools.mjs
main.mjs

The module in file-tools.mjs exports its function isTextFilePath():

export function isTextFilePath(filePath) {
  return filePath.endsWith('.txt');
}

The module in main.mjs imports the whole module path and the function isTextFilePath():

// Import whole module as namespace object `path`
import * as path from 'path';
// Import a single export of module file-tools.mjs
import {isTextFilePath} from './file-tools.mjs';

7.1.3 Classes

class Person {
  constructor(name) {
    this.name = name;
  }
  describe() {
    return `Person named ${this.name}`;
  }
  static logNames(persons) {
    for (const person of persons) {
      console.log(person.name);
    }
  }
}

class Employee extends Person {
  constructor(name, title) {
    super(name);
    this.title = title;
  }
  describe() {
    return super.describe() +
      ` (${this.title})`;
  }
}

const jane = new Employee('Jane', 'CTO');
assert.equal(
  jane.describe(),
  'Person named Jane (CTO)');

7.1.4 Exception handling

function throwsException() {
  throw new Error('Problem!');
}

function catchesException() {
  try {
    throwsException();
  } catch (err) {
    assert.ok(err instanceof Error);
    assert.equal(err.message, 'Problem!');
  }
}

Note:

try-finally and try-catch-finally are also supported.

We can throw any value, but features such as stack traces are only supported by Error and its subclasses.

7.1.5 Legal variable and property names

The grammatical category of variable names and property names is called identifier.

Identifiers are allowed to have the following characters:

Unicode letters: A–Z, a–z (etc.)

$, _

Unicode digits: 0–9 (etc.)

Variable names can’t start with a digit

Some words have special meaning in JavaScript and are called reserved. Examples include: if, true, const.

Reserved words can’t be used as variable names:

const if = 123;
  // SyntaxError: Unexpected token if

But they are allowed as names of properties:

> const obj = { if: 123 };
> obj.if
123

7.1.6 Casing styles

Common casing styles for concatenating words are:

Camel case: threeConcatenatedWords

Underscore case (also called snake case): three_concatenated_words

Dash case (also called kebab case): three-concatenated-words

7.1.7 Capitalization of names

In general, JavaScript uses camel case, except for constants.

Lowercase:

Functions, variables: myFunction

Methods: obj.myMethod

CSS:

CSS entity: special-class

Corresponding JavaScript variable: specialClass

Uppercase:

Classes: MyClass

Constants: MY_CONSTANT

Constants are also often written in camel case: myConstant

7.1.8 More naming conventions

The following naming conventions are popular in JavaScript.

If the name of a parameter starts with an underscore (or is an underscore) it means that this parameter is not used – for example:

arr.map((_x, i) => i)

If the name of a property of an object starts with an underscore then that property is considered private:

class ValueWrapper {
  constructor(value) {
    this._value = value;
  }
}

7.1.9 Where to put semicolons?

At the end of a statement:

const x = 123;
func();

But not if that statement ends with a curly brace:

while (false) {
  // ···
} // no semicolon

function func() {
  // ···
} // no semicolon

However, adding a semicolon after such a statement is not a syntax error – it is interpreted as an empty statement:

// Function declaration followed by empty statement:
function func() {
  // ···
};

  Quiz: basic

See quiz app.

7.2 (Advanced)

All remaining sections of this chapter are advanced.

7.3 Identifiers

7.3.1 Valid identifiers (variable names, etc.)

First character:

Unicode letter (including accented characters such as é and ü and characters from non-latin alphabets, such as α)

$

_

Subsequent characters:

Legal first characters

Unicode digits (including Eastern Arabic numerals)

Some other Unicode marks and punctuations

Examples:

const ε = 0.0001;
const строка = '';
let _tmp = 0;
const $foo2 = true;

7.3.2 Reserved words

Reserved words can’t be variable names, but they can be property names.

All JavaScript keywords are reserved words:

await break case catch class const continue debugger default delete do else export extends finally for function if import in instanceof let new return static super switch this throw try typeof var void while with yield

The following tokens are also keywords, but currently not used in the language:

enum implements package protected interface private public

The following literals are reserved words:

true false null

Technically, these words are not reserved, but you should avoid them, too, because they effectively are keywords:

Infinity NaN undefined async

You shouldn’t use the names of global variables (String, Math, etc.) for your own variables and parameters, either.

7.4 Statement vs. expression

In this section, we explore how JavaScript distinguishes two kinds of syntactic constructs: statements and expressions. Afterward, we’ll see that that can cause problems because the same syntax can mean different things, depending on where it is used.

  We pretend there are only statements and expressions

For the sake of simplicity, we pretend that there are only statements and expressions in JavaScript.

7.4.1 Statements

A statement is a piece of code that can be executed and performs some kind of action. For example, if is a statement:

let myStr;
if (myBool) {
  myStr = 'Yes';
} else {
  myStr = 'No';
}

One more example of a statement: a function declaration.

function twice(x) {
  return x + x;
}

7.4.2 Expressions

An expression is a piece of code that can be evaluated to produce a value. For example, the code between the parentheses is an expression:

let myStr = (myBool ? 'Yes' : 'No');

The operator _?_:_ used between the parentheses is called the ternary operator. It is the expression version of the if statement.

Let’s look at more examples of expressions. We enter expressions and the REPL evaluates them for us:

> 'ab' + 'cd'
'abcd'
> Number('123')
123
> true || false
true

7.4.3 What is allowed where?

The current location within JavaScript source code determines which kind of syntactic constructs you are allowed to use:

The body of a function must be a sequence of statements:

function max(x, y) {
  if (x > y) {
    return x;
  } else {
    return y;
  }
}

The arguments of a function call or a method call must be expressions:

console.log('ab' + 'cd', Number('123'));

However, expressions can be used as statements. Then they are called expression statements. The opposite is not true: when the context requires an expression, you can’t use a statement.

The following code demonstrates that any expression bar() can be either expression or statement – it depends on the context:

function f() {
  console.log(bar()); // bar() is expression
  bar(); // bar(); is (expression) statement
}

7.5 Ambiguous syntax

JavaScript has several programming constructs that are syntactically ambiguous: the same syntax is interpreted differently, depending on whether it is used in statement context or in expression context. This section explores the phenomenon and the pitfalls it causes.

7.5.1 Same syntax: function declaration and function expression

A function declaration is a statement:

function id(x) {
  return x;
}

A function expression is an expression (right-hand side of =):

const id = function me(x) {
  return x;
};

7.5.2 Same syntax: object literal and block

In the following code, {} is an object literal: an expression that creates an empty object.

const obj = {};

This is an empty code block (a statement):

{
}

7.5.3 Disambiguation

The ambiguities are only a problem in statement context: If the JavaScript parser encounters ambiguous syntax, it doesn’t know if it’s a plain statement or an expression statement. For example:

If a statement starts with function: Is it a function declaration or a function expression?

If a statement starts with {: Is it an object literal or a code block?

To resolve the ambiguity, statements starting with function or { are never interpreted as expressions. If you want an expression statement to start with either one of these tokens, you must wrap it in parentheses:

(function (x) { console.log(x) })('abc');

// Output:
// 'abc'

In this code:

We first create a function via a function expression:

function (x) { console.log(x) }

Then we invoke that function: ('abc')

The code fragment shown in (1) is only interpreted as an expression because we wrap it in parentheses. If we didn’t, we would get a syntax error because then JavaScript expects a function declaration and complains about the missing function name. Additionally, you can’t put a function call immediately after a function declaration.

Later in this book, we’ll see more examples of pitfalls caused by syntactic ambiguity:

Assigning via object destructuring

Returning an object literal from an arrow function

7.6 Semicolons

7.6.1 Rule of thumb for semicolons

Each statement is terminated by a semicolon:

const x = 3;
someFunction('abc');
i++;

except statements ending with blocks:

function foo() {
  // ···
}
if (y > 0) {
  // ···
}

The following case is slightly tricky:

const func = () => {}; // semicolon!

The whole const declaration (a statement) ends with a semicolon, but inside it, there is an arrow function expression. That is, it’s not the statement per se that ends with a curly brace; it’s the embedded arrow function expression. That’s why there is a semicolon at the end.

7.6.2 Semicolons: control statements

The body of a control statement is itself a statement. For example, this is the syntax of the while loop:

while (condition)
  statement

The body can be a single statement:

while (a > 0) a--;

But blocks are also statements and therefore legal bodies of control statements:

while (a > 0) {
  a--;
}

If you want a loop to have an empty body, your first option is an empty statement (which is just a semicolon):

while (processNextItem() > 0);

Your second option is an empty block:

while (processNextItem() > 0) {}

7.7 Automatic semicolon insertion (ASI)

While I recommend to always write semicolons, most of them are optional in JavaScript. The mechanism that makes this possible is called automatic semicolon insertion (ASI). In a way, it corrects syntax errors.

ASI works as follows. Parsing of a statement continues until there is either:

A semicolon

A line terminator followed by an illegal token

In other words, ASI can be seen as inserting semicolons at line breaks. The next subsections cover the pitfalls of ASI.

7.7.1 ASI triggered unexpectedly

The good news about ASI is that – if you don’t rely on it and always write semicolons – there is only one pitfall that you need to be aware of. It is that JavaScript forbids line breaks after some tokens. If you do insert a line break, a semicolon will be inserted, too.

The token where this is most practically relevant is return. Consider, for example, the following code:

return
{
  first: 'jane'
};

This code is parsed as:

return;
{
  first: 'jane';
}
;

That is:

Return statement without operand: return;

Start of code block: {

Expression statement 'jane'; with label first:

End of code block: }

Empty statement: ;

Why does JavaScript do this? It protects against accidentally returning a value in a line after a return.

7.7.2 ASI unexpectedly not triggered

In some cases, ASI is not triggered when you think it should be. That makes life more complicated for people who don’t like semicolons because they need to be aware of those cases. The following are three examples. There are more.

Example 1: Unintended function call.

a = b + c
(d + e).print()

Parsed as:

a = b + c(d + e).print();

Example 2: Unintended division.

a = b
/hi/g.exec(c).map(d)

Parsed as:

a = b / hi / g.exec(c).map(d);

Example 3: Unintended property access.

someFunction()
['ul', 'ol'].map(x => x + x)

Executed as:

const propKey = ('ul','ol'); // comma operator
assert.equal(propKey, 'ol');

someFunction()[propKey].map(x => x + x);

7.8 Semicolons: best practices

I recommend that you always write semicolons:

I like the visual structure it gives code – you clearly see where a statement ends.

There are less rules to keep in mind.

The majority of JavaScript programmers use semicolons.

However, there are also many people who don’t like the added visual clutter of semicolons. If you are one of them: Code without them is legal. I recommend that you use tools to help you avoid mistakes. The following are two examples:

The automatic code formatter Prettier can be configured to not use semicolons. It then automatically fixes problems. For example, if it encounters a line that starts with a square bracket, it prefixes that line with a semicolon.

The static checker ESLint has a rule that you tell your preferred style (always semicolons or as few semicolons as possible) and that warns you about critical issues.

7.9 Strict mode vs. sloppy mode

Starting with ECMAScript 5, JavaScript has two modes in which JavaScript can be executed:

Normal “sloppy” mode is the default in scripts (code fragments that are a precursor to modules and supported by browsers).

Strict mode is the default in modules and classes, and can be switched on in scripts (how is explained later). In this mode, several pitfalls of normal mode are removed and more exceptions are thrown.

You’ll rarely encounter sloppy mode in modern JavaScript code, which is almost always located in modules. In this book, I assume that strict mode is always switched on.

7.9.1 Switching on strict mode

In script files and CommonJS modules, you switch on strict mode for a complete file, by putting the following code in the first line:

'use strict';

The neat thing about this “directive” is that ECMAScript versions before 5 simply ignore it: it’s an expression statement that does nothing.

You can also switch on strict mode for just a single function:

function functionInStrictMode() {
  'use strict';
}

7.9.2 Improvements in strict mode

Let’s look at three things that strict mode does better than sloppy mode. Just in this one section, all code fragments are executed in sloppy mode.

7.9.2.1 Sloppy mode pitfall: changing an undeclared variable creates a global variable

In non-strict mode, changing an undeclared variable creates a global variable.

function sloppyFunc() {
  undeclaredVar1 = 123;
}
sloppyFunc();
// Created global variable `undeclaredVar1`:
assert.equal(undeclaredVar1, 123);

Strict mode does it better and throws a ReferenceError. That makes it easier to detect typos.

function strictFunc() {
  'use strict';
  undeclaredVar2 = 123;
}
assert.throws(
  () => strictFunc(),
  {
    name: 'ReferenceError',
    message: 'undeclaredVar2 is not defined',
  });

The assert.throws() states that its first argument, a function, throws a ReferenceError when it is called.

7.9.2.2 Function declarations are block-scoped in strict mode, function-scoped in sloppy mode

In strict mode, a variable created via a function declaration only exists within the innermost enclosing block:

function strictFunc() {
  'use strict';
  {
    function foo() { return 123 }
  }
  return foo(); // ReferenceError
}
assert.throws(
  () => strictFunc(),
  {
    name: 'ReferenceError',
    message: 'foo is not defined',
  });

In sloppy mode, function declarations are function-scoped:

function sloppyFunc() {
  {
    function foo() { return 123 }
  }
  return foo(); // works
}
assert.equal(sloppyFunc(), 123);

7.9.2.3 Sloppy mode doesn’t throw exceptions when changing immutable data

In strict mode, you get an exception if you try to change immutable data:

function strictFunc() {
  'use strict';
  true.prop = 1; // TypeError
}
assert.throws(
  () => strictFunc(),
  {
    name: 'TypeError',
    message: "Cannot create property 'prop' on boolean 'true'",
  });

In sloppy mode, the assignment fails silently:

function sloppyFunc() {
  true.prop = 1; // fails silently
  return true.prop;
}
assert.equal(sloppyFunc(), undefined);

  Further reading: sloppy mode

For more information on how sloppy mode differs from strict mode, see MDN.

  Quiz: advanced

See quiz app.

8 Consoles: interactive JavaScript command lines

8.1 Trying out JavaScript code

8.1.1 Browser consoles

8.1.2 The Node.js REPL

8.1.3 Other options

8.2 The console.* API: printing data and more

8.2.1 Printing values: console.log() (stdout)

8.2.2 Printing error information: console.error() (stderr)

8.2.3 Printing nested objects via JSON.stringify()

8.1 Trying out JavaScript code

You have many options for quickly running pieces of JavaScript code. The following subsections describe a few of them.

8.1.1 Browser consoles

Web browsers have so-called consoles: interactive command lines to which you can print text via console.log() and where you can run pieces of code. How to open the console differs from browser to browser. Fig. 3 shows the console of Google Chrome.

To find out how to open the console in your web browser, you can do a web search for “console «name-of-your-browser»”. These are pages for a few commonly used web browsers:

Apple Safari

Google Chrome

Microsoft Edge

Mozilla Firefox

Figure 3: The console of the web browser “Google Chrome” is open (in the bottom half of window) while visiting a web page.

8.1.2 The Node.js REPL

REPL stands for read-eval-print loop and basically means command line. To use it, you must first start Node.js from an operating system command line, via the command node. Then an interaction with it looks as depicted in fig. 4: The text after > is input from the user; everything else is output from Node.js.

Figure 4: Starting and using the Node.js REPL (interactive command line).

  Reading: REPL interactions

I occasionally demonstrate JavaScript via REPL interactions. Then I also use greater-than symbols (>) to mark input – for example:

> 3 + 5
8

8.1.3 Other options

Other options include:

There are many web apps that let you experiment with JavaScript in web browsers – for example, Babel’s REPL.

There are also native apps and IDE plugins for running JavaScript.

  Consoles often run in non-strict mode

In modern JavaScript, most code (e.g., modules) is executed in strict mode. However, consoles often run in non-strict mode. Therefore, you may occasionally get slightly different results when using a console to execute code from this book.

8.2 The console.* API: printing data and more

In browsers, the console is something you can bring up that is normally hidden. For Node.js, the console is the terminal that Node.js is currently running in.

The full console.* API is documented on MDN web docs and on the Node.js website. It is not part of the JavaScript language standard, but much functionality is supported by both browsers and Node.js.

In this chapter, we only look at the following two methods for printing data (“printing” means displaying in the console):

console.log()

console.error()

8.2.1 Printing values: console.log() (stdout)

There are two variants of this operation:

console.log(...values: any[]): void
console.log(pattern: string, ...values: any[]): void

8.2.1.1 Printing multiple values

The first variant prints (text representations of) values on the console:

console.log('abc', 123, true);
// Output:
// abc 123 true

At the end, console.log() always prints a newline. Therefore, if you call it with zero arguments, it just prints a newline.

8.2.1.2 Printing a string with substitutions

The second variant performs string substitution:

console.log('Test: %s %j', 123, 'abc');
// Output:
// Test: 123 "abc"

These are some of the directives you can use for substitutions:

%s converts the corresponding value to a string and inserts it.

console.log('%s %s', 'abc', 123);
// Output:
// abc 123

%o inserts a string representation of an object.

console.log('%o', {foo: 123, bar: 'abc'});
// Output:
// { foo: 123, bar: 'abc' }

%j converts a value to a JSON string and inserts it.

console.log('%j', {foo: 123, bar: 'abc'});
// Output:
// {"foo":123,"bar":"abc"}

%% inserts a single %.

console.log('%s%%', 99);
// Output:
// 99%

8.2.2 Printing error information: console.error() (stderr)

console.error() works the same as console.log(), but what it logs is considered error information. For Node.js, that means that the output goes to stderr instead of stdout on Unix.

8.2.3 Printing nested objects via JSON.stringify()

JSON.stringify() is occasionally useful for printing nested objects:

console.log(JSON.stringify({first: 'Jane', last: 'Doe'}, null, 2));

Output:

{
  "first": "Jane",
  "last": "Doe"
}

9 Assertion API

9.1 Assertions in software development

9.2 How assertions are used in this book

9.2.1 Documenting results in code examples via assertions

9.2.2 Implementing test-driven exercises via assertions

9.3 Normal comparison vs. deep comparison

9.4 Quick reference: module assert

9.4.1 Normal equality

9.4.2 Deep equality

9.4.3 Expecting exceptions

9.4.4 Another tool function

9.1 Assertions in software development

In software development, assertions state facts about values or pieces of code that must be true. If they aren’t, an exception is thrown. Node.js supports assertions via its built-in module assert – for example:

import * as assert from 'assert/strict';
assert.equal(3 + 5, 8);

This assertion states that the expected result of 3 plus 5 is 8. The import statement uses the recommended strict version of assert.

9.2 How assertions are used in this book

In this book, assertions are used in two ways: to document results in code examples and to implement test-driven exercises.

9.2.1 Documenting results in code examples via assertions

In code examples, assertions express expected results. Take, for example, the following function:

function id(x) {
  return x;
}

id() returns its parameter. We can show it in action via an assertion:

assert.equal(id('abc'), 'abc');

In the examples, I usually omit the statement for importing assert.

The motivation behind using assertions is:

You can specify precisely what is expected.

Code examples can be tested automatically, which ensures that they really work.

9.2.2 Implementing test-driven exercises via assertions

The exercises for this book are test-driven, via the test framework Mocha. Checks inside the tests are made via methods of assert.

The following is an example of such a test:

// For the exercise, you must implement the function hello().
// The test checks if you have done it properly.
test('First exercise', () => {
  assert.equal(hello('world'), 'Hello world!');
  assert.equal(hello('Jane'), 'Hello Jane!');
  assert.equal(hello('John'), 'Hello John!');
  assert.equal(hello(''), 'Hello !');
});

For more information, consult §10 “Getting started with quizzes and exercises”.

9.3 Normal comparison vs. deep comparison

The strict equal() uses === to compare values. Therefore, an object is only equal to itself – even if another object has the same content (because === does not compare the contents of objects, only their identities):

assert.notEqual({foo: 1}, {foo: 1});

deepEqual() is a better choice for comparing objects:

assert.deepEqual({foo: 1}, {foo: 1});

This method works for Arrays, too:

assert.notEqual(['a', 'b', 'c'], ['a', 'b', 'c']);
assert.deepEqual(['a', 'b', 'c'], ['a', 'b', 'c']);

9.4 Quick reference: module assert

For the full documentation, see the Node.js docs.

9.4.1 Normal equality

function equal(actual: any, expected: any, message?: string): void

actual === expected must be true. If not, an AssertionError is thrown.

assert.equal(3+3, 6);

function notEqual(actual: any, expected: any, message?: string): void

actual !== expected must be true. If not, an AssertionError is thrown.

assert.notEqual(3+3, 22);

The optional last parameter message can be used to explain what is asserted. If the assertion fails, the message is used to set up the AssertionError that is thrown.

let e;
try {
  const x = 3;
  assert.equal(x, 8, 'x must be equal to 8')
} catch (err) {
  assert.equal(
    String(err),
    'AssertionError [ERR_ASSERTION]: x must be equal to 8');
}

9.4.2 Deep equality

function deepEqual(actual: any, expected: any, message?: string): void

actual must be deeply equal to expected. If not, an AssertionError is thrown.

assert.deepEqual([1,2,3], [1,2,3]);
assert.deepEqual([], []);

// To .equal(), an object is only equal to itself:
assert.notEqual([], []);

function notDeepEqual(actual: any, expected: any, message?: string): void

actual must not be deeply equal to expected. If it is, an AssertionError is thrown.

assert.notDeepEqual([1,2,3], [1,2]);

9.4.3 Expecting exceptions

If you want to (or expect to) receive an exception, you need throws(): This function calls its first parameter, the function block, and only succeeds if it throws an exception. Additional parameters can be used to specify what that exception must look like.

function throws(block: Function, message?: string): void

assert.throws(
  () => {
    null.prop;
  }
);

function throws(block: Function, error: Function, message?: string): void

assert.throws(
  () => {
    null.prop;
  },
  TypeError
);

function throws(block: Function, error: RegExp, message?: string): void

assert.throws(
  () => {
    null.prop;
  },
  /^TypeError: Cannot read property 'prop' of null$/
);

function throws(block: Function, error: Object, message?: string): void

assert.throws(
  () => {
    null.prop;
  },
  {
    name: 'TypeError',
    message: `Cannot read property 'prop' of null`,
  }
);

9.4.4 Another tool function

function fail(message: string | Error): never

Always throws an AssertionError when it is called. That is occasionally useful for unit testing.

try {
  functionThatShouldThrow();
  assert.fail();
} catch (_) {
  // Success
}

  Quiz

See quiz app.

10 Getting started with quizzes and exercises

10.1 Quizzes

10.2 Exercises

10.2.1 Installing the exercises

10.2.2 Running exercises

10.3 Unit tests in JavaScript

10.3.1 A typical test

10.3.2 Asynchronous tests in Mocha

Throughout most chapters, there are quizzes and exercises. These are a paid feature, but a comprehensive preview is available. This chapter explains how to get started with them.

10.1 Quizzes

Installation:

Download and unzip impatient-js-quiz.zip

Running the quiz app:

Open impatient-js-quiz/index.html in a web browser

You’ll see a TOC of all the quizzes.

10.2 Exercises

10.2.1 Installing the exercises

To install the exercises:

Download and unzip impatient-js-code.zip

Follow the instructions in README.txt

10.2.2 Running exercises

Exercises are referred to by path in this book.

For example: exercises/quizzes-exercises/first_module_test.mjs

Within each file:

The first line contains the command for running the exercise.

The following lines describe what you have to do.

10.3 Unit tests in JavaScript

All exercises in this book are tests that are run via the test framework Mocha. This section gives a brief introduction.

10.3.1 A typical test

Typical test code is split into two parts:

Part 1: the code to be tested.

Part 2: the tests for the code.

Take, for example, the following two files:

id.mjs (code to be tested)

id_test.mjs (tests)

10.3.1.1 Part 1: the code

The code itself resides in id.mjs:

export function id(x) {
  return x;
}

The key thing here is: everything we want to test must be exported. Otherwise, the test code can’t access it.

10.3.1.2 Part 2: the tests

  Don’t worry about the exact details of tests

You don’t need to worry about the exact details of tests: They are always implemented for you. Therefore, you only need to read them, but not write them.

The tests for the code reside in id_test.mjs:

// npm t demos/quizzes-exercises/id_test.mjs
suite('id_test.mjs');

import * as assert from 'assert/strict'; // (A)
import {id} from './id.mjs'; // (B)

test('My test', () => { // (C)
  assert.equal(id('abc'), 'abc'); // (D)
});

The core of this test file is line D – an assertion: assert.equal() specifies that the expected result of id('abc') is 'abc'.

As for the other lines:

The comment at the very beginning shows the shell command for running the test.

Line A: We import the Node.js assertion library (in strict assertion mode).

Line B: We import the function to test.

Line C: We define a test. This is done by calling the function test():

First parameter: the name of the test.

Second parameter: the test code, which is provided via an arrow function. The parameter t gives us access to AVA’s testing API (assertions, etc.).

To run the test, we execute the following in a command line:

npm t demos/quizzes-exercises/id_test.mjs

The t is an abbreviation for test. That is, the long version of this command is:

npm test demos/quizzes-exercises/id_test.mjs

  Exercise: Your first exercise

The following exercise gives you a first taste of what exercises are like:

exercises/quizzes-exercises/first_module_test.mjs

10.3.2 Asynchronous tests in Mocha

  Reading

You may want to postpone reading this section until you get to the chapters on asynchronous programming.

Writing tests for asynchronous code requires extra work: The test receives its results later and has to signal to Mocha that it isn’t finished yet when it returns. The following subsections examine three ways of doing so.

10.3.2.1 Asynchronicity via callbacks

If the callback we pass to test() has a parameter (e.g., done), Mocha switches to callback-based asynchronicity. When we are done with our asynchronous work, we have to call done:

test('divideCallback', (done) => {
  divideCallback(8, 4, (error, result) => {
    if (error) {
      done(error);
    } else {
      assert.strictEqual(result, 2);
      done();
    }
  });
});

This is what divideCallback() looks like:

function divideCallback(x, y, callback) {
  if (y === 0) {
    callback(new Error('Division by zero'));
  } else {
    callback(null, x / y);
  }
}

10.3.2.2 Asynchronicity via Promises

If a test returns a Promise, Mocha switches to Promise-based asynchronicity. A test is considered successful if the Promise is fulfilled and failed if the Promise is rejected or if a settlement takes longer than a timeout.

test('dividePromise 1', () => {
  return dividePromise(8, 4)
  .then(result => {
    assert.strictEqual(result, 2);
  });
});

dividePromise() is implemented as follows:

function dividePromise(x, y) {
  return new Promise((resolve, reject) => {
    if (y === 0) {
      reject(new Error('Division by zero'));
    } else {
      resolve(x / y);
    }
  });
}

10.3.2.3 Async functions as test “bodies”

Async functions always return Promises. Therefore, an async function is a convenient way of implementing an asynchronous test. The following code is equivalent to the previous example.

test('dividePromise 2', async () => {
  const result = await dividePromise(8, 4);
  assert.strictEqual(result, 2);
  // No explicit return necessary!
});

We don’t need to explicitly return anything: The implicitly returned undefined is used to fulfill the Promise returned by this async function. And if the test code throws an exception, then the async function takes care of rejecting the returned Promise.

11 Variables and assignment

11.1 let

11.2 const

11.2.1 const and immutability

11.2.2 const and loops

11.3 Deciding between const and let

11.4 The scope of a variable

11.4.1 Shadowing variables

11.5 (Advanced)

11.6 Terminology: static vs. dynamic

11.6.1 Static phenomenon: scopes of variables

11.6.2 Dynamic phenomenon: function calls

11.7 Global variables and the global object

11.7.1 globalThis [ES2020]

11.8 Declarations: scope and activation

11.8.1 const and let: temporal dead zone

11.8.2 Function declarations and early activation

11.8.3 Class declarations are not activated early

11.8.4 var: hoisting (partial early activation)

11.9 Closures

11.9.1 Bound variables vs. free variables

11.9.2 What is a closure?

11.9.3 Example: A factory for incrementors

11.9.4 Use cases for closures

These are JavaScript’s main ways of declaring variables:

let declares mutable variables.

const declares constants (immutable variables).

Before ES6, there was also var. But it has several quirks, so it’s best to avoid it in modern JavaScript. You can read more about it in Speaking JavaScript.

11.1 let

Variables declared via let are mutable:

let i;
i = 0;
i = i + 1;
assert.equal(i, 1);

You can also declare and assign at the same time:

let i = 0;

11.2 const

Variables declared via const are immutable. You must always initialize immediately:

const i = 0; // must initialize

assert.throws(
  () => { i = i + 1 },
  {
    name: 'TypeError',
    message: 'Assignment to constant variable.',
  }
);

11.2.1 const and immutability

In JavaScript, const only means that the binding (the association between variable name and variable value) is immutable. The value itself may be mutable, like obj in the following example.

const obj = { prop: 0 };

// Allowed: changing properties of `obj`
obj.prop = obj.prop + 1;
assert.equal(obj.prop, 1);

// Not allowed: assigning to `obj`
assert.throws(
  () => { obj = {} },
  {
    name: 'TypeError',
    message: 'Assignment to constant variable.',
  }
);

11.2.2 const and loops

You can use const with for-of loops, where a fresh binding is created for each iteration:

const arr = ['hello', 'world'];
for (const elem of arr) {
  console.log(elem);
}
// Output:
// 'hello'
// 'world'

In plain for loops, you must use let, however:

const arr = ['hello', 'world'];
for (let i=0; i<arr.length; i++) {
  const elem = arr[i];
  console.log(elem);
}

11.3 Deciding between const and let

I recommend the following rules to decide between const and let:

const indicates an immutable binding and that a variable never changes its value. Prefer it.

let indicates that the value of a variable changes. Use it only when you can’t use const.

  Exercise: const

exercises/variables-assignment/const_exrc.mjs

11.4 The scope of a variable

The scope of a variable is the region of a program where it can be accessed. Consider the following code.

{ // // Scope A. Accessible: x
  const x = 0;
  assert.equal(x, 0);
  { // Scope B. Accessible: x, y
    const y = 1;
    assert.equal(x, 0);
    assert.equal(y, 1);
    { // Scope C. Accessible: x, y, z
      const z = 2;
      assert.equal(x, 0);
      assert.equal(y, 1);
      assert.equal(z, 2);
    }
  }
}
// Outside. Not accessible: x, y, z
assert.throws(
  () => console.log(x),
  {
    name: 'ReferenceError',
    message: 'x is not defined',
  }
);

Scope A is the (direct) scope of x.

Scopes B and C are inner scopes of scope A.

Scope A is an outer scope of scope B and scope C.

Each variable is accessible in its direct scope and all scopes nested within that scope.

The variables declared via const and let are called block-scoped because their scopes are always the innermost surrounding blocks.

11.4.1 Shadowing variables

You can’t declare the same variable twice at the same level:

assert.throws(
  () => {
    eval('let x = 1; let x = 2;');
  },
  {
    name: 'SyntaxError',
    message: "Identifier 'x' has already been declared",
  });

  Why eval()?

eval() delays parsing (and therefore the SyntaxError), until the callback of assert.throws() is executed. If we didn’t use it, we’d already get an error when this code is parsed and assert.throws() wouldn’t even be executed.

You can, however, nest a block and use the same variable name x that you used outside the block:

const x = 1;
assert.equal(x, 1);
{
  const x = 2;
  assert.equal(x, 2);
}
assert.equal(x, 1);

Inside the block, the inner x is the only accessible variable with that name. The inner x is said to shadow the outer x. Once you leave the block, you can access the old value again.

  Quiz: basic

See quiz app.

11.5 (Advanced)

All remaining sections are advanced.

11.6 Terminology: static vs. dynamic

These two adjectives describe phenomena in programming languages:

Static means that something is related to source code and can be determined without executing code.

Dynamic means at runtime.

Let’s look at examples for these two terms.

11.6.1 Static phenomenon: scopes of variables

Variable scopes are a static phenomenon. Consider the following code:

function f() {
  const x = 3;
  // ···
}

x is statically (or lexically) scoped. That is, its scope is fixed and doesn’t change at runtime.

Variable scopes form a static tree (via static nesting).

11.6.2 Dynamic phenomenon: function calls

Function calls are a dynamic phenomenon. Consider the following code:

function g(x) {}
function h(y) {
  if (Math.random()) g(y); // (A)
}

Whether or not the function call in line A happens, can only be decided at runtime.

Function calls form a dynamic tree (via dynamic calls).

11.7 Global variables and the global object

JavaScript’s variable scopes are nested. They form a tree:

The outermost scope is the root of the tree.

The scopes directly contained in that scope are the children of the root.

And so on.

The root is also called the global scope. In web browsers, the only location where one is directly in that scope is at the top level of a script. The variables of the global scope are called global variables and accessible everywhere. There are two kinds of global variables:

Global declarative variables are normal variables.

They can only be created while at the top level of a script, via const, let, and class declarations.

Global object variables are stored in properties of the so-called global object.

They are created in the top level of a script, via var and function declarations.

The global object can be accessed via the global variable globalThis. It can be used to create, read, and delete global object variables.

Other than that, global object variables work like normal variables.

The following HTML fragment demonstrates globalThis and the two kinds of global variables.

<script>
  const declarativeVariable = 'd';
  var objectVariable = 'o';
</script>
<script>
  // All scripts share the same top-level scope:
  console.log(declarativeVariable); // 'd'
  console.log(objectVariable); // 'o'

  // Not all declarations create properties of the global object:
  console.log(globalThis.declarativeVariable); // undefined
  console.log(globalThis.objectVariable); // 'o'
</script>

Each ECMAScript module has its own scope. Therefore, variables that exist at the top level of a module are not global. Fig. 5 illustrates how the various scopes are related.

Figure 5: The global scope is JavaScript’s outermost scope. It has two kinds of variables: object variables (managed via the global object) and normal declarative variables. Each ECMAScript module has its own scope which is contained in the global scope.

11.7.1 globalThis [ES2020]

The global variable globalThis is the new standard way of accessing the global object. It got its name from the fact that it has the same value as this in global scope.

  globalThis does not always directly point to the global object

For example, in browsers, there is an indirection. That indirection is normally not noticable, but it is there and can be observed.

11.7.1.1 Alternatives to globalThis

Older ways of accessing the global object depend on the platform:

Global variable window: is the classic way of referring to the global object. But it doesn’t work in Node.js and in Web Workers.

Global variable self: is available in Web Workers and browsers in general. But it isn’t supported by Node.js.

Global variable global: is only available in Node.js.

11.7.1.2 Use cases for globalThis

The global object is now considered a mistake that JavaScript can’t get rid of, due to backward compatibility. It affects performance negatively and is generally confusing.

ECMAScript 6 introduced several features that make it easier to avoid the global object – for example:

const, let, and class declarations don’t create global object properties when used in global scope.

Each ECMAScript module has its own local scope.

It is usually better to access global object variables via variables and not via properties of globalThis. The former has always worked the same on all JavaScript platforms.

Tutorials on the web occasionally access global variables globVar via window.globVar. But the prefix “window.” is not necessary and I recommend to omit it:

window.encodeURIComponent(str); // no
encodeURIComponent(str); // yes

Therefore, there are relatively few use cases for globalThis – for example:

Polyfills that add new features to old JavaScript engines.

Feature detection, to find out what features a JavaScript engine supports.

11.8 Declarations: scope and activation

These are two key aspects of declarations:

Scope: Where can a declared entity be seen? This is a static trait.

Activation: When can I access an entity? This is a dynamic trait. Some entities can be accessed as soon as we enter their scopes. For others, we have to wait until execution reaches their declarations.

Tbl. 1 summarizes how various declarations handle these aspects.

Table 1: Aspects of declarations. “Duplicates” describes if a declaration can be used twice with the same name (per scope). “Global prop.” describes if a declaration adds a property to the global object, when it is executed in the global scope of a script. TDZ means temporal dead zone (which is explained later). (*) Function declarations are normally block-scoped, but function-scoped in sloppy mode.

| Scope | Activation | Duplicates | Global prop.
const | Block | decl. (TDZ) | ✘ | ✘
let | Block | decl. (TDZ) | ✘ | ✘
function | Block (*) | start | ✔ | ✔
class | Block | decl. (TDZ) | ✘ | ✘
import | Module | same as export | ✘ | ✘
var | Function | start, partially | ✔ | ✔

import is described in §27.5 “ECMAScript modules”. The following sections describe the other constructs in more detail.

11.8.1 const and let: temporal dead zone

For JavaScript, TC39 needed to decide what happens if you access a constant in its direct scope, before its declaration:

{
  console.log(x); // What happens here?
  const x;
}

Some possible approaches are:

The name is resolved in the scope surrounding the current scope.

You get undefined.

There is an error.

Approach 1 was rejected because there is no precedent in the language for this approach. It would therefore not be intuitive to JavaScript programmers.

Approach 2 was rejected because then x wouldn’t be a constant – it would have different values before and after its declaration.

let uses the same approach 3 as const, so that both work similarly and it’s easy to switch between them.

The time between entering the scope of a variable and executing its declaration is called the temporal dead zone (TDZ) of that variable:

During this time, the variable is considered to be uninitialized (as if that were a special value it has).

If you access an uninitialized variable, you get a ReferenceError.

Once you reach a variable declaration, the variable is set to either the value of the initializer (specified via the assignment symbol) or undefined – if there is no initializer.

The following code illustrates the temporal dead zone:

if (true) { // entering scope of `tmp`, TDZ starts
  // `tmp` is uninitialized:
  assert.throws(() => (tmp = 'abc'), ReferenceError);
  assert.throws(() => console.log(tmp), ReferenceError);

  let tmp; // TDZ ends
  assert.equal(tmp, undefined);
}

The next example shows that the temporal dead zone is truly temporal (related to time):

if (true) { // entering scope of `myVar`, TDZ starts
  const func = () => {
    console.log(myVar); // executed later
  };

  // We are within the TDZ:
  // Accessing `myVar` causes `ReferenceError`

  let myVar = 3; // TDZ ends
  func(); // OK, called outside TDZ
}

Even though func() is located before the declaration of myVar and uses that variable, we can call func(). But we have to wait until the temporal dead zone of myVar is over.

11.8.2 Function declarations and early activation

  More information on functions

In this section, we are using functions – before we had a chance to learn them properly. Hopefully, everything still makes sense. Whenever it doesn’t, please see §25 “Callable values”.

A function declaration is always executed when entering its scope, regardless of where it is located within that scope. That enables you to call a function foo() before it is declared:

assert.equal(foo(), 123); // OK
function foo() { return 123; }

The early activation of foo() means that the previous code is equivalent to:

function foo() { return 123; }
assert.equal(foo(), 123);

If you declare a function via const or let, then it is not activated early. In the following example, you can only use bar() after its declaration.

assert.throws(
  () => bar(), // before declaration
  ReferenceError);

const bar = () => { return 123; };

assert.equal(bar(), 123); // after declaration

11.8.2.1 Calling ahead without early activation

Even if a function g() is not activated early, it can be called by a preceding function f() (in the same scope) if we adhere to the following rule: f() must be invoked after the declaration of g().

const f = () => g();
const g = () => 123;

// We call f() after g() was declared:
assert.equal(f(), 123);

The functions of a module are usually invoked after its complete body is executed. Therefore, in modules, you rarely need to worry about the order of functions.

Lastly, note how early activation automatically keeps the aforementioned rule: when entering a scope, all function declarations are executed first, before any calls are made.

11.8.2.2 A pitfall of early activation

If you rely on early activation to call a function before its declaration, then you need to be careful that it doesn’t access data that isn’t activated early.

funcDecl();

const MY_STR = 'abc';
function funcDecl() {
  assert.throws(
    () => MY_STR,
    ReferenceError);
}

The problem goes away if you make the call to funcDecl() after the declaration of MY_STR.

11.8.2.3 The pros and cons of early activation

We have seen that early activation has a pitfall and that you can get most of its benefits without using it. Therefore, it is better to avoid early activation. But I don’t feel strongly about this and, as mentioned before, often use function declarations because I like their syntax.

11.8.3 Class declarations are not activated early

Even though they are similar to function declarations in some ways, class declarations are not activated early:

assert.throws(
  () => new MyClass(),
  ReferenceError);

class MyClass {}

assert.equal(new MyClass() instanceof MyClass, true);

Why is that? Consider the following class declaration:

class MyClass extends Object {}

The operand of extends is an expression. Therefore, you can do things like this:

const identity = x => x;
class MyClass extends identity(Object) {}

Evaluating such an expression must be done at the location where it is mentioned. Anything else would be confusing. That explains why class declarations are not activated early.

11.8.4 var: hoisting (partial early activation)

var is an older way of declaring variables that predates const and let (which are preferred now). Consider the following var declaration.

var x = 123;

This declaration has two parts:

Declaration var x: The scope of a var-declared variable is the innermost surrounding function and not the innermost surrounding block, as for most other declarations. Such a variable is already active at the beginning of its scope and initialized with undefined.

Assignment x = 123: The assignment is always executed in place.

The following code demonstrates the effects of var:

function f() {
  // Partial early activation:
  assert.equal(x, undefined);
  if (true) {
    var x = 123;
    // The assignment is executed in place:
    assert.equal(x, 123);
  }
  // Scope is function, not block:
  assert.equal(x, 123);
}

11.9 Closures

Before we can explore closures, we need to learn about bound variables and free variables.

11.9.1 Bound variables vs. free variables

Per scope, there is a set of variables that are mentioned. Among these variables we distinguish:

Bound variables are declared within the scope. They are parameters and local variables.

Free variables are declared externally. They are also called non-local variables.

Consider the following code:

function func(x) {
  const y = 123;
  console.log(z);
}

In the body of func(), x and y are bound variables. z is a free variable.

11.9.2 What is a closure?

What is a closure then?

A closure is a function plus a connection to the variables that exist at its “birth place”.

What is the point of keeping this connection? It provides the values for the free variables of the function – for example:

function funcFactory(value) {
  return () => {
    return value;
  };
}

const func = funcFactory('abc');
assert.equal(func(), 'abc'); // (A)

funcFactory returns a closure that is assigned to func. Because func has the connection to the variables at its birth place, it can still access the free variable value when it is called in line A (even though it “escaped” its scope).

  All functions in JavaScript are closures

Static scoping is supported via closures in JavaScript. Therefore, every function is a closure.

11.9.3 Example: A factory for incrementors

The following function returns incrementors (a name that I just made up). An incrementor is a function that internally stores a number. When it is called, it updates that number by adding the argument to it and returns the new value.

function createInc(startValue) {
  return (step) => { // (A)
    startValue += step;
    return startValue;
  };
}
const inc = createInc(5);
assert.equal(inc(2), 7);

We can see that the function created in line A keeps its internal number in the free variable startValue. This time, we don’t just read from the birth scope, we use it to store data that we change and that persists across function calls.

We can create more storage slots in the birth scope, via local variables:

function createInc(startValue) {
  let index = -1;
  return (step) => {
    startValue += step;
    index++;
    return [index, startValue];
  };
}
const inc = createInc(5);
assert.deepEqual(inc(2), [0, 7]);
assert.deepEqual(inc(2), [1, 9]);
assert.deepEqual(inc(2), [2, 11]);

11.9.4 Use cases for closures

What are closures good for?

For starters, they are simply an implementation of static scoping. As such, they provide context data for callbacks.

They can also be used by functions to store state that persists across function calls. createInc() is an example of that.

And they can provide private data for objects (produced via literals or classes). The details of how that works are explained in Exploring ES6.

  Quiz: advanced

See quiz app.

12 Values

12.1 What’s a type?

12.2 JavaScript’s type hierarchy

12.3 The types of the language specification

12.4 Primitive values vs. objects

12.4.1 Primitive values (short: primitives)

12.4.2 Objects

12.5 The operators typeof and instanceof: what’s the type of a value?

12.5.1 typeof

12.5.2 instanceof

12.6 Classes and constructor functions

12.6.1 Constructor functions associated with primitive types

12.7 Converting between types

12.7.1 Explicit conversion between types

12.7.2 Coercion (automatic conversion between types)

In this chapter, we’ll examine what kinds of values JavaScript has.

  Supporting tool: ===

In this chapter, we’ll occasionally use the strict equality operator. a === b evaluates to true if a and b are equal. What exactly that means is explained in §13.4.2 “Strict equality (=== and !==)”.

12.1 What’s a type?

For this chapter, I consider types to be sets of values – for example, the type boolean is the set { false, true }.

12.2 JavaScript’s type hierarchy

Figure 6: A partial hierarchy of JavaScript’s types. Missing are the classes for errors, the classes associated with primitive types, and more. The diagram hints at the fact that not all objects are instances of Object.

Fig. 6 shows JavaScript’s type hierarchy. What do we learn from that diagram?

JavaScript distinguishes two kinds of values: primitive values and objects. We’ll see soon what the difference is.

The diagram differentiates objects and instances of class Object. Each instance of Object is also an object, but not vice versa. However, virtually all objects that you’ll encounter in practice are instances of Object – for example, objects created via object literals. More details on this topic are explained in §29.4.3.4 “Objects that aren’t instances of Object”.

12.3 The types of the language specification

The ECMAScript specification only knows a total of eight types. The names of those types are (I’m using TypeScript’s names, not the spec’s names):

undefined with the only element undefined

null with the only element null

boolean with the elements false and true

number the type of all numbers (e.g., -123, 3.141)

bigint the type of all big integers (e.g., -123n)

string the type of all strings (e.g., 'abc')

symbol the type of all symbols (e.g., Symbol('My Symbol'))

object the type of all objects (different from Object, the type of all instances of class Object and its subclasses)

12.4 Primitive values vs. objects

The specification makes an important distinction between values:

Primitive values are the elements of the types undefined, null, boolean, number, bigint, string, symbol.

All other values are objects.

In contrast to Java (that inspired JavaScript here), primitive values are not second-class citizens. The difference between them and objects is more subtle. In a nutshell:

Primitive values: are atomic building blocks of data in JavaScript.

They are passed by value: when primitive values are assigned to variables or passed to functions, their contents are copied.

They are compared by value: when comparing two primitive values, their contents are compared.

Objects: are compound pieces of data.

They are passed by identity (my term): when objects are assigned to variables or passed to functions, their identities (think pointers) are copied.

They are compared by identity (my term): when comparing two objects, their identities are compared.

Other than that, primitive values and objects are quite similar: they both have properties (key-value entries) and can be used in the same locations.

Next, we’ll look at primitive values and objects in more depth.

12.4.1 Primitive values (short: primitives)

12.4.1.1 Primitives are immutable

You can’t change, add, or remove properties of primitives:

let str = 'abc';
assert.equal(str.length, 3);
assert.throws(
  () => { str.length = 1 },
  /^TypeError: Cannot assign to read only property 'length'/
);

12.4.1.2 Primitives are passed by value

Primitives are passed by value: variables (including parameters) store the contents of the primitives. When assigning a primitive value to a variable or passing it as an argument to a function, its content is copied.

let x = 123;
let y = x;
assert.equal(y, 123);

12.4.1.3 Primitives are compared by value

Primitives are compared by value: when comparing two primitive values, we compare their contents.

assert.equal(123 === 123, true);
assert.equal('abc' === 'abc', true);

To see what’s so special about this way of comparing, read on and find out how objects are compared.

12.4.2 Objects

Objects are covered in detail in §28 “Single objects” and the following chapter. Here, we mainly focus on how they differ from primitive values.

Let’s first explore two common ways of creating objects:

Object literal:

const obj = {
  first: 'Jane',
  last: 'Doe',
};

The object literal starts and ends with curly braces {}. It creates an object with two properties. The first property has the key 'first' (a string) and the value 'Jane'. The second property has the key 'last' and the value 'Doe'. For more information on object literals, consult §28.2.1 “Object literals: properties”.

Array literal:

const fruits = ['strawberry', 'apple'];

The Array literal starts and ends with square brackets []. It creates an Array with two elements: 'strawberry' and 'apple'. For more information on Array literals, consult [content not included].

12.4.2.1 Objects are mutable by default

By default, you can freely change, add, and remove the properties of objects:

const obj = {};

obj.count = 2; // add a property
assert.equal(obj.count, 2);

obj.count = 3; // change a property
assert.equal(obj.count, 3);

12.4.2.2 Objects are passed by identity

Objects are passed by identity (my term): variables (including parameters) store the identities of objects.

The identity of an object is like a pointer (or a transparent reference) to the object’s actual data on the heap (think shared main memory of a JavaScript engine).

When assigning an object to a variable or passing it as an argument to a function, its identity is copied. Each object literal creates a fresh object on the heap and returns its identity.

const a = {}; // fresh empty object
// Pass the identity in `a` to `b`:
const b = a;

// Now `a` and `b` point to the same object
// (they “share” that object):
assert.equal(a === b, true);

// Changing `a` also changes `b`:
a.name = 'Tessa';
assert.equal(b.name, 'Tessa');

JavaScript uses garbage collection to automatically manage memory:

let obj = { prop: 'value' };
obj = {};

Now the old value { prop: 'value' } of obj is garbage (not used anymore). JavaScript will automatically garbage-collect it (remove it from memory), at some point in time (possibly never if there is enough free memory).

  Details: passing by identity

“Passing by identity” means that the identity of an object (a transparent reference) is passed by value. This approach is also called “passing by sharing”.

12.4.2.3 Objects are compared by identity

Objects are compared by identity (my term): two variables are only equal if they contain the same object identity. They are not equal if they refer to different objects with the same content.

const obj = {}; // fresh empty object
assert.equal(obj === obj, true); // same identity
assert.equal({} === {}, false); // different identities, same content

12.5 The operators typeof and instanceof: what’s the type of a value?

The two operators typeof and instanceof let you determine what type a given value x has:

if (typeof x === 'string') ···
if (x instanceof Array) ···

How do they differ?

typeof distinguishes the 7 types of the specification (minus one omission, plus one addition).

instanceof tests which class created a given value.

  Rule of thumb: typeof is for primitive values; instanceof is for objects

12.5.1 typeof

Table 2: The results of the typeof operator.

x | typeof x
undefined | 'undefined'
null | 'object'
Boolean | 'boolean'
Number | 'number'
Bigint | 'bigint'
String | 'string'
Symbol | 'symbol'
Function | 'function'
All other objects | 'object'

Tbl. 2 lists all results of typeof. They roughly correspond to the 7 types of the language specification. Alas, there are two differences, and they are language quirks:

typeof null returns 'object' and not 'null'. That’s a bug. Unfortunately, it can’t be fixed. TC39 tried to do that, but it broke too much code on the web.

typeof of a function should be 'object' (functions are objects). Introducing a separate category for functions is confusing.

These are a few examples of using typeof:

> typeof undefined
'undefined'
> typeof 123n
'bigint'
> typeof 'abc'
'string'
> typeof {}
'object'

  Exercises: Two exercises on typeof

exercises/values/typeof_exrc.mjs

Bonus: exercises/values/is_object_test.mjs

12.5.2 instanceof

This operator answers the question: has a value x been created by a class C?

x instanceof C

For example:

> (function() {}) instanceof Function
true
> ({}) instanceof Object
true
> [] instanceof Array
true

Primitive values are not instances of anything:

> 123 instanceof Number
false
> '' instanceof String
false
> '' instanceof Object
false

  Exercise: instanceof

exercises/values/instanceof_exrc.mjs

12.6 Classes and constructor functions

JavaScript’s original factories for objects are constructor functions: ordinary functions that return “instances” of themselves if you invoke them via the new operator.

ES6 introduced classes, which are mainly better syntax for constructor functions.

In this book, I’m using the terms constructor function and class interchangeably.

Classes can be seen as partitioning the single type object of the specification into subtypes – they give us more types than the limited 7 ones of the specification. Each class is the type of the objects that were created by it.

12.6.1 Constructor functions associated with primitive types

Each primitive type (except for the spec-internal types for undefined and null) has an associated constructor function (think class):

The constructor function Boolean is associated with booleans.

The constructor function Number is associated with numbers.

The constructor function String is associated with strings.

The constructor function Symbol is associated with symbols.

Each of these functions plays several roles – for example, Number:

You can use it as a function and convert values to numbers:

assert.equal(Number('123'), 123);

Number.prototype provides the properties for numbers – for example, method .toString():

assert.equal((123).toString, Number.prototype.toString);

Number is a namespace/container object for tool functions for numbers – for example:

assert.equal(Number.isInteger(123), true);

Lastly, you can also use Number as a class and create number objects. These objects are different from real numbers and should be avoided.

assert.notEqual(new Number(123), 123);
assert.equal(new Number(123).valueOf(), 123);

12.6.1.1 Wrapping primitive values

The constructor functions related to primitive types are also called wrapper types because they provide the canonical way of converting primitive values to objects. In the process, primitive values are “wrapped” in objects.

const prim = true;
assert.equal(typeof prim, 'boolean');
assert.equal(prim instanceof Boolean, false);

const wrapped = Object(prim);
assert.equal(typeof wrapped, 'object');
assert.equal(wrapped instanceof Boolean, true);

assert.equal(wrapped.valueOf(), prim); // unwrap

Wrapping rarely matters in practice, but it is used internally in the language specification, to give primitives properties.

12.7 Converting between types

There are two ways in which values are converted to other types in JavaScript:

Explicit conversion: via functions such as String().

Coercion (automatic conversion): happens when an operation receives operands/parameters that it can’t work with.

12.7.1 Explicit conversion between types

The function associated with a primitive type explicitly converts values to that type:

> Boolean(0)
false
> Number('123')
123
> String(123)
'123'

You can also use Object() to convert values to objects:

> typeof Object(123)
'object'

The following table describes in more detail how this conversion works:

x | Object(x)
undefined | {}
null | {}
boolean | new Boolean(x)
number | new Number(x)
bigint | An instance of BigInt (new throws TypeError)
string | new String(x)
symbol | An instance of Symbol (new throws TypeError)
object | x

12.7.2 Coercion (automatic conversion between types)

For many operations, JavaScript automatically converts the operands/parameters if their types don’t fit. This kind of automatic conversion is called coercion.

For example, the multiplication operator coerces its operands to numbers:

> '7' * '3'
21

Many built-in functions coerce, too. For example, parseInt() coerces its parameter to string (parsing stops at the first character that is not a digit):

> parseInt(123.45)
123

  Exercise: Converting values to primitives

exercises/values/conversion_exrc.mjs

  Quiz

See quiz app.

13 Operators

13.1 Making sense of operators

13.1.1 Operators coerce their operands to appropriate types

13.1.2 Most operators only work with primitive values

13.2 The plus operator (+)

13.3 Assignment operators

13.3.1 The plain assignment operator

13.3.2 Compound assignment operators

13.4 Equality: == vs. ===

13.4.1 Loose equality (== and !=)

13.4.2 Strict equality (=== and !==)

13.4.3 Recommendation: always use strict equality

13.4.4 Even stricter than ===: Object.is()

13.5 Ordering operators

13.6 Various other operators

13.6.1 Comma operator

13.6.2 void operator

13.1 Making sense of operators

JavaScript’s operators may seem quirky. With the following two rules, they are easier to understand:

Operators coerce their operands to appropriate types

Most operators only work with primitive values

13.1.1 Operators coerce their operands to appropriate types

If an operator gets operands that don’t have the proper types, it rarely throws an exception. Instead, it coerces (automatically converts) the operands so that it can work with them. Let’s look at two examples.

First, the multiplication operator can only work with numbers. Therefore, it converts strings to numbers before computing its result.

> '7' * '3'
21

Second, the square brackets operator ([ ]) for accessing the properties of an object can only handle strings and symbols. All other values are coerced to string:

const obj = {};
obj['true'] = 123;

// Coerce true to the string 'true'
assert.equal(obj[true], 123);

13.1.2 Most operators only work with primitive values

As mentioned before, most operators only work with primitive values. If an operand is an object, it is usually coerced to a primitive value – for example:

> [1,2,3] + [4,5,6]
'1,2,34,5,6'

Why? The plus operator first coerces its operands to primitive values:

> String([1,2,3])
'1,2,3'
> String([4,5,6])
'4,5,6'

Next, it concatenates the two strings:

> '1,2,3' + '4,5,6'
'1,2,34,5,6'

13.2 The plus operator (+)

The plus operator works as follows in JavaScript:

First, it converts both operands to primitive values. Then it switches to one of two modes:

String mode: If one of the two primitive values is a string, then it converts the other one to a string, concatenates both strings, and returns the result.

Number mode: Otherwise, It converts both operands to numbers, adds them, and returns the result.

String mode lets us use + to assemble strings:

> 'There are ' + 3 + ' items'
'There are 3 items'

Number mode means that if neither operand is a string (or an object that becomes a string) then everything is coerced to numbers:

> 4 + true
5

Number(true) is 1.

13.3 Assignment operators

13.3.1 The plain assignment operator

The plain assignment operator is used to change storage locations:

x = value; // assign to a previously declared variable
obj.propKey = value; // assign to a property
arr[index] = value; // assign to an Array element

Initializers in variable declarations can also be viewed as a form of assignment:

const x = value;
let y = value;

13.3.2 Compound assignment operators

JavaScript supports the following assignment operators:

Arithmetic assignment operators: += -= *= /= %= [ES1]

+= can also be used for string concatenation

Introduced later: **= [ES2016]

Bitwise assignment operators: &= ^= |= [ES1]

Bitwise shift assignment operators: <<= >>= >>>= [ES1]

Logical assignment operators: ||= &&= ??= [ES2021]

13.3.2.1 Logical assignment operators

Logical assignment operators work differently from other compound assignment operators:

Assignment operator | Equivalent to | Only assigns if a is
a ||= b | a || (a = b) | Falsy
a &&= b | a && (a = b) | Truthy
a ??= b | a ?? (a = b) | Nullish

Why is a ||= b equivalent to the following expression?

a || (a = b)

Why not to this expression?

a = a || b

The former expression has the benefit of short-circuiting: The assignment is only evaluated if a evaluates to false. Therefore, the assignment is only performed if it’s necessary. In contrast, the latter expression always performs an assignment.

For more on ??=, see §14.4.5 “The nullish coalescing assignment operator (??=)”.

13.3.2.2 The remaining compound assignment operators

For operators op other than || && ??, the following two ways of assigning are equivalent:

myvar op= value
myvar = myvar op value

If, for example, op is +, then we get the operator += that works as follows.

let str = '';
str += '<b>';
str += 'Hello!';
str += '</b>';

assert.equal(str, '<b>Hello!</b>');

13.4 Equality: == vs. ===

JavaScript has two kinds of equality operators: loose equality (==) and strict equality (===). The recommendation is to always use the latter.

  Other names for == and ===

== is also called double equals. Its official name in the language specification is abstract equality comparison.

=== is also called triple equals.

13.4.1 Loose equality (== and !=)

Loose equality is one of JavaScript’s quirks. It often coerces operands. Some of those coercions make sense:

> '123' == 123
true
> false == 0
true

Others less so:

> '' == 0
true

Objects are coerced to primitives if (and only if!) the other operand is primitive:

> [1, 2, 3] == '1,2,3'
true
> ['1', '2', '3'] == '1,2,3'
true

If both operands are objects, they are only equal if they are the same object:

> [1, 2, 3] == ['1', '2', '3']
false
> [1, 2, 3] == [1, 2, 3]
false

> const arr = [1, 2, 3];
> arr == arr
true

Lastly, == considers undefined and null to be equal:

> undefined == null
true

13.4.2 Strict equality (=== and !==)

Strict equality never coerces. Two values are only equal if they have the same type. Let’s revisit our previous interaction with the == operator and see what the === operator does:

> false === 0
false
> '123' === 123
false

An object is only equal to another value if that value is the same object:

> [1, 2, 3] === '1,2,3'
false
> ['1', '2', '3'] === '1,2,3'
false

> [1, 2, 3] === ['1', '2', '3']
false
> [1, 2, 3] === [1, 2, 3]
false

> const arr = [1, 2, 3];
> arr === arr
true

The === operator does not consider undefined and null to be equal:

> undefined === null
false

13.4.3 Recommendation: always use strict equality

I recommend to always use ===. It makes your code easier to understand and spares you from having to think about the quirks of ==.

Let’s look at two use cases for == and what I recommend to do instead.

13.4.3.1 Use case for ==: comparing with a number or a string

== lets you check if a value x is a number or that number as a string – with a single comparison:

if (x == 123) {
  // x is either 123 or '123'
}

I prefer either of the following two alternatives:

if (x === 123 || x === '123') ···
if (Number(x) === 123) ···

You can also convert x to a number when you first encounter it.

13.4.3.2 Use case for ==: comparing with undefined or null

Another use case for == is to check if a value x is either undefined or null:

if (x == null) {
  // x is either null or undefined
}

The problem with this code is that you can’t be sure if someone meant to write it that way or if they made a typo and meant === null.

I prefer either of the following two alternatives:

if (x === undefined || x === null) ···
if (!x) ···

A downside of the second alternative is that it accepts values other than undefined and null, but it is a well-established pattern in JavaScript (to be explained in detail in §15.3 “Truthiness-based existence checks”).

The following three conditions are also roughly equivalent:

if (x != null) ···
if (x !== undefined && x !== null) ···
if (x) ···

13.4.4 Even stricter than ===: Object.is()

Method Object.is() compares two values:

> Object.is(123, 123)
true
> Object.is(123, '123')
false

It is even stricter than ===. For example, it considers NaN, the error value for computations involving numbers, to be equal to itself:

> Object.is(NaN, NaN)
true
> NaN === NaN
false

That is occasionally useful. For example, you can use it to implement an improved version of the Array method .indexOf():

const myIndexOf = (arr, elem) => {
  return arr.findIndex(x => Object.is(x, elem));
};

myIndexOf() finds NaN in an Array, while .indexOf() doesn’t:

> myIndexOf([0,NaN,2], NaN)
1
> [0,NaN,2].indexOf(NaN)
-1

The result -1 means that .indexOf() couldn’t find its argument in the Array.

13.5 Ordering operators

Table 3: JavaScript’s ordering operators.

Operator | name
< | less than
<= | Less than or equal
> | Greater than
>= | Greater than or equal

JavaScript’s ordering operators (tbl. 3) work for both numbers and strings:

> 5 >= 2
true
> 'bar' < 'foo'
true

<= and >= are based on strict equality.

  The ordering operators don’t work well for human languages

The ordering operators don’t work well for comparing text in a human language, e.g., when capitalization or accents are involved. The details are explained in §20.5 “Comparing strings”.

13.6 Various other operators

The following operators are covered elsewhere in this book:

Operators for booleans, numbers, strings, objects

The nullish coalescing operator (??) for default values

The next two subsections discuss two operators that are rarely used.

13.6.1 Comma operator

The comma operator has two operands, evaluates both of them and returns the second one:

> 'a', 'b'
'b'

For more information on this operator, see Speaking JavaScript.

13.6.2 void operator

The void operator evaluates its operand and returns undefined:

> void (3 + 2)
undefined

For more information on this operator, see Speaking JavaScript.

  Quiz

See quiz app.

14 The non-values undefined and null

14.1 undefined vs. null

14.2 Occurrences of undefined and null

14.2.1 Occurrences of undefined

14.2.2 Occurrences of null

14.3 Checking for undefined or null

14.4 The nullish coalescing operator (??) for default values [ES2020]

14.4.1 Example: counting matches

14.4.2 Example: specifying a default value for a property

14.4.3 Using destructuring for default values

14.4.4 Legacy approach: using logical Or (||) for default values

14.4.5 The nullish coalescing assignment operator (??=)

14.5 undefined and null don’t have properties

14.6 The history of undefined and null

Many programming languages have one “non-value” called null. It indicates that a variable does not currently point to an object – for example, when it hasn’t been initialized yet.

In contrast, JavaScript has two of them: undefined and null.

14.1 undefined vs. null

Both values are very similar and often used interchangeably. How they differ is therefore subtle. The language itself makes the following distinction:

undefined means “not initialized” (e.g., a variable) or “not existing” (e.g., a property of an object).

null means “the intentional absence of any object value” (a quote from the language specification).

Programmers may make the following distinction:

undefined is the non-value used by the language (when something is uninitialized, etc.).

null means “explicitly switched off”. That is, it helps implement a type that comprises both meaningful values and a meta-value that stands for “no meaningful value”. Such a type is called option type or maybe type in functional programming.

14.2 Occurrences of undefined and null

The following subsections describe where undefined and null appear in the language. We’ll encounter several mechanisms that are explained in more detail later in this book.

14.2.1 Occurrences of undefined

Uninitialized variable myVar:

let myVar;
assert.equal(myVar, undefined);

Parameter x is not provided:

function func(x) {
  return x;
}
assert.equal(func(), undefined);

Property .unknownProp is missing:

const obj = {};
assert.equal(obj.unknownProp, undefined);

If we don’t explicitly specify the result of a function via a return statement, JavaScript returns undefined for us:

function func() {}
assert.equal(func(), undefined);

14.2.2 Occurrences of null

The prototype of an object is either an object or, at the end of a chain of prototypes, null. Object.prototype does not have a prototype:

> Object.getPrototypeOf(Object.prototype)
null

If we match a regular expression (such as /a/) against a string (such as 'x'), we either get an object with matching data (if matching was successful) or null (if matching failed):

> /a/.exec('x')
null

The JSON data format does not support undefined, only null:

> JSON.stringify({a: undefined, b: null})
'{"b":null}'

14.3 Checking for undefined or null

Checking for either:

if (x === null) ···
if (x === undefined) ···

Does x have a value?

if (x !== undefined && x !== null) {
  // ···
}
if (x) { // truthy?
  // x is neither: undefined, null, false, 0, NaN, ''
}

Is x either undefined or null?

if (x === undefined || x === null) {
  // ···
}
if (!x) { // falsy?
  // x is: undefined, null, false, 0, NaN, ''
}

Truthy means “is true if coerced to boolean”. Falsy means “is false if coerced to boolean”. Both concepts are explained properly in §15.2 “Falsy and truthy values”.

14.4 The nullish coalescing operator (??) for default values [ES2020]

Sometimes we receive a value and only want to use it if it isn’t either null or undefined. Otherwise, we’d like to use a default value, as a fallback. We can do that via the nullish coalescing operator (??):

const valueToUse = receivedValue ?? defaultValue;

The following two expressions are equivalent:

a ?? b
a !== undefined && a !== null ? a : b

14.4.1 Example: counting matches

The following code shows a real-world example:

function countMatches(regex, str) {
  const matchResult = str.match(regex); // null or Array
  return (matchResult ?? []).length;
}

assert.equal(
  countMatches(/a/g, 'ababa'), 3);
assert.equal(
  countMatches(/b/g, 'ababa'), 2);
assert.equal(
  countMatches(/x/g, 'ababa'), 0);

If there are one or more matches for regex inside str, then .match() returns an Array. If there are no matches, it unfortunately returns null (and not the empty Array). We fix that via the ?? operator.

We also could have used optional chaining:

return matchResult?.length ?? 0;

14.4.2 Example: specifying a default value for a property

function getTitle(fileDesc) {
  return fileDesc.title ?? '(Untitled)';
}

const files = [
  {path: 'index.html', title: 'Home'},
  {path: 'tmp.html'},
];
assert.deepEqual(
  files.map(f => getTitle(f)),
  ['Home', '(Untitled)']);

14.4.3 Using destructuring for default values

In some cases, destructuring can also be used for default values – for example:

function getTitle(fileDesc) {
  const {title = '(Untitled)'} = fileDesc;
  return title;
}

14.4.4 Legacy approach: using logical Or (||) for default values

Before ECMAScript 2020 and the nullish coalescing operator, logical Or was used for default values. That has a downside.

|| works as expected for undefined and null:

> undefined || 'default'
'default'
> null || 'default'
'default'

But it also returns the default for all other falsy values – for example:

> false || 'default'
'default'
> 0 || 'default'
'default'
> 0n || 'default'
'default'
> '' || 'default'
'default'

Compare that to how ?? works:

> undefined ?? 'default'
'default'
> null ?? 'default'
'default'

> false ?? 'default'
false
> 0 ?? 'default'
0
> 0n ?? 'default'
0n
> '' ?? 'default'
''

14.4.5 The nullish coalescing assignment operator (??=)

??= is a logical assignment operator. The following two expressions are roughly equivalent:

a ??= b
a ?? (a = b)

That means that ??= is short-circuiting: The assignment is only made if a is undefined or null.

14.4.5.1 Example: using ??= to add missing properties

const books = [
  {
    isbn: '123',
  },
  {
    title: 'ECMAScript Language Specification',
    isbn: '456',
  },
];

// Add property .title where it’s missing
for (const book of books) {
  book.title ??= '(Untitled)';
}

assert.deepEqual(
  books,
  [
    {
      isbn: '123',
      title: '(Untitled)',
    },
    {
      title: 'ECMAScript Language Specification',
      isbn: '456',
    },
  ]);

14.5 undefined and null don’t have properties

undefined and null are the only two JavaScript values where we get an exception if we try to read a property. To explore this phenomenon, let’s use the following function, which reads (“gets”) property .foo and returns the result.

function getFoo(x) {
  return x.foo;
}

If we apply getFoo() to various values, we can see that it only fails for undefined and null:

> getFoo(undefined)
TypeError: Cannot read property 'foo' of undefined
> getFoo(null)
TypeError: Cannot read property 'foo' of null

> getFoo(true)
undefined
> getFoo({})
undefined

14.6 The history of undefined and null

In Java (which inspired many aspects of JavaScript), initialization values depend on the static type of a variable:

Variables with object types are initialized with null.

Each primitive type has its own initialization value. For example, int variables are initialized with 0.

In JavaScript, each variable can hold both object values and primitive values. Therefore, if null means “not an object”, JavaScript also needs an initialization value that means “neither an object nor a primitive value”. That initialization value is undefined.

  Quiz

See quiz app.

15 Booleans

15.1 Converting to boolean

15.2 Falsy and truthy values

15.2.1 Checking for truthiness or falsiness

15.3 Truthiness-based existence checks

15.3.1 Pitfall: truthiness-based existence checks are imprecise

15.3.2 Use case: was a parameter provided?

15.3.3 Use case: does a property exist?

15.4 Conditional operator (? :)

15.5 Binary logical operators: And (x && y), Or (x || y)

15.5.1 Value-preservation

15.5.2 Short-circuiting

15.5.3 Logical And (x && y)

15.5.4 Logical Or (||)

15.6 Logical Not (!)

The primitive type boolean comprises two values – false and true:

> typeof false
'boolean'
> typeof true
'boolean'

15.1 Converting to boolean

  The meaning of “converting to [type]”

“Converting to [type]” is short for “Converting arbitrary values to values of type [type]”.

These are three ways in which you can convert an arbitrary value x to a boolean.

Boolean(x)
Most descriptive; recommended.

x ? true : false
Uses the conditional operator (explained later in this chapter).

!!x
Uses the logical Not operator (!). This operator coerces its operand to boolean. It is applied a second time to get a non-negated result.

Tbl. 4 describes how various values are converted to boolean.

Table 4: Converting values to booleans.

x | Boolean(x)
undefined | false
null | false
boolean | x (no change)
number | 0 → false, NaN → false
| Other numbers → true
bigint | 0 → false
| Other numbers → true
string | '' → false
| Other strings → true
symbol | true
object | Always true

15.2 Falsy and truthy values

When checking the condition of an if statement, a while loop, or a do-while loop, JavaScript works differently than you may expect. Take, for example, the following condition:

if (value) {}

In many programming languages, this condition is equivalent to:

if (value === true) {}

However, in JavaScript, it is equivalent to:

if (Boolean(value) === true) {}

That is, JavaScript checks if value is true when converted to boolean. This kind of check is so common that the following names were introduced:

A value is called truthy if it is true when converted to boolean.

A value is called falsy if it is false when converted to boolean.

Each value is either truthy or falsy. Consulting tbl. 4, we can make an exhaustive list of falsy values:

undefined

null

Boolean: false

Numbers: 0, NaN

Bigint: 0n

String: ''

All other values (including all objects) are truthy:

> Boolean('abc')
true
> Boolean([])
true
> Boolean({})
true

15.2.1 Checking for truthiness or falsiness

if (x) {
  // x is truthy
}

if (!x) {
  // x is falsy
}

if (x) {
  // x is truthy
} else {
  // x is falsy
}

const result = x ? 'truthy' : 'falsy';

The conditional operator that is used in the last line, is explained later in this chapter.

  Exercise: Truthiness

exercises/booleans/truthiness_exrc.mjs

15.3 Truthiness-based existence checks

In JavaScript, if you read something that doesn’t exist (e.g., a missing parameter or a missing property), you usually get undefined as a result. In these cases, an existence check amounts to comparing a value with undefined. For example, the following code checks if object obj has the property .prop:

if (obj.prop !== undefined) {
  // obj has property .prop
}

Due to undefined being falsy, we can shorten this check to:

if (obj.prop) {
  // obj has property .prop
}

15.3.1 Pitfall: truthiness-based existence checks are imprecise

Truthiness-based existence checks have one pitfall: they are not very precise. Consider this previous example:

if (obj.prop) {
  // obj has property .prop
}

The body of the if statement is skipped if:

obj.prop is missing (in which case, JavaScript returns undefined).

However, it is also skipped if:

obj.prop is undefined.

obj.prop is any other falsy value (null, 0, '', etc.).

In practice, this rarely causes problems, but you have to be aware of this pitfall.

15.3.2 Use case: was a parameter provided?

A truthiness check is often used to determine if the caller of a function provided a parameter:

function func(x) {
  if (!x) {
    throw new Error('Missing parameter x');
  }
  // ···
}

On the plus side, this pattern is established and short. It correctly throws errors for undefined and null.

On the minus side, there is the previously mentioned pitfall: the code also throws errors for all other falsy values.

An alternative is to check for undefined:

if (x === undefined) {
  throw new Error('Missing parameter x');
}

15.3.3 Use case: does a property exist?

Truthiness checks are also often used to determine if a property exists:

function readFile(fileDesc) {
  if (!fileDesc.path) {
    throw new Error('Missing property: .path');
  }
  // ···
}
readFile({ path: 'foo.txt' }); // no error

This pattern is also established and has the usual caveat: it not only throws if the property is missing, but also if it exists and has any of the falsy values.

If you truly want to check if the property exists, you have to use the in operator:

if (! ('path' in fileDesc)) {
  throw new Error('Missing property: .path');
}

15.4 Conditional operator (? :)

The conditional operator is the expression version of the if statement. Its syntax is:

«condition» ? «thenExpression» : «elseExpression»

It is evaluated as follows:

If condition is truthy, evaluate and return thenExpression.

Otherwise, evaluate and return elseExpression.

The conditional operator is also called ternary operator because it has three operands.

Examples:

> true ? 'yes' : 'no'
'yes'
> false ? 'yes' : 'no'
'no'
> '' ? 'yes' : 'no'
'no'

The following code demonstrates that whichever of the two branches “then” and “else” is chosen via the condition, only that branch is evaluated. The other branch isn’t.

const x = (true ? console.log('then') : console.log('else'));

// Output:
// 'then'

15.5 Binary logical operators: And (x && y), Or (x || y)

The binary logical operators && and || are value-preserving and short-circuiting.

15.5.1 Value-preservation

Value-preservation means that operands are interpreted as booleans but returned unchanged:

> 12 || 'hello'
12
> 0 || 'hello'
'hello'

15.5.2 Short-circuiting

Short-circuiting means if the first operand already determines the result, then the second operand is not evaluated. The only other operator that delays evaluating its operands is the conditional operator. Usually, all operands are evaluated before performing an operation.

For example, logical And (&&) does not evaluate its second operand if the first one is falsy:

const x = false && console.log('hello');
// No output

If the first operand is truthy, console.log() is executed:

const x = true && console.log('hello');

// Output:
// 'hello'

15.5.3 Logical And (x && y)

The expression a && b (“a And b”) is evaluated as follows:

Evaluate a.

Is the result falsy? Return it.

Otherwise, evaluate b and return the result.

In other words, the following two expressions are roughly equivalent:

a && b
!a ? a : b

Examples:

> false && true
false
> false && 'abc'
false

> true && false
false
> true && 'abc'
'abc'

> '' && 'abc'
''

15.5.4 Logical Or (||)

The expression a || b (“a Or b”) is evaluated as follows:

Evaluate a.

Is the result truthy? Return it.

Otherwise, evaluate b and return the result.

In other words, the following two expressions are roughly equivalent:

a || b
a ? a : b

Examples:

> true || false
true
> true || 'abc'
true

> false || true
true
> false || 'abc'
'abc'

> 'abc' || 'def'
'abc'

15.5.4.1 Legacy use case for logical Or (||): providing default values

ECMAScript 2020 introduced the nullish coalescing operator (??) for default values. Before that, logical Or was used for this purpose:

const valueToUse = receivedValue || defaultValue;

See §14.4 “The nullish coalescing operator (??) for default values [ES2020]” for more information on ?? and the downsides of || in this case.

  Legacy exercise: Default values via the Or operator (||)

exercises/booleans/default_via_or_exrc.mjs

15.6 Logical Not (!)

The expression !x (“Not x”) is evaluated as follows:

Evaluate x.

Is it truthy? Return false.

Otherwise, return true.

Examples:

> !false
true
> !true
false

> !0
true
> !123
false

> !''
true
> !'abc'
false

  Quiz

See quiz app.

16 Numbers

16.1 Numbers are used for both floating point numbers and integers

16.2 Number literals

16.2.1 Integer literals

16.2.2 Floating point literals

16.2.3 Syntactic pitfall: properties of integer literals

16.2.4 Underscores (_) as separators in number literals [ES2021]

16.3 Arithmetic operators

16.3.1 Binary arithmetic operators

16.3.2 Unary plus (+) and negation (-)

16.3.3 Incrementing (++) and decrementing (--)

16.4 Converting to number

16.5 Error values

16.5.1 Error value: NaN

16.5.2 Error value: Infinity

16.6 The precision of numbers: careful with decimal fractions

16.7 (Advanced)

16.8 Background: floating point precision

16.8.1 A simplified representation of floating point numbers

16.9 Integer numbers in JavaScript

16.9.1 Converting to integer

16.9.2 Ranges of integer numbers in JavaScript

16.9.3 Safe integers

16.10 Bitwise operators

16.10.1 Internally, bitwise operators work with 32-bit integers

16.10.2 Bitwise Not

16.10.3 Binary bitwise operators

16.10.4 Bitwise shift operators

16.10.5 b32(): displaying unsigned 32-bit integers in binary notation

16.11 Quick reference: numbers

16.11.1 Global functions for numbers

16.11.2 Static properties of Number

16.11.3 Static methods of Number

16.11.4 Methods of Number.prototype

16.11.5 Sources

JavaScript has two kinds of numeric values:

Numbers are 64-bit floating point numbers and are also used for smaller integers (within a range of plus/minus 53 bits).

Bigints represent integers with an arbitrary precision.

This chapter covers numbers. Bigints are covered later in this book.

16.1 Numbers are used for both floating point numbers and integers

The type number is used for both integers and floating point numbers in JavaScript:

98
123.45

However, all numbers are doubles, 64-bit floating point numbers implemented according to the IEEE Standard for Floating-Point Arithmetic (IEEE 754).

Integer numbers are simply floating point numbers without a decimal fraction:

> 98 === 98.0
true

Note that, under the hood, most JavaScript engines are often able to use real integers, with all associated performance and storage size benefits.

16.2 Number literals

Let’s examine literals for numbers.

16.2.1 Integer literals

Several integer literals let us express integers with various bases:

// Binary (base 2)
assert.equal(0b11, 3); // ES6

// Octal (base 8)
assert.equal(0o10, 8); // ES6

// Decimal (base 10)
assert.equal(35, 35);

// Hexadecimal (base 16)
assert.equal(0xE7, 231);

16.2.2 Floating point literals

Floating point numbers can only be expressed in base 10.

Fractions:

> 35.0
35

Exponent: eN means ×10N

> 3e2
300
> 3e-2
0.03
> 0.3e2
30

16.2.3 Syntactic pitfall: properties of integer literals

Accessing a property of an integer literal entails a pitfall: If the integer literal is immediately followed by a dot, then that dot is interpreted as a decimal dot:

7.toString(); // syntax error

There are four ways to work around this pitfall:

7.0.toString()
(7).toString()
7..toString()
7 .toString()  // space before dot

16.2.4 Underscores (_) as separators in number literals [ES2021]

Grouping digits to make long numbers more readable has a long tradition. For example:

In 1825, London had 1,335,000 inhabitants.

The distance between Earth and Sun is 149,600,000 km.

Since ES2021, we can use underscores as separators in number literals:

const inhabitantsOfLondon = 1_335_000;
const distanceEarthSunInKm = 149_600_000;

With other bases, grouping is important, too:

const fileSystemPermission = 0b111_111_000;
const bytes = 0b1111_10101011_11110000_00001101;
const words = 0xFAB_F00D;

We can also use the separator in fractions and exponents:

const massOfElectronInKg = 9.109_383_56e-31;
const trillionInShortScale = 1e1_2;

16.2.4.1 Where can we put separators?

The locations of separators are restricted in two ways:

We can only put underscores between two digits. Therefore, all of the following number literals are illegal:

3_.141
3._141

1_e12
1e_12

_1464301  // valid variable name!
1464301_

0_b111111000
0b_111111000

We can’t use more than one underscore in a row:

123__456 // two underscores – not allowed

The motivation behind these restrictions is to keep parsing simple and to avoid strange edge cases.

16.2.4.2 Parsing numbers with separators

The following functions for parsing numbers do not support separators:

Number()

parseInt()

parseFloat()

For example:

> Number('123_456')
NaN
> parseInt('123_456')
123

The rationale is that numeric separators are for code. Other kinds of input should be processed differently.

16.3 Arithmetic operators

16.3.1 Binary arithmetic operators

Tbl. 5 lists JavaScript’s binary arithmetic operators.

Table 5: Binary arithmetic operators.

Operator | Name |  | Example
n + m | Addition | ES1 | 3 + 4 → 7
n - m | Subtraction | ES1 | 9 - 1 → 8
n * m | Multiplication | ES1 | 3 * 2.25 → 6.75
n / m | Division | ES1 | 5.625 / 5 → 1.125
n % m | Remainder | ES1 | 8 % 5 → 3
|  |  | -8 % 5 → -3
n ** m | Exponentiation | ES2016 | 4 ** 2 → 16

16.3.1.1 % is a remainder operator

% is a remainder operator, not a modulo operator. Its result has the sign of the first operand:

> 5 % 3
2
> -5 % 3
-2

For more information on the difference between remainder and modulo, see the blog post “Remainder operator vs. modulo operator (with JavaScript code)” on 2ality.

16.3.2 Unary plus (+) and negation (-)

Tbl. 6 summarizes the two operators unary plus (+) and negation (-).

Table 6: The operators unary plus (+) and negation (-).

Operator | Name |  | Example
+n | Unary plus | ES1 | +(-7) → -7
-n | Unary negation | ES1 | -(-7) → 7

Both operators coerce their operands to numbers:

> +'5'
5
> +'-12'
-12
> -'9'
-9

Thus, unary plus lets us convert arbitrary values to numbers.

16.3.3 Incrementing (++) and decrementing (--)

The incrementation operator ++ exists in a prefix version and a suffix version. In both versions, it destructively adds one to its operand. Therefore, its operand must be a storage location that can be changed.

The decrementation operator -- works the same, but subtracts one from its operand. The next two examples explain the difference between the prefix and the suffix version.

Tbl. 7 summarizes the incrementation and decrementation operators.

Table 7: Incrementation operators and decrementation operators.

Operator | Name |  | Example
v++ | Increment | ES1 | let v=0; [v++, v] → [0, 1]
++v | Increment | ES1 | let v=0; [++v, v] → [1, 1]
v-- | Decrement | ES1 | let v=1; [v--, v] → [1, 0]
--v | Decrement | ES1 | let v=1; [--v, v] → [0, 0]

Next, we’ll look at examples of these operators in use.

Prefix ++ and prefix -- change their operands and then return them.

let foo = 3;
assert.equal(++foo, 4);
assert.equal(foo, 4);

let bar = 3;
assert.equal(--bar, 2);
assert.equal(bar, 2);

Suffix ++ and suffix -- return their operands and then change them.

let foo = 3;
assert.equal(foo++, 3);
assert.equal(foo, 4);

let bar = 3;
assert.equal(bar--, 3);
assert.equal(bar, 2);

16.3.3.1 Operands: not just variables

We can also apply these operators to property values:

const obj = { a: 1 };
++obj.a;
assert.equal(obj.a, 2);

And to Array elements:

const arr = [ 4 ];
arr[0]++;
assert.deepEqual(arr, [5]);

  Exercise: Number operators

exercises/numbers-math/is_odd_test.mjs

16.4 Converting to number

These are three ways of converting values to numbers:

Number(value)

+value

parseFloat(value) (avoid; different than the other two!)

Recommendation: use the descriptive Number(). Tbl. 8 summarizes how it works.

Table 8: Converting values to numbers.

x | Number(x)
undefined | NaN
null | +0
boolean | false → +0, true → 1
number | x (no change)
bigint | Throws TypeError
string | '' → +0
| Other → parsed number, ignoring leading/trailing whitespace
symbol | Throws TypeError
object | Configurable (e.g. via .valueOf())

Examples:

assert.equal(Number(123.45), 123.45);

assert.equal(Number(''), 0);
assert.equal(Number('\n 123.45 \t'), 123.45);
assert.equal(Number('xyz'), NaN);

How objects are converted to numbers can be configured – for example, by overriding .valueOf():

> Number({ valueOf() { return 123 } })
123

  Exercise: Converting to number

exercises/numbers-math/parse_number_test.mjs

16.5 Error values

Two number values are returned when errors happen:

NaN

Infinity

16.5.1 Error value: NaN

NaN is an abbreviation of “not a number”. Ironically, JavaScript considers it to be a number:

> typeof NaN
'number'

When is NaN returned?

NaN is returned if a number can’t be parsed:

> Number('$$$')
NaN
> Number(undefined)
NaN

NaN is returned if an operation can’t be performed:

> Math.log(-1)
NaN
> Math.sqrt(-1)
NaN

NaN is returned if an operand or argument is NaN (to propagate errors):

> NaN - 3
NaN
> 7 ** NaN
NaN

16.5.1.1 Checking for NaN

NaN is the only JavaScript value that is not strictly equal to itself:

const n = NaN;
assert.equal(n === n, false);

These are several ways of checking if a value x is NaN:

const x = NaN;

assert.equal(Number.isNaN(x), true); // preferred
assert.equal(Object.is(x, NaN), true);
assert.equal(x !== x, true);

In the last line, we use the comparison quirk to detect NaN.

16.5.1.2 Finding NaN in Arrays

Some Array methods can’t find NaN:

> [NaN].indexOf(NaN)
-1

Others can:

> [NaN].includes(NaN)
true
> [NaN].findIndex(x => Number.isNaN(x))
0
> [NaN].find(x => Number.isNaN(x))
NaN

Alas, there is no simple rule of thumb. We have to check for each method how it handles NaN.

16.5.2 Error value: Infinity

When is the error value Infinity returned?

Infinity is returned if a number is too large:

> Math.pow(2, 1023)
8.98846567431158e+307
> Math.pow(2, 1024)
Infinity

Infinity is returned if there is a division by zero:

> 5 / 0
Infinity
> -5 / 0
-Infinity

16.5.2.1 Infinity as a default value

Infinity is larger than all other numbers (except NaN), making it a good default value:

function findMinimum(numbers) {
  let min = Infinity;
  for (const n of numbers) {
    if (n < min) min = n;
  }
  return min;
}

assert.equal(findMinimum([5, -1, 2]), -1);
assert.equal(findMinimum([]), Infinity);

16.5.2.2 Checking for Infinity

These are two common ways of checking if a value x is Infinity:

const x = Infinity;

assert.equal(x === Infinity, true);
assert.equal(Number.isFinite(x), false);

  Exercise: Comparing numbers

exercises/numbers-math/find_max_test.mjs

16.6 The precision of numbers: careful with decimal fractions

Internally, JavaScript floating point numbers are represented with base 2 (according to the IEEE 754 standard). That means that decimal fractions (base 10) can’t always be represented precisely:

> 0.1 + 0.2
0.30000000000000004
> 1.3 * 3
3.9000000000000004
> 1.4 * 100000000000000
139999999999999.98

We therefore need to take rounding errors into consideration when performing arithmetic in JavaScript.

Read on for an explanation of this phenomenon.

  Quiz: basic

See quiz app.

16.7 (Advanced)

All remaining sections of this chapter are advanced.

16.8 Background: floating point precision

In JavaScript, computations with numbers don’t always produce correct results – for example:

> 0.1 + 0.2
0.30000000000000004

To understand why, we need to explore how JavaScript represents floating point numbers internally. It uses three integers to do so, which take up a total of 64 bits of storage (double precision):

Component | Size | Integer range
Sign | 1 bit | [0, 1]
Fraction | 52 bits | [0, 2⁵²−1]
Exponent | 11 bits | [−1023, 1024]

The floating point number represented by these integers is computed as follows:

(–1)sign × 0b1.fraction × 2exponent

This representation can’t encode a zero because its second component (involving the fraction) always has a leading 1. Therefore, a zero is encoded via the special exponent −1023 and a fraction 0.

16.8.1 A simplified representation of floating point numbers

To make further discussions easier, we simplify the previous representation:

Instead of base 2 (binary), we use base 10 (decimal) because that’s what most people are more familiar with.

The fraction is a natural number that is interpreted as a fraction (digits after a point). We switch to a mantissa, an integer that is interpreted as itself. As a consequence, the exponent is used differently, but its fundamental role doesn’t change.

As the mantissa is an integer (with its own sign), we don’t need a separate sign, anymore.

The new representation works like this:

mantissa × 10exponent

Let’s try out this representation for a few floating point numbers.

For the integer −123, we mainly need the mantissa:

> -123 * (10 ** 0)
-123

For the number 1.5, we imagine there being a point after the mantissa. We use a negative exponent to move that point one digit to the left:

> 15 * (10 ** -1)
1.5

For the number 0.25, we move the point two digits to the left:

> 25 * (10 ** -2)
0.25

Representations with negative exponents can also be written as fractions with positive exponents in the denominators:

> 15 * (10 ** -1) === 15 / (10 ** 1)
true
> 25 * (10 ** -2) === 25 / (10 ** 2)
true

These fractions help with understanding why there are numbers that our encoding cannot represent:

1/10 can be represented. It already has the required format: a power of 10 in the denominator.

1/2 can be represented as 5/10. We turned the 2 in the denominator into a power of 10 by multiplying the numerator and denominator by 5.

1/4 can be represented as 25/100. We turned the 4 in the denominator into a power of 10 by multiplying the numerator and denominator by 25.

1/3 cannot be represented. There is no way to turn the denominator into a power of 10. (The prime factors of 10 are 2 and 5. Therefore, any denominator that only has these prime factors can be converted to a power of 10, by multiplying both the numerator and denominator by enough twos and fives. If a denominator has a different prime factor, then there’s nothing we can do.)

To conclude our excursion, we switch back to base 2:

0.5 = 1/2 can be represented with base 2 because the denominator is already a power of 2.

0.25 = 1/4 can be represented with base 2 because the denominator is already a power of 2.

0.1 = 1/10 cannot be represented because the denominator cannot be converted to a power of 2.

0.2 = 2/10 cannot be represented because the denominator cannot be converted to a power of 2.

Now we can see why 0.1 + 0.2 doesn’t produce a correct result: internally, neither of the two operands can be represented precisely.

The only way to compute precisely with decimal fractions is by internally switching to base 10. For many programming languages, base 2 is the default and base 10 an option. For example, Java has the class BigDecimal and Python has the module decimal. There are tentative plans to add something similar to JavaScript: the ECMAScript proposal “Decimal” is currently at stage 0.

16.9 Integer numbers in JavaScript

Integer numbers are normal (floating point) numbers without decimal fractions:

> 1 === 1.0
true
> Number.isInteger(1.0)
true

In this section, we’ll look at a few tools for working with these pseudo-integers. JavaScript also supports bigints, which are real integers.

16.9.1 Converting to integer

The recommended way of converting numbers to integers is to use one of the rounding methods of the Math object:

Math.floor(n): returns the largest integer i ≤ n

> Math.floor(2.1)
2
> Math.floor(2.9)
2

Math.ceil(n): returns the smallest integer i ≥ n

> Math.ceil(2.1)
3
> Math.ceil(2.9)
3

Math.round(n): returns the integer that is “closest” to n with __.5 being rounded up – for example:

> Math.round(2.4)
2
> Math.round(2.5)
3

Math.trunc(n): removes any decimal fraction (after the point) that n has, therefore turning it into an integer.

> Math.trunc(2.1)
2
> Math.trunc(2.9)
2

For more information on rounding, consult §17.3 “Rounding”.

16.9.2 Ranges of integer numbers in JavaScript

These are important ranges of integer numbers in JavaScript:

Safe integers: can be represented “safely” by JavaScript (more on what that means in the next subsection)

Precision: 53 bits plus sign

Range: (−2⁵³, 2⁵³)

Array indices

Precision: 32 bits, unsigned

Range: [0, 2³²−1) (excluding the maximum length)

Typed Arrays have a larger range of 53 bits (safe and unsigned)

Bitwise operators (bitwise Or, etc.)

Precision: 32 bits

Range of unsigned right shift (>>>): unsigned, [0, 2³²)

Range of all other bitwise operators: signed, [−2³¹, 2³¹)

16.9.3 Safe integers

This is the range of integer numbers that are safe in JavaScript (53 bits plus a sign):

[–2⁵³–1, 2⁵³–1]

An integer is safe if it is represented by exactly one JavaScript number. Given that JavaScript numbers are encoded as a fraction multiplied by 2 to the power of an exponent, higher integers can also be represented, but then there are gaps between them.

For example (18014398509481984 is 2⁵⁴):

> 18014398509481984
18014398509481984
> 18014398509481985
18014398509481984
> 18014398509481986
18014398509481984
> 18014398509481987
18014398509481988

The following properties of Number help determine if an integer is safe:

assert.equal(Number.MAX_SAFE_INTEGER, (2 ** 53) - 1);
assert.equal(Number.MIN_SAFE_INTEGER, -Number.MAX_SAFE_INTEGER);

assert.equal(Number.isSafeInteger(5), true);
assert.equal(Number.isSafeInteger('5'), false);
assert.equal(Number.isSafeInteger(5.1), false);
assert.equal(Number.isSafeInteger(Number.MAX_SAFE_INTEGER), true);
assert.equal(Number.isSafeInteger(Number.MAX_SAFE_INTEGER+1), false);

  Exercise: Detecting safe integers

exercises/numbers-math/is_safe_integer_test.mjs

16.9.3.1 Safe computations

Let’s look at computations involving unsafe integers.

The following result is incorrect and unsafe, even though both of its operands are safe:

> 9007199254740990 + 3
9007199254740992

The following result is safe, but incorrect. The first operand is unsafe; the second operand is safe:

> 9007199254740995 - 10
9007199254740986

Therefore, the result of an expression a op b is correct if and only if:

isSafeInteger(a) && isSafeInteger(b) && isSafeInteger(a op b)

That is, both operands and the result must be safe.

16.10 Bitwise operators

16.10.1 Internally, bitwise operators work with 32-bit integers

Internally, JavaScript’s bitwise operators work with 32-bit integers. They produce their results in the following steps:

Input (JavaScript numbers): The 1–2 operands are first converted to JavaScript numbers (64-bit floating point numbers) and then to 32-bit integers.

Computation (32-bit integers): The actual operation processes 32-bit integers and produces a 32-bit integer.

Output (JavaScript number): Before returning the result, it is converted back to a JavaScript number.

16.10.1.1 The types of operands and results

For each bitwise operator, this book mentions the types of its operands and its result. Each type is always one of the following two:

Type | Description | Size | Range
Int32 | signed 32-bit integer | 32 bits incl. sign | [−2³¹, 2³¹)
Uint32 | unsigned 32-bit integer | 32 bits | [0, 2³²)

Considering the previously mentioned steps, I recommend to pretend that bitwise operators internally work with unsigned 32-bit integers (step “computation”) and that Int32 and Uint32 only affect how JavaScript numbers are converted to and from integers (steps “input” and “output”).

16.10.1.2 Displaying JavaScript numbers as unsigned 32-bit integers

While exploring the bitwise operators, it occasionally helps to display JavaScript numbers as unsigned 32-bit integers in binary notation. That’s what b32() does (whose implementation is shown later):

assert.equal(
  b32(-1),
  '11111111111111111111111111111111');
assert.equal(
  b32(1),
  '00000000000000000000000000000001');
assert.equal(
  b32(2 ** 31),
  '10000000000000000000000000000000');

16.10.2 Bitwise Not

Table 9: The bitwise Not operator.

Operation | Name | Type signature |
~num | Bitwise Not, ones’ complement | Int32 → Int32 | ES1

The bitwise Not operator (tbl. 9) inverts each binary digit of its operand:

> b32(~0b100)
'11111111111111111111111111111011'

This so-called ones’ complement is similar to a negative for some arithmetic operations. For example, adding an integer to its ones’ complement is always -1:

> 4 + ~4
-1
> -11 + ~-11
-1

16.10.3 Binary bitwise operators

Table 10: Binary bitwise operators.

Operation | Name | Type signature |
num1 & num2 | Bitwise And | Int32 × Int32 → Int32 | ES1
num1 ¦ num2 | Bitwise Or | Int32 × Int32 → Int32 | ES1
num1 ^ num2 | Bitwise Xor | Int32 × Int32 → Int32 | ES1

The binary bitwise operators (tbl. 10) combine the bits of their operands to produce their results:

> (0b1010 & 0b0011).toString(2).padStart(4, '0')
'0010'
> (0b1010 | 0b0011).toString(2).padStart(4, '0')
'1011'
> (0b1010 ^ 0b0011).toString(2).padStart(4, '0')
'1001'

16.10.4 Bitwise shift operators

Table 11: Bitwise shift operators.

Operation | Name | Type signature |
num << count | Left shift | Int32 × Uint32 → Int32 | ES1
num >> count | Signed right shift | Int32 × Uint32 → Int32 | ES1
num >>> count | Unsigned right shift | Uint32 × Uint32 → Uint32 | ES1

The shift operators (tbl. 11) move binary digits to the left or to the right:

> (0b10 << 1).toString(2)
'100'

>> preserves highest bit, >>> doesn’t:

> b32(0b10000000000000000000000000000010 >> 1)
'11000000000000000000000000000001'
> b32(0b10000000000000000000000000000010 >>> 1)
'01000000000000000000000000000001'

16.10.5 b32(): displaying unsigned 32-bit integers in binary notation

We have now used b32() a few times. The following code is an implementation of it:

/**
 * Return a string representing n as a 32-bit unsigned integer,
 * in binary notation.
 */
function b32(n) {
  // >>> ensures highest bit isn’t interpreted as a sign
  return (n >>> 0).toString(2).padStart(32, '0');
}
assert.equal(
  b32(6),
  '00000000000000000000000000000110');

n >>> 0 means that we are shifting n zero bits to the right. Therefore, in principle, the >>> operator does nothing, but it still coerces n to an unsigned 32-bit integer:

> 12 >>> 0
12
> -12 >>> 0
4294967284
> (2**32 + 1) >>> 0
1

16.11 Quick reference: numbers

16.11.1 Global functions for numbers

JavaScript has the following four global functions for numbers:

isFinite()

isNaN()

parseFloat()

parseInt()

However, it is better to use the corresponding methods of Number (Number.isFinite(), etc.), which have fewer pitfalls. They were introduced with ES6 and are discussed below.

16.11.2 Static properties of Number

.EPSILON: number [ES6]

The difference between 1 and the next representable floating point number. In general, a machine epsilon provides an upper bound for rounding errors in floating point arithmetic.

Approximately: 2.2204460492503130808472633361816 × 10⁻¹⁶

.MAX_SAFE_INTEGER: number [ES6]

The largest integer that JavaScript can represent unambiguously (2⁵³−1).

.MAX_VALUE: number [ES1]

The largest positive finite JavaScript number.

Approximately: 1.7976931348623157 × 10³⁰⁸

.MIN_SAFE_INTEGER: number [ES6]

The smallest integer that JavaScript can represent unambiguously (−2⁵³+1).

.MIN_VALUE: number [ES1]

The smallest positive JavaScript number. Approximately 5 × 10⁻³²⁴.

.NaN: number [ES1]

The same as the global variable NaN.

.NEGATIVE_INFINITY: number [ES1]

The same as -Number.POSITIVE_INFINITY.

.POSITIVE_INFINITY: number [ES1]

The same as the global variable Infinity.

16.11.3 Static methods of Number

.isFinite(num: number): boolean [ES6]

Returns true if num is an actual number (neither Infinity nor -Infinity nor NaN).

> Number.isFinite(Infinity)
false
> Number.isFinite(-Infinity)
false
> Number.isFinite(NaN)
false
> Number.isFinite(123)
true

.isInteger(num: number): boolean [ES6]

Returns true if num is a number and does not have a decimal fraction.

> Number.isInteger(-17)
true
> Number.isInteger(33)
true
> Number.isInteger(33.1)
false
> Number.isInteger('33')
false
> Number.isInteger(NaN)
false
> Number.isInteger(Infinity)
false

.isNaN(num: number): boolean [ES6]

Returns true if num is the value NaN:

> Number.isNaN(NaN)
true
> Number.isNaN(123)
false
> Number.isNaN('abc')
false

.isSafeInteger(num: number): boolean [ES6]

Returns true if num is a number and unambiguously represents an integer.

.parseFloat(str: string): number [ES6]

Coerces its parameter to string and parses it as a floating point number. For converting strings to numbers, Number() (which ignores leading and trailing whitespace) is usually a better choice than Number.parseFloat() (which ignores leading whitespace and illegal trailing characters and can hide problems).

> Number.parseFloat(' 123.4#')
123.4
> Number(' 123.4#')
NaN

.parseInt(str: string, radix=10): number [ES6]

Coerces its parameter to string and parses it as an integer, ignoring leading whitespace and illegal trailing characters:

> Number.parseInt('  123#')
123

The parameter radix specifies the base of the number to be parsed:

> Number.parseInt('101', 2)
5
> Number.parseInt('FF', 16)
255

Do not use this method to convert numbers to integers: coercing to string is inefficient. And stopping before the first non-digit is not a good algorithm for removing the fraction of a number. Here is an example where it goes wrong:

> Number.parseInt(1e21, 10) // wrong
1

It is better to use one of the rounding functions of Math to convert a number to an integer:

> Math.trunc(1e21) // correct
1e+21

16.11.4 Methods of Number.prototype

(Number.prototype is where the methods of numbers are stored.)

.toExponential(fractionDigits?: number): string [ES3]

Returns a string that represents the number via exponential notation. With fractionDigits, we can specify, how many digits should be shown of the number that is multiplied with the exponent (the default is to show as many digits as necessary).

Example: number too small to get a positive exponent via .toString().

> 1234..toString()
'1234'

> 1234..toExponential() // 3 fraction digits
'1.234e+3'
> 1234..toExponential(5)
'1.23400e+3'
> 1234..toExponential(1)
'1.2e+3'

Example: fraction not small enough to get a negative exponent via .toString().

> 0.003.toString()
'0.003'
> 0.003.toExponential()
'3e-3'

.toFixed(fractionDigits=0): string [ES3]

Returns an exponent-free representation of the number, rounded to fractionDigits digits.

> 0.00000012.toString() // with exponent
'1.2e-7'

> 0.00000012.toFixed(10) // no exponent
'0.0000001200'
> 0.00000012.toFixed()
'0'

If the number is 10²¹ or greater, even .toFixed() uses an exponent:

> (10 ** 21).toFixed()
'1e+21'

.toPrecision(precision?: number): string [ES3]

Works like .toString(), but precision specifies how many digits should be shown. If precision is missing, .toString() is used.

> 1234..toPrecision(3)  // requires exponential notation
'1.23e+3'

> 1234..toPrecision(4)
'1234'

> 1234..toPrecision(5)
'1234.0'

> 1.234.toPrecision(3)
'1.23'

.toString(radix=10): string [ES1]

Returns a string representation of the number.

By default, we get a base 10 numeral as a result:

> 123.456.toString()
'123.456'

If we want the numeral to have a different base, we can specify it via radix:

> 4..toString(2) // binary (base 2)
'100'
> 4.5.toString(2)
'100.1'

> 255..toString(16) // hexadecimal (base 16)
'ff'
> 255.66796875.toString(16)
'ff.ab'

> 1234567890..toString(36)
'kf12oi'

parseInt() provides the inverse operation: it converts a string that contains an integer (no fraction!) numeral with a given base, to a number.

> parseInt('kf12oi', 36)
1234567890

16.11.5 Sources

Wikipedia

TypeScript’s built-in typings

MDN web docs for JavaScript

ECMAScript language specification

  Quiz: advanced

See quiz app.

17 Math

17.1 Data properties

17.2 Exponents, roots, logarithms

17.3 Rounding

17.4 Trigonometric Functions

17.5 Various other functions

17.6 Sources

Math is an object with data properties and methods for processing numbers. You can see it as a poor man’s module: It was created long before JavaScript had modules.

17.1 Data properties

Math.E: number [ES1]

Euler’s number, base of the natural logarithms, approximately 2.7182818284590452354.

Math.LN10: number [ES1]

The natural logarithm of 10, approximately 2.302585092994046.

Math.LN2: number [ES1]

The natural logarithm of 2, approximately 0.6931471805599453.

Math.LOG10E: number [ES1]

The logarithm of e to base 10, approximately 0.4342944819032518.

Math.LOG2E: number [ES1]

The logarithm of e to base 2, approximately 1.4426950408889634.

Math.PI: number [ES1]

The mathematical constant π, ratio of a circle’s circumference to its diameter, approximately 3.1415926535897932.

Math.SQRT1_2: number [ES1]

The square root of 1/2, approximately 0.7071067811865476.

Math.SQRT2: number [ES1]

The square root of 2, approximately 1.4142135623730951.

17.2 Exponents, roots, logarithms

Math.cbrt(x: number): number [ES6]

Returns the cube root of x.

> Math.cbrt(8)
2

Math.exp(x: number): number [ES1]

Returns ex (e being Euler’s number). The inverse of Math.log().

> Math.exp(0)
1
> Math.exp(1) === Math.E
true

Math.expm1(x: number): number [ES6]

Returns Math.exp(x)-1. The inverse of Math.log1p(). Very small numbers (fractions close to 0) are represented with a higher precision. Therefore, this function returns more precise values whenever .exp() returns values close to 1.

Math.log(x: number): number [ES1]

Returns the natural logarithm of x (to base e, Euler’s number). The inverse of Math.exp().

> Math.log(1)
0
> Math.log(Math.E)
1
> Math.log(Math.E ** 2)
2

Math.log1p(x: number): number [ES6]

Returns Math.log(1 + x). The inverse of Math.expm1(). Very small numbers (fractions close to 0) are represented with a higher precision. Therefore, you can provide this function with a more precise argument whenever the argument for .log() is close to 1.

Math.log10(x: number): number [ES6]

Returns the logarithm of x to base 10. The inverse of 10 ** x.

> Math.log10(1)
0
> Math.log10(10)
1
> Math.log10(100)
2

Math.log2(x: number): number [ES6]

Returns the logarithm of x to base 2. The inverse of 2 ** x.

> Math.log2(1)
0
> Math.log2(2)
1
> Math.log2(4)
2

Math.pow(x: number, y: number): number [ES1]

Returns xy, x to the power of y. The same as x ** y.

> Math.pow(2, 3)
8
> Math.pow(25, 0.5)
5

Math.sqrt(x: number): number [ES1]

Returns the square root of x. The inverse of x ** 2.

> Math.sqrt(9)
3

17.3 Rounding

Rounding means converting an arbitrary number to an integer (a number without a decimal fraction). The following functions implement different approaches to rounding.

Math.ceil(x: number): number [ES1]

Returns the smallest (closest to −∞) integer i with x ≤ i.

> Math.ceil(2.1)
3
> Math.ceil(2.9)
3

Math.floor(x: number): number [ES1]

Returns the largest (closest to +∞) integer i with i ≤ x.

> Math.floor(2.1)
2
> Math.floor(2.9)
2

Math.round(x: number): number [ES1]

Returns the integer that is closest to x. If the decimal fraction of x is .5 then .round() rounds up (to the integer closer to positive infinity):

> Math.round(2.4)
2
> Math.round(2.5)
3

Math.trunc(x: number): number [ES6]

Removes the decimal fraction of x and returns the resulting integer.

> Math.trunc(2.1)
2
> Math.trunc(2.9)
2

Tbl. 12 shows the results of the rounding functions for a few representative inputs.

Table 12: Rounding functions of Math. Note how things change with negative numbers because “larger” always means “closer to positive infinity”.

| -2.9 | -2.5 | -2.1 | 2.1 | 2.5 | 2.9
Math.floor | -3 | -3 | -3 | 2 | 2 | 2
Math.ceil | -2 | -2 | -2 | 3 | 3 | 3
Math.round | -3 | -2 | -2 | 2 | 3 | 3
Math.trunc | -2 | -2 | -2 | 2 | 2 | 2

17.4 Trigonometric Functions

All angles are specified in radians. Use the following two functions to convert between degrees and radians.

function degreesToRadians(degrees) {
  return degrees / 180 * Math.PI;
}
assert.equal(degreesToRadians(90), Math.PI/2);

function radiansToDegrees(radians) {
  return radians / Math.PI * 180;
}
assert.equal(radiansToDegrees(Math.PI), 180);

Math.acos(x: number): number [ES1]

Returns the arc cosine (inverse cosine) of x.

> Math.acos(0)
1.5707963267948966
> Math.acos(1)
0

Math.acosh(x: number): number [ES6]

Returns the inverse hyperbolic cosine of x.

Math.asin(x: number): number [ES1]

Returns the arc sine (inverse sine) of x.

> Math.asin(0)
0
> Math.asin(1)
1.5707963267948966

Math.asinh(x: number): number [ES6]

Returns the inverse hyperbolic sine of x.

Math.atan(x: number): number [ES1]

Returns the arc tangent (inverse tangent) of x.

Math.atanh(x: number): number [ES6]

Returns the inverse hyperbolic tangent of x.

Math.atan2(y: number, x: number): number [ES1]

Returns the arc tangent of the quotient y/x.

Math.cos(x: number): number [ES1]

Returns the cosine of x.

> Math.cos(0)
1
> Math.cos(Math.PI)
-1

Math.cosh(x: number): number [ES6]

Returns the hyperbolic cosine of x.

Math.hypot(...values: number[]): number [ES6]

Returns the square root of the sum of the squares of values (Pythagoras’ theorem):

> Math.hypot(3, 4)
5

Math.sin(x: number): number [ES1]

Returns the sine of x.

> Math.sin(0)
0
> Math.sin(Math.PI / 2)
1

Math.sinh(x: number): number [ES6]

Returns the hyperbolic sine of x.

Math.tan(x: number): number [ES1]

Returns the tangent of x.

> Math.tan(0)
0
> Math.tan(1)
1.5574077246549023

Math.tanh(x: number): number; [ES6]

Returns the hyperbolic tangent of x.

17.5 Various other functions

Math.abs(x: number): number [ES1]

Returns the absolute value of x.

> Math.abs(3)
3
> Math.abs(-3)
3
> Math.abs(0)
0

Math.clz32(x: number): number [ES6]

Counts the leading zero bits in the 32-bit integer x. Used in DSP algorithms.

> Math.clz32(0b01000000000000000000000000000000)
1
> Math.clz32(0b00100000000000000000000000000000)
2
> Math.clz32(2)
30
> Math.clz32(1)
31

Math.max(...values: number[]): number [ES1]

Converts values to numbers and returns the largest one.

> Math.max(3, -5, 24)
24

Math.min(...values: number[]): number [ES1]

Converts values to numbers and returns the smallest one.

> Math.min(3, -5, 24)
-5

Math.random(): number [ES1]

Returns a pseudo-random number n where 0 ≤ n < 1.

/** Returns a random integer i with 0 <= i < max */
function getRandomInteger(max) {
  return Math.floor(Math.random() * max);
}

Math.sign(x: number): number [ES6]

Returns the sign of a number:

> Math.sign(-8)
-1
> Math.sign(0)
0
> Math.sign(3)
1

17.6 Sources

Wikipedia

TypeScript’s built-in typings

MDN web docs for JavaScript

ECMAScript language specification

18 Bigints – arbitrary-precision integers [ES2020] (advanced)

18.1 Why bigints?

18.2 Bigints

18.2.1 Going beyond 53 bits for integers

18.2.2 Example: using bigints

18.3 Bigint literals

18.3.1 Underscores (_) as separators in bigint literals [ES2021]

18.4 Reusing number operators for bigints (overloading)

18.4.1 Arithmetic operators

18.4.2 Ordering operators

18.4.3 Bitwise operators

18.4.4 Loose equality (==) and inequality (!=)

18.4.5 Strict equality (===) and inequality (!==)

18.5 The wrapper constructor BigInt

18.5.1 BigInt as a constructor and as a function

18.5.2 BigInt.prototype.* methods

18.5.3 BigInt.* methods

18.5.4 Casting and 64-bit integers

18.6 Coercing bigints to other primitive types

18.7 TypedArrays and DataView operations for 64-bit values

18.8 Bigints and JSON

18.8.1 Stringifying bigints

18.8.2 Parsing bigints

18.9 FAQ: Bigints

18.9.1 How do I decide when to use numbers and when to use bigints?

18.9.2 Why not just increase the precision of numbers in the same manner as is done for bigints?

In this chapter, we take a look at bigints, JavaScript’s integers whose storage space grows and shrinks as needed.

18.1 Why bigints?

Before ECMAScript 2020, JavaScript handled integers as follows:

There only was a single type for floating point numbers and integers: 64-bit floating point numbers (IEEE 754 double precision).

Under the hood, most JavaScript engines transparently supported integers: If a number has no decimal digits and is within a certain range, it can internally be stored as a genuine integer. This representation is called small integer and usually fits into 32 bits. For example, the range of small integers on the 64-bit version of the V8 engine is from −2³¹ to 2³¹−1 (source).

JavaScript numbers could also represent integers beyond the small integer range, as floating point numbers. Here, the safe range is plus/minus 53 bits. For more information on this topic, see §16.9.3 “Safe integers”.

Sometimes, we need more than signed 53 bits – for example:

Twitter uses 64-bit integers as IDs for tweets (source). In JavaScript, these IDs had to be stored in strings.

Financial technology uses so-called big integers (integers with arbitrary precision) to represent amounts of money. Internally, the amounts are multiplied so that the decimal numbers disappear. For example, USD amounts are multiplied by 100 so that the cents disappear.

18.2 Bigints

Bigint is a new primitive data type for integers. Bigints don’t have a fixed storage size in bits; their sizes adapt to the integers they represent:

Small integers are represented with fewer bits than large integers.

There is no negative lower limit or positive upper limit for the integers that can be represented.

A bigint literal is a sequence of one or more digits, suffixed with an n – for example:

123n

Operators such as - and * are overloaded and work with bigints:

> 123n * 456n
56088n

Bigints are primitive values. typeof returns a new result for them:

> typeof 123n
'bigint'

18.2.1 Going beyond 53 bits for integers

JavaScript numbers are internally represented as a fraction multiplied by an exponent (see §16.8 “Background: floating point precision” for details). As a consequence, if we go beyond the highest safe integer 2⁵³−1, there are still some integers that can be represented, but with gaps between them:

> 2**53 - 2 // safe
9007199254740990
> 2**53 - 1 // safe
9007199254740991

> 2**53 // unsafe, same as next integer
9007199254740992
> 2**53 + 1
9007199254740992
> 2**53 + 2
9007199254740994
> 2**53 + 3
9007199254740996
> 2**53 + 4
9007199254740996
> 2**53 + 5
9007199254740996

Bigints enable us to go beyond 53 bits:

> 2n**53n
9007199254740992n
> 2n**53n + 1n
9007199254740993n
> 2n**53n + 2n
9007199254740994n

18.2.2 Example: using bigints

This is what using bigints looks like (code based on an example in the proposal):

/**
 * Takes a bigint as an argument and returns a bigint
 */
function nthPrime(nth) {
  if (typeof nth !== 'bigint') {
    throw new TypeError();
  }
  function isPrime(p) {
    for (let i = 2n; i < p; i++) {
      if (p % i === 0n) return false;
    }
    return true;
  }
  for (let i = 2n; ; i++) {
    if (isPrime(i)) {
      if (--nth === 0n) return i;
    }
  }
}

assert.deepEqual(
  [1n, 2n, 3n, 4n, 5n].map(nth => nthPrime(nth)),
  [2n, 3n, 5n, 7n, 11n]
);

18.3 Bigint literals

Like number literals, bigint literals support several bases:

Decimal: 123n

Hexadecimal: 0xFFn

Binary: 0b1101n

Octal: 0o777n

Negative bigints are produced by prefixing the unary minus operator: -0123n

18.3.1 Underscores (_) as separators in bigint literals [ES2021]

Just like in number literals, we can use underscores (_) as separators in bigint literals:

const massOfEarthInKg = 6_000_000_000_000_000_000_000_000n;

Bigints are often used to represent money in the financial technical sector. Separators can help here, too:

const priceInCents = 123_000_00; // 123 thousand dollars

As with number literals, two restrictions apply:

We can only put an underscore between two digits.

We can use at most one underscore in a row.

18.4 Reusing number operators for bigints (overloading)

With most operators, we are not allowed to mix bigints and numbers. If we do, exceptions are thrown:

> 2n + 1
TypeError: Cannot mix BigInt and other types, use explicit conversions

The reason for this rule is that there is no general way of coercing a number and a bigint to a common type: numbers can’t represent bigints beyond 53 bits, bigints can’t represent fractions. Therefore, the exceptions warn us about typos that may lead to unexpected results.

For example, should the result of the following expression be 9007199254740993n or 9007199254740992?

2**53 + 1n

It is also not clear what the result of the following expression should be:

2n**53n * 3.3

18.4.1 Arithmetic operators

Binary +, binary -, *, ** work as expected:

> 7n * 3n
21n

It is OK to mix bigints and strings:

> 6n + ' apples'
'6 apples'

/, % round towards zero (like Math.trunc()):

> 1n / 2n
0n

Unary - works as expected:

> -(-64n)
64n

Unary + is not supported for bigints because much code relies on it coercing its operand to number:

> +23n
TypeError: Cannot convert a BigInt value to a number

18.4.2 Ordering operators

Ordering operators <, >, >=, <= work as expected:

> 17n <= 17n
true
> 3n > -1n
true

Comparing bigints and numbers does not pose any risks. Therefore, we can mix bigints and numbers:

> 3n > -1
true

18.4.3 Bitwise operators

18.4.3.1 Bitwise operators for numbers

Bitwise operators interpret numbers as 32-bit integers. These integers are either unsigned or signed. If they are signed, the negative of an integer is its two’s complement (adding an integer to its two’s complement – while ignoring overflow – produces zero):

> 2**32-1 >> 0
-1

Due to these integers having a fixed size, their highest bits indicate their signs:

> 2**31 >> 0 // highest bit is 1
-2147483648
> 2**31 - 1 >> 0 // highest bit is 0
2147483647

18.4.3.2 Bitwise operators for bigints

For bigints, bitwise operators interpret a negative sign as an infinite two’s complement – for example:

-1 is ···111111 (ones extend infinitely to the left)

-2 is ···111110

-3 is ···111101

-4 is ···111100

That is, a negative sign is more of an external flag and not represented as an actual bit.

18.4.3.3 Bitwise Not (~)

Bitwise Not (~) inverts all bits:

> ~0b10n
-3n
> ~0n
-1n
> ~-2n
1n

18.4.3.4 Binary bitwise operators (&, |, ^)

Applying binary bitwise operators to bigints works analogously to applying them to numbers:

> (0b1010n |  0b0111n).toString(2)
'1111'
> (0b1010n &  0b0111n).toString(2)
'10'

> (0b1010n | -1n).toString(2)
'-1'
> (0b1010n & -1n).toString(2)
'1010'

18.4.3.5 Bitwise signed shift operators (<< and >>)

The signed shift operators for bigints preserve the sign of a number:

> 2n << 1n
4n
> -2n << 1n
-4n

> 2n >> 1n
1n
> -2n >> 1n
-1n

Recall that -1n is a sequence of ones that extends infinitely to the left. That’s why shifting it left doesn’t change it:

> -1n >> 20n
-1n

18.4.3.6 Bitwise unsigned right shift operator (>>>)

There is no unsigned right shift operator for bigints:

> 2n >>> 1n
TypeError: BigInts have no unsigned right shift, use >> instead

Why? The idea behind unsigned right shifting is that a zero is shifted in “from the left”. In other words, the assumption is that there is a finite amount of binary digits.

However, with bigints, there is no “left”, their binary digits extend infinitely. This is especially important with negative numbers.

Signed right shift works even with an infinite number of digits because the highest digit is preserved. Therefore, it can be adapted to bigints.

18.4.4 Loose equality (==) and inequality (!=)

Loose equality (==) and inequality (!=) coerce values:

> 0n == false
true
> 1n == true
true

> 123n == 123
true

> 123n == '123'
true

18.4.5 Strict equality (===) and inequality (!==)

Strict equality (===) and inequality (!==) only consider values to be equal if they have the same type:

> 123n === 123
false
> 123n === 123n
true

18.5 The wrapper constructor BigInt

Analogously to numbers, bigints have the associated wrapper constructor BigInt.

18.5.1 BigInt as a constructor and as a function

new BigInt(): throws a TypeError.

BigInt(x) converts arbitrary values x to bigint. This works similarly to Number(), with several differences which are summarized in tbl. 13 and explained in more detail in the following subsections.

Table 13: Converting values to bigints.

x | BigInt(x)
undefined | Throws TypeError
null | Throws TypeError
boolean | false → 0n, true → 1n
number | Example: 123 → 123n
| Non-integer → throws RangeError
bigint | x (no change)
string | Example: '123' → 123n
| Unparsable → throws SyntaxError
symbol | Throws TypeError
object | Configurable (e.g. via .valueOf())

18.5.1.1 Converting undefined and null

A TypeError is thrown if x is either undefined or null:

> BigInt(undefined)
TypeError: Cannot convert undefined to a BigInt
> BigInt(null)
TypeError: Cannot convert null to a BigInt

18.5.1.2 Converting strings

If a string does not represent an integer, BigInt() throws a SyntaxError (whereas Number() returns the error value NaN):

> BigInt('abc')
SyntaxError: Cannot convert abc to a BigInt

The suffix 'n' is not allowed:

> BigInt('123n')
SyntaxError: Cannot convert 123n to a BigInt

All bases of bigint literals are allowed:

> BigInt('123')
123n
> BigInt('0xFF')
255n
> BigInt('0b1101')
13n
> BigInt('0o777')
511n

18.5.1.3 Non-integer numbers produce exceptions

> BigInt(123.45)
RangeError: The number 123.45 cannot be converted to a BigInt because
it is not an integer
> BigInt(123)
123n

18.5.1.4 Converting objects

How objects are converted to bigints can be configured – for example, by overriding .valueOf():

> BigInt({valueOf() {return 123n}})
123n

18.5.2 BigInt.prototype.* methods

BigInt.prototype holds the methods “inherited” by primitive bigints:

BigInt.prototype.toLocaleString(reserved1?, reserved2?)

BigInt.prototype.toString(radix?)

BigInt.prototype.valueOf()

18.5.3 BigInt.* methods

BigInt.asIntN(width, theInt)
Casts theInt to width bits (signed). This influences how the value is represented internally.

BigInt.asUintN(width, theInt)
Casts theInt to width bits (unsigned).

18.5.4 Casting and 64-bit integers

Casting allows us to create integer values with a specific number of bits. If we want to restrict ourselves to just 64-bit integers, we have to always cast:

const uint64a = BigInt.asUintN(64, 12345n);
const uint64b = BigInt.asUintN(64, 67890n);
const result = BigInt.asUintN(64, uint64a * uint64b);

18.6 Coercing bigints to other primitive types

This table show what happens if we convert bigints to other primitive types:

Convert to | Explicit conversion | Coercion (implicit conversion)
boolean | Boolean(0n) → false | !0n → true
| Boolean(int) → true | !int → false
number | Number(7n) → 7 (example) | +int → TypeError (1)
string | String(7n) → '7' (example) | ''+7n → '7' (example)

Footnote:

(1) Unary + is not supported for bigints, because much code relies on it coercing its operand to number.

18.7 TypedArrays and DataView operations for 64-bit values

Thanks to bigints, Typed Arrays and DataViews can support 64-bit values:

Typed Array constructors:

BigInt64Array

BigUint64Array

DataView methods:

DataView.prototype.getBigInt64()

DataView.prototype.setBigInt64()

DataView.prototype.getBigUint64()

DataView.prototype.setBigUint64()

18.8 Bigints and JSON

The JSON standard is fixed and won’t change. The upside is that old JSON parsing code will never be outdated. The downside is that JSON can’t be extended to contain bigints.

Stringifying bigints throws exceptions:

> JSON.stringify(123n)
TypeError: Do not know how to serialize a BigInt
> JSON.stringify([123n])
TypeError: Do not know how to serialize a BigInt

18.8.1 Stringifying bigints

Therefore, our best option is to store bigints in strings:

const bigintPrefix = '[[bigint]]';

function bigintReplacer(_key, value) {
  if (typeof value === 'bigint') {
    return bigintPrefix + value;
  }
  return value;
}

const data = { value: 9007199254740993n };
assert.equal(
  JSON.stringify(data, bigintReplacer),
  '{"value":"[[bigint]]9007199254740993"}'
);

18.8.2 Parsing bigints

The following code shows how to parse strings such as the one that we have produced in the previous example.

function bigintReviver(_key, value) {
  if (typeof value === 'string' && value.startsWith(bigintPrefix)) {
    return BigInt(value.slice(bigintPrefix.length));
  }
  return value;
}

const str = '{"value":"[[bigint]]9007199254740993"}';
assert.deepEqual(
  JSON.parse(str, bigintReviver),
  { value: 9007199254740993n }
);

18.9 FAQ: Bigints

18.9.1 How do I decide when to use numbers and when to use bigints?

My recommendations:

Use numbers for up to 53 bits and for Array indices. Rationale: They already appear everywhere and are handled efficiently by most engines (especially if they fit into 31 bits). Appearances include:

Array.prototype.forEach()

Array.prototype.entries()

Use bigints for large numeric values: If your fraction-less values don’t fit into 53 bits, you have no choice but to move to bigints.

All existing web APIs return and accept only numbers and will only upgrade to bigint on a case-by-case basis.

18.9.2 Why not just increase the precision of numbers in the same manner as is done for bigints?

One could conceivably split number into integer and double, but that would add many new complexities to the language (several integer-only operators etc.). I’ve sketched the consequences in a Gist.

Acknowledgements:

Thanks to Daniel Ehrenberg for reviewing an earlier version of this content.

Thanks to Dan Callahan for reviewing an earlier version of this content.

19 Unicode – a brief introduction (advanced)

19.1 Code points vs. code units

19.1.1 Code points

19.1.2 Encoding Unicode code points: UTF-32, UTF-16, UTF-8

19.2 Encodings used in web development: UTF-16 and UTF-8

19.2.1 Source code internally: UTF-16

19.2.2 Strings: UTF-16

19.2.3 Source code in files: UTF-8

19.3 Grapheme clusters – the real characters

Unicode is a standard for representing and managing text in most of the world’s writing systems. Virtually all modern software that works with text, supports Unicode. The standard is maintained by the Unicode Consortium. A new version of the standard is published every year (with new emojis, etc.). Unicode version 1.0.0 was published in October 1991.

19.1 Code points vs. code units

Two concepts are crucial for understanding Unicode:

Code points are numbers that represent Unicode characters.

Code units are numbers that encode code points, to store or transmit Unicode text. One or more code units encode a single code point. Each code unit has the same size, which depends on the encoding format that is used. The most popular format, UTF-8, has 8-bit code units.

19.1.1 Code points

The first version of Unicode had 16-bit code points. Since then, the number of characters has grown considerably and the size of code points was extended to 21 bits. These 21 bits are partitioned in 17 planes, with 16 bits each:

Plane 0: Basic Multilingual Plane (BMP), 0x0000–0xFFFF

Contains characters for almost all modern languages (Latin characters, Asian characters, etc.) and many symbols.

Plane 1: Supplementary Multilingual Plane (SMP), 0x10000–0x1FFFF

Supports historic writing systems (e.g., Egyptian hieroglyphs and cuneiform) and additional modern writing systems.

Supports emojis and many other symbols.

Plane 2: Supplementary Ideographic Plane (SIP), 0x20000–0x2FFFF

Contains additional CJK (Chinese, Japanese, Korean) ideographs.

Plane 3–13: Unassigned

Plane 14: Supplementary Special-Purpose Plane (SSP), 0xE0000–0xEFFFF

Contains non-graphical characters such as tag characters and glyph variation selectors.

Plane 15–16: Supplementary Private Use Area (S PUA A/B), 0x0F0000–0x10FFFF

Available for character assignment by parties outside the ISO and the Unicode Consortium. Not standardized.

Planes 1-16 are called supplementary planes or astral planes.

Let’s check the code points of a few characters:

> 'A'.codePointAt(0).toString(16)
'41'
> 'ü'.codePointAt(0).toString(16)
'fc'
> 'π'.codePointAt(0).toString(16)
'3c0'
> '🙂'.codePointAt(0).toString(16)
'1f642'

The hexadecimal numbers of the code points tell us that the first three characters reside in plane 0 (within 16 bits), while the emoji resides in plane 1.

19.1.2 Encoding Unicode code points: UTF-32, UTF-16, UTF-8

The main ways of encoding code points are three Unicode Transformation Formats (UTFs): UTF-32, UTF-16, UTF-8. The number at the end of each format indicates the size (in bits) of its code units.

19.1.2.1 UTF-32 (Unicode Transformation Format 32)

UTF-32 uses 32 bits to store code units, resulting in one code unit per code point. This format is the only one with fixed-length encoding; all others use a varying number of code units to encode a single code point.

19.1.2.2 UTF-16 (Unicode Transformation Format 16)

UTF-16 uses 16-bit code units. It encodes code points as follows:

The BMP (first 16 bits of Unicode) is stored in single code units.

Astral planes: The BMP comprises 0x10_000 code points. Given that Unicode has a total of 0x110_000 code points, we still need to encode the remaining 0x100_000 code points (20 bits). The BMP has two ranges of unassigned code points that provide the necessary storage:

Most significant 10 bits (leading surrogate): 0xD800-0xDBFF

Least significant 10 bits (trailing surrogate): 0xDC00-0xDFFF

In other words, the two hexadecimal digits at the end contribute 8 bits. But we can only use those 8 bits if a BMP starts with one of the following 2-digit pairs:

D8, D9, DA, DB

DC, DD, DE, DF

Per surrogate, we have a choice between 4 pairs, which is where the remaining 2 bits come from.

As a consequence, each UTF-16 code unit is always either a leading surrogate, a trailing surrogate, or encodes a BMP code point.

These are two examples of UTF-16-encoded code points:

Code point 0x03C0 (π) is in the BMP and can therefore be represented by a single UTF-16 code unit: 0x03C0.

Code point 0x1F642 (🙂) is in an astral plane and represented by two code units: 0xD83D and 0xDE42.

19.1.2.3 UTF-8 (Unicode Transformation Format 8)

UTF-8 has 8-bit code units. It uses 1–4 code units to encode a code point:

Code points | Code units
0000–007F | 0bbbbbbb (7 bits)
0080–07FF | 110bbbbb, 10bbbbbb (5+6 bits)
0800–FFFF | 1110bbbb, 10bbbbbb, 10bbbbbb (4+6+6 bits)
10000–1FFFFF | 11110bbb, 10bbbbbb, 10bbbbbb, 10bbbbbb (3+6+6+6 bits)

Notes:

The bit prefix of each code unit tells us:

Is it first in a series of code units? If yes, how many code units will follow?

Is it second or later in a series of code units?

The character mappings in the 0000–007F range are the same as ASCII, which leads to a degree of backward compatibility with older software.

Three examples:

Character | Code point | Code units
A | 0x0041 | 01000001
π | 0x03C0 | 11001111, 10000000
🙂 | 0x1F642 | 11110000, 10011111, 10011001, 10000010

19.2 Encodings used in web development: UTF-16 and UTF-8

The Unicode encoding formats that are used in web development are: UTF-16 and UTF-8.

19.2.1 Source code internally: UTF-16

The ECMAScript specification internally represents source code as UTF-16.

19.2.2 Strings: UTF-16

The characters in JavaScript strings are based on UTF-16 code units:

> const smiley = '🙂';
> smiley.length
2
> smiley === '\uD83D\uDE42' // code units
true

For more information on Unicode and strings, consult §20.6 “Atoms of text: Unicode characters, JavaScript characters, grapheme clusters”.

19.2.3 Source code in files: UTF-8

HTML and JavaScript are almost always encoded as UTF-8 these days.

For example, this is how HTML files usually start now:

<!doctype html>
<html>
<head>
  <meta charset="UTF-8">
···

For HTML modules loaded in web browsers, the standard encoding is also UTF-8.

19.3 Grapheme clusters – the real characters

The concept of a character becomes remarkably complex once you consider many of the world’s writing systems.

On one hand, there are Unicode characters, as represented by code points.

On the other hand, there are grapheme clusters. A grapheme cluster corresponds most closely to a symbol displayed on screen or paper. It is defined as “a horizontally segmentable unit of text”. Therefore, official Unicode documents also call it a user-perceived character. One or more code point characters are needed to encode a grapheme cluster.

For example, the Devanagari kshi is encoded by 4 code points. We use spreading (...) to split a string into an Array with code point characters (for details, consult §20.6.1 “Working with code points”):

Flag emojis are also grapheme clusters and composed of two code point characters – for example, the flag of Japan:

  More information on grapheme clusters

For more information, consult “Let’s Stop Ascribing Meaning to Code Points” by Manish Goregaokar.

  Quiz

See quiz app.

20 Strings

20.1 Plain string literals

20.1.1 Escaping

20.2 Accessing characters and code points

20.2.1 Accessing JavaScript characters

20.2.2 Accessing Unicode code point characters via for-of and spreading

20.3 String concatenation via +

20.4 Converting to string

20.4.1 Stringifying objects

20.4.2 Customizing the stringification of objects

20.4.3 An alternate way of stringifying values

20.5 Comparing strings

20.6 Atoms of text: Unicode characters, JavaScript characters, grapheme clusters

20.6.1 Working with code points

20.6.2 Working with code units (char codes)

20.6.3 ASCII escapes

20.6.4 Caveat: grapheme clusters

20.7 Quick reference: Strings

20.7.1 Converting to string

20.7.2 Numeric values of characters

20.7.3 String operators

20.7.4 String.prototype: finding and matching

20.7.5 String.prototype: extracting

20.7.6 String.prototype: combining

20.7.7 String.prototype: transforming

20.7.8 Sources

Strings are primitive values in JavaScript and immutable. That is, string-related operations always produce new strings and never change existing strings.

20.1 Plain string literals

Plain string literals are delimited by either single quotes or double quotes:

const str1 = 'abc';
const str2 = "abc";
assert.equal(str1, str2);

Single quotes are used more often because it makes it easier to mention HTML, where double quotes are preferred.

The next chapter covers template literals, which give us:

String interpolation

Multiple lines

Raw string literals (backslash has no special meaning)

20.1.1 Escaping

The backslash lets us create special characters:

Unix line break: '\n'

Windows line break: '\r\n'

Tab: '\t'

Backslash: '\\'

The backslash also lets us use the delimiter of a string literal inside that literal:

assert.equal(
  'She said: "Let\'s go!"',
  "She said: \"Let's go!\"");

20.2 Accessing characters and code points

20.2.1 Accessing JavaScript characters

JavaScript has no extra data type for characters – characters are always represented as strings.

const str = 'abc';

// Reading a character at a given index
assert.equal(str[1], 'b');

// Counting the characters in a string:
assert.equal(str.length, 3);

20.2.2 Accessing Unicode code point characters via for-of and spreading

Iterating over strings via for-of or spreading (...) visits Unicode code point characters. Each code point character is encoded by 1–2 JavaScript characters. For more information, see §20.6 “Atoms of text: Unicode characters, JavaScript characters, grapheme clusters”.

This is how we iterate over the code point characters of a string via for-of:

for (const ch of 'x🙂y') {
  console.log(ch);
}
// Output:
// 'x'
// '🙂'
// 'y'

And this is how we convert a string into an Array of code point characters via spreading:

assert.deepEqual([...'x🙂y'], ['x', '🙂', 'y']);

20.3 String concatenation via +

If at least one operand is a string, the plus operator (+) converts any non-strings to strings and concatenates the result:

assert.equal(3 + ' times ' + 4, '3 times 4');

The assignment operator += is useful if we want to assemble a string, piece by piece:

let str = ''; // must be `let`!
str += 'Say it';
str += ' one more';
str += ' time';

assert.equal(str, 'Say it one more time');

  Concatenating via + is efficient

Using + to assemble strings is quite efficient because most JavaScript engines internally optimize it.

  Exercise: Concatenating strings

exercises/strings/concat_string_array_test.mjs

20.4 Converting to string

These are three ways of converting a value x to a string:

String(x)

''+x

x.toString() (does not work for undefined and null)

Recommendation: use the descriptive and safe String().

Examples:

assert.equal(String(undefined), 'undefined');
assert.equal(String(null), 'null');

assert.equal(String(false), 'false');
assert.equal(String(true), 'true');

assert.equal(String(123.45), '123.45');

Pitfall for booleans: If we convert a boolean to a string via String(), we generally can’t convert it back via Boolean():

> String(false)
'false'
> Boolean('false')
true

The only string for which Boolean() returns false, is the empty string.

20.4.1 Stringifying objects

Plain objects have a default string representation that is not very useful:

> String({a: 1})
'[object Object]'

Arrays have a better string representation, but it still hides much information:

> String(['a', 'b'])
'a,b'
> String(['a', ['b']])
'a,b'

> String([1, 2])
'1,2'
> String(['1', '2'])
'1,2'

> String([true])
'true'
> String(['true'])
'true'
> String(true)
'true'

Stringifying functions, returns their source code:

> String(function f() {return 4})
'function f() {return 4}'

20.4.2 Customizing the stringification of objects

We can override the built-in way of stringifying objects by implementing the method toString():

const obj = {
  toString() {
    return 'hello';
  }
};

assert.equal(String(obj), 'hello');

20.4.3 An alternate way of stringifying values

The JSON data format is a text representation of JavaScript values. Therefore, JSON.stringify() can also be used to convert values to strings:

> JSON.stringify({a: 1})
'{"a":1}'
> JSON.stringify(['a', ['b']])
'["a",["b"]]'

The caveat is that JSON only supports null, booleans, numbers, strings, Arrays, and objects (which it always treats as if they were created by object literals).

Tip: The third parameter lets us switch on multiline output and specify how much to indent – for example:

console.log(JSON.stringify({first: 'Jane', last: 'Doe'}, null, 2));

This statement produces the following output:

{
  "first": "Jane",
  "last": "Doe"
}

20.5 Comparing strings

Strings can be compared via the following operators:

< <= > >=

There is one important caveat to consider: These operators compare based on the numeric values of JavaScript characters. That means that the order that JavaScript uses for strings is different from the one used in dictionaries and phone books:

> 'A' < 'B' // ok
true
> 'a' < 'B' // not ok
false
> 'ä' < 'b' // not ok
false

Properly comparing text is beyond the scope of this book. It is supported via the ECMAScript Internationalization API (Intl).

20.6 Atoms of text: Unicode characters, JavaScript characters, grapheme clusters

Quick recap of §19 “Unicode – a brief introduction”:

Unicode characters are represented by code points – numbers which have a range of 21 bits.

In JavaScript strings, Unicode is implemented via code units based on the encoding format UTF-16. Each code unit is a 16-bit number. One to two of code units are needed to encode a single code point.

Therefore, each JavaScript character is represented by a code unit. In the JavaScript standard library, code units are also called char codes. Which is what they are: numbers for JavaScript characters.

Grapheme clusters (user-perceived characters) are written symbols, as displayed on screen or paper. One or more Unicode characters are needed to encode a single grapheme cluster.

The following code demonstrates that a single Unicode character comprises one or two JavaScript characters. We count the latter via .length:

// 3 Unicode characters, 3 JavaScript characters:
assert.equal('abc'.length, 3);

// 1 Unicode character, 2 JavaScript characters:
assert.equal('🙂'.length, 2);

The following table summarizes the concepts we have just explored:

Entity | Numeric representation | Size | Encoded via
Grapheme cluster |  |  | 1+ code points
Unicode character | Code point | 21 bits | 1–2 code units
JavaScript character | UTF-16 code unit | 16 bits | –

20.6.1 Working with code points

Let’s explore JavaScript’s tools for working with code points.

A Unicode code point escape lets us specify a code point hexadecimally (1–5 digits). It produces one or two JavaScript characters.

> '\u{1F642}'
'🙂'

  Unicode escape sequences

In the ECMAScript language specification, Unicode code point escapes and Unicode code unit escapes (which we’ll encounter later) are called Unicode escape sequences.

String.fromCodePoint() converts a single code point to 1–2 JavaScript characters:

> String.fromCodePoint(0x1F642)
'🙂'

.codePointAt() converts 1–2 JavaScript characters to a single code point:

> '🙂'.codePointAt(0).toString(16)
'1f642'

We can iterate over a string, which visits Unicode characters (not JavaScript characters). Iteration is described later in this book. One way of iterating is via a for-of loop:

const str = '🙂a';
assert.equal(str.length, 3);

for (const codePointChar of str) {
  console.log(codePointChar);
}

// Output:
// '🙂'
// 'a'

Spreading (...) into Array literals is also based on iteration and visits Unicode characters:

> [...'🙂a']
[ '🙂', 'a' ]

That makes it a good tool for counting Unicode characters:

> [...'🙂a'].length
2
> '🙂a'.length
3

20.6.2 Working with code units (char codes)

Indices and lengths of strings are based on JavaScript characters (as represented by UTF-16 code units).

To specify a code unit hexadecimally, we can use a Unicode code unit escape with exactly four hexadecimal digits:

> '\uD83D\uDE42'
'🙂'

And we can use String.fromCharCode(). Char code is the standard library’s name for code unit:

> String.fromCharCode(0xD83D) + String.fromCharCode(0xDE42)
'🙂'

To get the char code of a character, use .charCodeAt():

> '🙂'.charCodeAt(0).toString(16)
'd83d'

20.6.3 ASCII escapes

If the code point of a character is below 256, we can refer to it via a ASCII escape with exactly two hexadecimal digits:

> 'He\x6C\x6Co'
'Hello'

(The official name of ASCII escapes is Hexadecimal escape sequences – it was the first escape that used hexadecimal numbers.)

20.6.4 Caveat: grapheme clusters

When working with text that may be written in any human language, it’s best to split at the boundaries of grapheme clusters, not at the boundaries of Unicode characters.

TC39 is working on Intl.Segmenter, a proposal for the ECMAScript Internationalization API to support Unicode segmentation (along grapheme cluster boundaries, word boundaries, sentence boundaries, etc.).

Until that proposal becomes a standard, we can use one of several libraries that are available (do a web search for “JavaScript grapheme”).

20.7 Quick reference: Strings

Strings are immutable; none of the string methods ever modify their strings.

20.7.1 Converting to string

Tbl. 14 describes how various values are converted to strings.

Table 14: Converting values to strings.

x | String(x)
undefined | 'undefined'
null | 'null'
boolean | false → 'false', true → 'true'
number | Example: 123 → '123'
bigint | Example: 123n → '123'
string | x (input, unchanged)
symbol | Example: Symbol('abc') → 'Symbol(abc)'
object | Configurable via, e.g., toString()

20.7.2 Numeric values of characters

Char code: represents a JavaScript character numerically. JavaScript’s name for Unicode code unit.

Size: 16 bits, unsigned

Convert number to character: String.fromCharCode() [ES1]

Convert character to number: string method .charCodeAt() [ES1]

Code point: represents a Unicode character numerically.

Size: 21 bits, unsigned (17 planes, 16 bits each)

Convert number to character: String.fromCodePoint() [ES6]

Convert character to number: string method .codePointAt() [ES6]

20.7.3 String operators

// Access characters via []
const str = 'abc';
assert.equal(str[1], 'b');

// Concatenate strings via +
assert.equal('a' + 'b' + 'c', 'abc');
assert.equal('take ' + 3 + ' oranges', 'take 3 oranges');

20.7.4 String.prototype: finding and matching

(String.prototype is where the methods of strings are stored.)

.endsWith(searchString: string, endPos=this.length): boolean [ES6]

Returns true if the string would end with searchString if its length were endPos. Returns false otherwise.

> 'foo.txt'.endsWith('.txt')
true
> 'abcde'.endsWith('cd', 4)
true

.includes(searchString: string, startPos=0): boolean [ES6]

Returns true if the string contains the searchString and false otherwise. The search starts at startPos.

> 'abc'.includes('b')
true
> 'abc'.includes('b', 2)
false

.indexOf(searchString: string, minIndex=0): number [ES1]

Returns the lowest index at which searchString appears within the string or -1, otherwise. Any returned index will beminIndex` or higher.

> 'abab'.indexOf('a')
0
> 'abab'.indexOf('a', 1)
2
> 'abab'.indexOf('c')
-1

.lastIndexOf(searchString: string, maxIndex=Infinity): number [ES1]

Returns the highest index at which searchString appears within the string or -1, otherwise. Any returned index will bemaxIndex` or lower.

> 'abab'.lastIndexOf('ab', 2)
2
> 'abab'.lastIndexOf('ab', 1)
0
> 'abab'.lastIndexOf('ab')
2

[1 of 2] .match(regExp: string | RegExp): RegExpMatchArray | null [ES3]

If regExp is a regular expression with flag /g not set, then .match() returns the first match for regExp within the string. Or null if there is no match. If regExp is a string, it is used to create a regular expression (think parameter of new RegExp()) before performing the previously mentioned steps.

The result has the following type:

interface RegExpMatchArray extends Array<string> {
  index: number;
  input: string;
  groups: undefined | {
    [key: string]: string
  };
}

Numbered capture groups become Array indices (which is why this type extends Array). Named capture groups (ES2018) become properties of .groups. In this mode, .match() works like RegExp.prototype.exec().

Examples:

> 'ababb'.match(/a(b+)/)
{ 0: 'ab', 1: 'b', index: 0, input: 'ababb', groups: undefined }
> 'ababb'.match(/a(?<foo>b+)/)
{ 0: 'ab', 1: 'b', index: 0, input: 'ababb', groups: { foo: 'b' } }
> 'abab'.match(/x/)
null

[2 of 2] .match(regExp: RegExp): string[] | null [ES3]

If flag /g of regExp is set, .match() returns either an Array with all matches or null if there was no match.

> 'ababb'.match(/a(b+)/g)
[ 'ab', 'abb' ]
> 'ababb'.match(/a(?<foo>b+)/g)
[ 'ab', 'abb' ]
> 'abab'.match(/x/g)
null

.search(regExp: string | RegExp): number [ES3]

Returns the index at which regExp occurs within the string. If regExp is a string, it is used to create a regular expression (think parameter of new RegExp()).

> 'a2b'.search(/[0-9]/)
1
> 'a2b'.search('[0-9]')
1

.startsWith(searchString: string, startPos=0): boolean [ES6]

Returns true if searchString occurs in the string at index startPos. Returns false otherwise.

> '.gitignore'.startsWith('.')
true
> 'abcde'.startsWith('bc', 1)
true

20.7.5 String.prototype: extracting

.slice(start=0, end=this.length): string [ES3]

Returns the substring of the string that starts at (including) index start and ends at (excluding) index end. If an index is negative, it is added to .length before it is used (-1 becomes this.length-1, etc.).

> 'abc'.slice(1, 3)
'bc'
> 'abc'.slice(1)
'bc'
> 'abc'.slice(-2)
'bc'

.split(separator: string | RegExp, limit?: number): string[] [ES3]

Splits the string into an Array of substrings – the strings that occur between the separators. The separator can be a string:

> 'a | b | c'.split('|')
[ 'a ', ' b ', ' c' ]

It can also be a regular expression:

> 'a : b : c'.split(/ *: */)
[ 'a', 'b', 'c' ]
> 'a : b : c'.split(/( *):( *)/)
[ 'a', ' ', ' ', 'b', ' ', ' ', 'c' ]

The last invocation demonstrates that captures made by groups in the regular expression become elements of the returned Array.

Warning: .split('') splits a string into JavaScript characters. That doesn’t work well when dealing with astral Unicode characters (which are encoded as two JavaScript characters). For example, emojis are astral:

> '🙂X🙂'.split('')
[ '\uD83D', '\uDE42', 'X', '\uD83D', '\uDE42' ]

Instead, it is better to use spreading:

> [...'🙂X🙂']
[ '🙂', 'X', '🙂' ]

.substring(start: number, end=this.length): string [ES1]

Use .slice() instead of this method. .substring() wasn’t implemented consistently in older engines and doesn’t support negative indices.

20.7.6 String.prototype: combining

.concat(...strings: string[]): string [ES3]

Returns the concatenation of the string and strings. 'a'.concat('b') is equivalent to 'a'+'b'. The latter is much more popular.

> 'ab'.concat('cd', 'ef', 'gh')
'abcdefgh'

.padEnd(len: number, fillString=' '): string [ES2017]

Appends (fragments of) fillString to the string until it has the desired length len. If it already has or exceeds len, then it is returned without any changes.

> '#'.padEnd(2)
'# '
> 'abc'.padEnd(2)
'abc'
> '#'.padEnd(5, 'abc')
'#abca'

.padStart(len: number, fillString=' '): string [ES2017]

Prepends (fragments of) fillString to the string until it has the desired length len. If it already has or exceeds len, then it is returned without any changes.

> '#'.padStart(2)
' #'
> 'abc'.padStart(2)
'abc'
> '#'.padStart(5, 'abc')
'abca#'

.repeat(count=0): string [ES6]

Returns the string, concatenated count times.

> '*'.repeat()
''
> '*'.repeat(3)
'***'

20.7.7 String.prototype: transforming

.normalize(form: 'NFC'|'NFD'|'NFKC'|'NFKD' = 'NFC'): string [ES6]

Normalizes the string according to the Unicode Normalization Forms.

[1 of 2] .replaceAll(searchValue: string | RegExp, replaceValue: string): string [ES2021]

  What to do if you can’t use .replaceAll()

If .replaceAll() isn’t available on your targeted platform, you can use .replace() instead. How is explained in [content not included].

Replaces all matches of searchValue with replaceValue. If searchValue is a regular expression without flag /g, a TypeError is thrown.

> 'x.x.'.replaceAll('.', '#')
'x#x#'
> 'x.x.'.replaceAll(/./g, '#')
'####'
> 'x.x.'.replaceAll(/./, '#')
TypeError: String.prototype.replaceAll called with
a non-global RegExp argument

Special characters in replaceValue are:

$$: becomes $

$n: becomes the capture of numbered group n (alas, $0 stands for the string '$0', it does not refer to the complete match)

$&: becomes the complete match

$`: becomes everything before the match

$': becomes everything after the match

Examples:

> 'a 1995-12 b'.replaceAll(/([0-9]{4})-([0-9]{2})/g, '|$2|')
'a |12| b'
> 'a 1995-12 b'.replaceAll(/([0-9]{4})-([0-9]{2})/g, '|$&|')
'a |1995-12| b'
> 'a 1995-12 b'.replaceAll(/([0-9]{4})-([0-9]{2})/g, '|$`|')
'a |a | b'

Named capture groups (ES2018) are supported, too:

$<name> becomes the capture of named group name

Example:

assert.equal(
  'a 1995-12 b'.replaceAll(
    /(?<year>[0-9]{4})-(?<month>[0-9]{2})/g, '|$<month>|'),
  'a |12| b');

[2 of 2] .replaceAll(searchValue: string | RegExp, replacer: (...args: any[]) => string): string [ES2021]

If the second parameter is a function, occurrences are replaced with the strings it returns. Its parameters args are:

matched: string. The complete match

g1: string|undefined. The capture of numbered group 1

g2: string|undefined. The capture of numbered group 2

(Etc.)

offset: number. Where was the match found in the input string?

input: string. The whole input string

const regexp = /([0-9]{4})-([0-9]{2})/g;
const replacer = (all, year, month) => '|' + all + '|';
assert.equal(
  'a 1995-12 b'.replaceAll(regexp, replacer),
  'a |1995-12| b');

Named capture groups (ES2018) are supported, too. If there are any, an argument is added at the end with an object whose properties contain the captures:

const regexp = /(?<year>[0-9]{4})-(?<month>[0-9]{2})/g;
const replacer = (...args) => {
  const groups=args.pop();
  return '|' + groups.month + '|';
};
assert.equal(
  'a 1995-12 b'.replaceAll(regexp, replacer),
  'a |12| b');

.replace(searchValue: string | RegExp, replaceValue: string): string [ES3]

.replace(searchValue: string | RegExp, replacer: (...args: any[]) => string): string [ES3]

.replace() works like .replaceAll(), but only replaces the first occurrence if searchValue is a string or a regular expression without /g:

> 'x.x.'.replace('.', '#')
'x#x.'
> 'x.x.'.replace(/./, '#')
'#.x.'

For more information on this method, see [content not included].

.toUpperCase(): string [ES1]

Returns a copy of the string in which all lowercase alphabetic characters are converted to uppercase. How well that works for various alphabets, depends on the JavaScript engine.

> '-a2b-'.toUpperCase()
'-A2B-'
> 'αβγ'.toUpperCase()
'ΑΒΓ'

.toLowerCase(): string [ES1]

Returns a copy of the string in which all uppercase alphabetic characters are converted to lowercase. How well that works for various alphabets, depends on the JavaScript engine.

> '-A2B-'.toLowerCase()
'-a2b-'
> 'ΑΒΓ'.toLowerCase()
'αβγ'

.trim(): string [ES5]

Returns a copy of the string in which all leading and trailing whitespace (spaces, tabs, line terminators, etc.) is gone.

> '\r\n#\t  '.trim()
'#'
> '  abc  '.trim()
'abc'

.trimEnd(): string [ES2019]

Similar to .trim() but only the end of the string is trimmed:

> '  abc  '.trimEnd()
'  abc'

.trimStart(): string [ES2019]

Similar to .trim() but only the beginning of the string is trimmed:

> '  abc  '.trimStart()
'abc  '

20.7.8 Sources

TypeScript’s built-in typings

MDN web docs for JavaScript

ECMAScript language specification

  Exercise: Using string methods

exercises/strings/remove_extension_test.mjs

  Quiz

See quiz app.

21 Using template literals and tagged templates

21.1 Disambiguation: “template”

21.2 Template literals

21.3 Tagged templates

21.3.1 Cooked vs. raw template strings (advanced)

21.4 Examples of tagged templates (as provided via libraries)

21.4.1 Tag function library: lit-html

21.4.2 Tag function library: re-template-tag

21.4.3 Tag function library: graphql-tag

21.5 Raw string literals

21.6 (Advanced)

21.7 Multiline template literals and indentation

21.7.1 Fix: template tag for dedenting

21.7.2 Fix: .trim()

21.8 Simple templating via template literals

21.8.1 A more complex example

21.8.2 Simple HTML-escaping

Before we dig into the two features template literal and tagged template, let’s first examine the multiple meanings of the term template.

21.1 Disambiguation: “template”

The following three things are significantly different despite all having template in their names and despite all of them looking similar:

A text template is a function from data to text. It is frequently used in web development and often defined via text files. For example, the following text defines a template for the library Handlebars:

<div class="entry">
  <h1>{{title}}</h1>
  <div class="body">
    {{body}}
  </div>
</div>

This template has two blanks to be filled in: title and body. It is used like this:

// First step: retrieve the template text, e.g. from a text file.
const tmplFunc = Handlebars.compile(TMPL_TEXT); // compile string
const data = {title: 'My page', body: 'Welcome to my page!'};
const html = tmplFunc(data);

A template literal is similar to a string literal, but has additional features – for example, interpolation. It is delimited by backticks:

const num = 5;
assert.equal(`Count: ${num}!`, 'Count: 5!');

Syntactically, a tagged template is a template literal that follows a function (or rather, an expression that evaluates to a function). That leads to the function being called. Its arguments are derived from the contents of the template literal.

const getArgs = (...args) => args;
assert.deepEqual(
  getArgs`Count: ${5}!`,
  [['Count: ', '!'], 5] );

Note that getArgs() receives both the text of the literal and the data interpolated via ${}.

21.2 Template literals

A template literal has two new features compared to a normal string literal.

First, it supports string interpolation: if we put a dynamically computed value inside a ${}, it is converted to a string and inserted into the string returned by the literal.

const MAX = 100;
function doSomeWork(x) {
  if (x > MAX) {
    throw new Error(`At most ${MAX} allowed: ${x}!`);
  }
  // ···
}
assert.throws(
  () => doSomeWork(101),
  {message: 'At most 100 allowed: 101!'});

Second, template literals can span multiple lines:

const str = `this is
a text with
multiple lines`;

Template literals always produce strings.

21.3 Tagged templates

The expression in line A is a tagged template. It is equivalent to invoking tagFunc() with the arguments listed in the Array in line B.

function tagFunc(...args) {
  return args;
}

const setting = 'dark mode';
const value = true;

assert.deepEqual(
  tagFunc`Setting ${setting} is ${value}!`, // (A)
  [['Setting ', ' is ', '!'], 'dark mode', true] // (B)
);

The function tagFunc before the first backtick is called a tag function. Its arguments are:

Template strings (first argument): an Array with the text fragments surrounding the interpolations ${}.

In the example: ['Setting ', ' is ', '!']

Substitutions (remaining arguments): the interpolated values.

In the example: 'dark mode' and true

The static (fixed) parts of the literal (the template strings) are kept separate from the dynamic parts (the substitutions).

A tag function can return arbitrary values.

21.3.1 Cooked vs. raw template strings (advanced)

So far, we have only seen the cooked interpretation of template strings. But tag functions actually get two interpretations:

A cooked interpretation where backslashes have special meaning. For example, \t produces a tab character. This interpretation of the template strings is stored as an Array in the first argument.

A raw interpretation where backslashes do not have special meaning. For example, \t produces two characters – a backslash and a t. This interpretation of the template strings is stored in property .raw of the first argument (an Array).

The raw interpretation enables raw string literals via String.raw (described later) and similar applications.

The following tag function cookedRaw uses both interpretations:

function cookedRaw(templateStrings, ...substitutions) {
  return {
    cooked: [...templateStrings], // copy only Array elements
    raw: templateStrings.raw,
    substitutions,
  };
}
assert.deepEqual(
  cookedRaw`\tab${'subst'}\newline\\`,
  {
    cooked: ['\tab', '\newline\\'],
    raw:    ['\\tab', '\\newline\\\\'],
    substitutions: ['subst'],
  });

We can also use Unicode code point escapes (\u{1F642}), Unicode code unit escapes (\u03A9), and ASCII escapes (\x52) in tagged templates:

assert.deepEqual(
  cookedRaw`\u{54}\u0065\x78t`,
  {
    cooked: ['Text'],
    raw:    ['\\u{54}\\u0065\\x78t'],
    substitutions: [],
  });

If the syntax of one of these escapes isn’t correct, the corresponding cooked template string is undefined, while the raw version is still verbatim:

assert.deepEqual(
  cookedRaw`\uu\xx ${1} after`,
  {
    cooked: [undefined, ' after'],
    raw:    ['\\uu\\xx ', ' after'],
    substitutions: [1],
  });

Incorrect escapes produce syntax errors in template literals and string literals. Why does JavaScript treat tagged templates differently? It lets us use tagged templates for text that we can’t put inside string literals – for example:

windowsPath`C:\uuu\xxx\111`
latex`\unicode`

21.4 Examples of tagged templates (as provided via libraries)

Tagged templates are great for supporting small embedded languages (so-called domain-specific languages). We’ll continue with a few examples.

21.4.1 Tag function library: lit-html

lit-html is a templating library that is based on tagged templates and used by the frontend framework Polymer:

import {html, render} from 'lit-html';

const template = (items) => html`
  <ul>
    ${
      repeat(items,
        (item) => item.id,
        (item, index) => html`<li>${index}. ${item.name}</li>`
      )
    }
  </ul>
`;

repeat() is a custom function for looping. Its 2nd parameter produces unique keys for the values returned by the 3rd parameter. Note the nested tagged template used by that parameter.

21.4.2 Tag function library: re-template-tag

re-template-tag is a simple library for composing regular expressions. Templates tagged with re produce regular expressions. The main benefit is that we can interpolate regular expressions and plain text via ${} (line A):

const RE_YEAR = re`(?<year>[0-9]{4})`;
const RE_MONTH = re`(?<month>[0-9]{2})`;
const RE_DAY = re`(?<day>[0-9]{2})`;
const RE_DATE = re`/${RE_YEAR}-${RE_MONTH}-${RE_DAY}/u`; // (A)

const match = RE_DATE.exec('2017-01-27');
assert.equal(match.groups.year, '2017');

21.4.3 Tag function library: graphql-tag

The library graphql-tag lets us create GraphQL queries via tagged templates:

import gql from 'graphql-tag';

const query = gql`
  {
    user(id: 5) {
      firstName
      lastName
    }
  }
  `;

Additionally, there are plugins for pre-compiling such queries in Babel, TypeScript, etc.

21.5 Raw string literals

Raw string literals are implemented via the tag function String.raw. They are string literals where backslashes don’t do anything special (such as escaping characters, etc.):

assert.equal(String.raw`\back`, '\\back');

This helps whenever data contains backslashes – for example, strings with regular expressions:

const regex1 = /^\./;
const regex2 = new RegExp('^\\.');
const regex3 = new RegExp(String.raw`^\.`);

All three regular expressions are equivalent. With a normal string literal, we have to write the backslash twice, to escape it for that literal. With a raw string literal, we don’t have to do that.

Raw string literals are also useful for specifying Windows filename paths:

const WIN_PATH = String.raw`C:\foo\bar`;
assert.equal(WIN_PATH, 'C:\\foo\\bar');

21.6 (Advanced)

All remaining sections are advanced

21.7 Multiline template literals and indentation

If we put multiline text in template literals, two goals are in conflict: On one hand, the template literal should be indented to fit inside the source code. On the other hand, the lines of its content should start in the leftmost column.

For example:

function div(text) {
  return `
    <div>
      ${text}
    </div>
  `;
}
console.log('Output:');
console.log(
  div('Hello!')
  // Replace spaces with mid-dots:
  .replace(/ /g, '·')
  // Replace \n with #\n:
  .replace(/\n/g, '#\n')
);

Due to the indentation, the template literal fits well into the source code. Alas, the output is also indented. And we don’t want the return at the beginning and the return plus two spaces at the end.

Output:
#
····<div>#
······Hello!#
····</div>#
··

There are two ways to fix this: via a tagged template or by trimming the result of the template literal.

21.7.1 Fix: template tag for dedenting

The first fix is to use a custom template tag that removes the unwanted whitespace. It uses the first line after the initial line break to determine in which column the text starts and shortens the indentation everywhere. It also removes the line break at the very beginning and the indentation at the very end. One such template tag is dedent by Desmond Brand:

import dedent from 'dedent';
function divDedented(text) {
  return dedent`
    <div>
      ${text}
    </div>
  `.replace(/\n/g, '#\n');
}
console.log('Output:');
console.log(divDedented('Hello!'));

This time, the output is not indented:

Output:
<div>#
  Hello!#
</div>

21.7.2 Fix: .trim()

The second fix is quicker, but also dirtier:

function divDedented(text) {
  return `
<div>
  ${text}
</div>
  `.trim().replace(/\n/g, '#\n');
}
console.log('Output:');
console.log(divDedented('Hello!'));

The string method .trim() removes the superfluous whitespace at the beginning and at the end, but the content itself must start in the leftmost column. The advantage of this solution is that we don’t need a custom tag function. The downside is that it looks ugly.

The output is the same as with dedent:

Output:
<div>#
  Hello!#
</div>

21.8 Simple templating via template literals

While template literals look like text templates, it is not immediately obvious how to use them for (text) templating: A text template gets its data from an object, while a template literal gets its data from variables. The solution is to use a template literal in the body of a function whose parameter receives the templating data – for example:

const tmpl = (data) => `Hello ${data.name}!`;
assert.equal(tmpl({name: 'Jane'}), 'Hello Jane!');

21.8.1 A more complex example

As a more complex example, we’d like to take an Array of addresses and produce an HTML table. This is the Array:

const addresses = [
  { first: '<Jane>', last: 'Bond' },
  { first: 'Lars', last: '<Croft>' },
];

The function tmpl() that produces the HTML table looks as follows:

const tmpl = (addrs) => `
<table>
  ${addrs.map(
    (addr) => `
      <tr>
        <td>${escapeHtml(addr.first)}</td>
        <td>${escapeHtml(addr.last)}</td>
      </tr>
      `.trim()
  ).join('')}
</table>
`.trim();

This code contains two templating functions:

The first one (line 1) takes addrs, an Array with addresses, and returns a string with a table.

The second one (line 4) takes addr, an object containing an address, and returns a string with a table row. Note the .trim() at the end, which removes unnecessary whitespace.

The first templating function produces its result by wrapping a table element around an Array that it joins into a string (line 10). That Array is produced by mapping the second templating function to each element of addrs (line 3). It therefore contains strings with table rows.

The helper function escapeHtml() is used to escape special HTML characters (line 6 and line 7). Its implementation is shown in the next subsection.

Let us call tmpl() with the addresses and log the result:

console.log(tmpl(addresses));

The output is:

<table>
  <tr>
        <td>&lt;Jane&gt;</td>
        <td>Bond</td>
      </tr><tr>
        <td>Lars</td>
        <td>&lt;Croft&gt;</td>
      </tr>
</table>

21.8.2 Simple HTML-escaping

The following function escapes plain text so that it is displayed verbatim in HTML:

function escapeHtml(str) {
  return str
    .replace(/&/g, '&amp;') // first!
    .replace(/>/g, '&gt;')
    .replace(/</g, '&lt;')
    .replace(/"/g, '&quot;')
    .replace(/'/g, '&#39;')
    .replace(/`/g, '&#96;')
    ;
}
assert.equal(
  escapeHtml('Rock & Roll'), 'Rock &amp; Roll');
assert.equal(
  escapeHtml('<blank>'), '&lt;blank&gt;');

  Exercise: HTML templating

Exercise with bonus challenge: exercises/template-literals/templating_test.mjs

  Quiz

See quiz app.

22 Symbols

22.1 Symbols are primitives that are also like objects

22.1.1 Symbols are primitive values

22.1.2 Symbols are also like objects

22.2 The descriptions of symbols

22.3 Use cases for symbols

22.3.1 Symbols as values for constants

22.3.2 Symbols as unique property keys

22.4 Publicly known symbols

22.5 Converting symbols

22.1 Symbols are primitives that are also like objects

Symbols are primitive values that are created via the factory function Symbol():

const mySymbol = Symbol('mySymbol');

The parameter is optional and provides a description, which is mainly useful for debugging.

22.1.1 Symbols are primitive values

Symbols are primitive values:

They have to be categorized via typeof:

const sym = Symbol();
assert.equal(typeof sym, 'symbol');

They can be property keys in objects:

const obj = {
  [sym]: 123,
};

22.1.2 Symbols are also like objects

Even though symbols are primitives, they are also like objects in that each value created by Symbol() is unique and not compared by value:

> Symbol() === Symbol()
false

Prior to symbols, objects were the best choice if we needed values that were unique (only equal to themselves):

const string1 = 'abc';
const string2 = 'abc';
assert.equal(
  string1 === string2, true); // not unique

const object1 = {};
const object2 = {};
assert.equal(
  object1 === object2, false); // unique

const symbol1 = Symbol();
const symbol2 = Symbol();
assert.equal(
  symbol1 === symbol2, false); // unique

22.2 The descriptions of symbols

The parameter we pass to the symbol factory function provides a description for the created symbol:

const mySymbol = Symbol('mySymbol');

The description can be accessed in two ways.

First, it is part of the string returned by .toString():

assert.equal(mySymbol.toString(), 'Symbol(mySymbol)');

Second, since ES2019, we can retrieve the description via the property .description:

assert.equal(mySymbol.description, 'mySymbol');

22.3 Use cases for symbols

The main use cases for symbols, are:

Values for constants

Unique property keys

22.3.1 Symbols as values for constants

Let’s assume you want to create constants representing the colors red, orange, yellow, green, blue, and violet. One simple way of doing so would be to use strings:

const COLOR_BLUE = 'Blue';

On the plus side, logging that constant produces helpful output. On the minus side, there is a risk of mistaking an unrelated value for a color because two strings with the same content are considered equal:

const MOOD_BLUE = 'Blue';
assert.equal(COLOR_BLUE, MOOD_BLUE);

We can fix that problem via symbols:

const COLOR_BLUE = Symbol('Blue');
const MOOD_BLUE = Symbol('Blue');

assert.notEqual(COLOR_BLUE, MOOD_BLUE);

Let’s use symbol-valued constants to implement a function:

const COLOR_RED    = Symbol('Red');
const COLOR_ORANGE = Symbol('Orange');
const COLOR_YELLOW = Symbol('Yellow');
const COLOR_GREEN  = Symbol('Green');
const COLOR_BLUE   = Symbol('Blue');
const COLOR_VIOLET = Symbol('Violet');

function getComplement(color) {
  switch (color) {
    case COLOR_RED:
      return COLOR_GREEN;
    case COLOR_ORANGE:
      return COLOR_BLUE;
    case COLOR_YELLOW:
      return COLOR_VIOLET;
    case COLOR_GREEN:
      return COLOR_RED;
    case COLOR_BLUE:
      return COLOR_ORANGE;
    case COLOR_VIOLET:
      return COLOR_YELLOW;
    default:
      throw new Exception('Unknown color: '+color);
  }
}
assert.equal(getComplement(COLOR_YELLOW), COLOR_VIOLET);

22.3.2 Symbols as unique property keys

The keys of properties (fields) in objects are used at two levels:

The program operates at a base level. The keys at that level reflect the problem domain – the area in which a program solves a problem – for example:

If a program manages employees, the property keys may be about job titles, salary categories, department IDs, etc.

If the program is a chess app, the property keys may be about chess pieces, chess boards, player colors, etc.

ECMAScript and many libraries operate at a meta-level. They manage data and provide services that are not part of the problem domain – for example:

The standard method .toString() is used by ECMAScript when creating a string representation of an object (line A):

const point = {
  x: 7,
  y: 4,
  toString() {
    return `(${this.x}, ${this.y})`;
  },
};
assert.equal(
  String(point), '(7, 4)'); // (A)

.x and .y are base-level properties – they are used to solve the problem of computing with points. .toString() is a meta-level property – it doesn’t have anything to do with the problem domain.

The standard ECMAScript method .toJSON()

const point = {
  x: 7,
  y: 4,
  toJSON() {
    return [this.x, this.y];
  },
};
assert.equal(
  JSON.stringify(point), '[7,4]');

.x and .y are base-level properties, .toJSON() is a meta-level property.

The base level and the meta-level of a program must be independent: Base-level property keys should not be in conflict with meta-level property keys.

If we use names (strings) as property keys, we are facing two challenges:

When a language is first created, it can use any meta-level names it wants. Base-level code is forced to avoid those names. Later, however, when much base-level code already exists, meta-level names can’t be chosen freely, anymore.

We could introduce naming rules to separate base level and meta-level. For example, Python brackets meta-level names with two underscores: __init__, __iter__, __hash__, etc. However, the meta-level names of the language and the meta-level names of libraries would still exist in the same namespace and can clash.

These are two examples of where the latter was an issue for JavaScript:

In May 2018, the Array method .flatten() had to be renamed to .flat() because the former name was already used by libraries (source).

In November 2020, the Array method .item() had to be renamed to .at() because the former name was already used by library (source).

Symbols, used as property keys, help us here: Each symbol is unique and a symbol key never clashes with any other string or symbol key.

22.3.2.1 Example: a library with a meta-level method

As an example, let’s assume we are writing a library that treats objects differently if they implement a special method. This is what defining a property key for such a method and implementing it for an object would look like:

const specialMethod = Symbol('specialMethod');
const obj = {
  _id: 'kf12oi',
  [specialMethod]() { // (A)
    return this._id;
  }
};
assert.equal(obj[specialMethod](), 'kf12oi');

The square brackets in line A enable us to specify that the method must have the key specialMethod. More details are explained in §28.6.2 “Computed property keys”.

22.4 Publicly known symbols

Symbols that play special roles within ECMAScript are called publicly known symbols. Examples include:

Symbol.iterator: makes an object iterable. It’s the key of a method that returns an iterator. For more information on this topic, see [content not included].

Symbol.hasInstance: customizes how instanceof works. If an object implements a method with that key, it can be used at the right-hand side of that operator. For example:

const PrimitiveNull = {
  [Symbol.hasInstance](x) {
    return x === null;
  }
};
assert.equal(null instanceof PrimitiveNull, true);

Symbol.toStringTag: influences the default .toString() method.

> String({})
'[object Object]'
> String({ [Symbol.toStringTag]: 'is no money' })
'[object is no money]'

Note: It’s usually better to override .toString().

  Exercises: Publicly known symbols

Symbol.toStringTag: exercises/symbols/to_string_tag_test.mjs

Symbol.hasInstance: exercises/symbols/has_instance_test.mjs

22.5 Converting symbols

What happens if we convert a symbol sym to another primitive type? Tbl. 15 has the answers.

Table 15: The results of converting symbols to other primitive types.

Convert to | Explicit conversion | Coercion (implicit conv.)
boolean | Boolean(sym) → OK | !sym → OK
number | Number(sym) → TypeError | sym*2 → TypeError
string | String(sym) → OK | ''+sym → TypeError
| sym.toString() → OK | `${sym}` → TypeError

One key pitfall with symbols is how often exceptions are thrown when converting them to something else. What is the thinking behind that? First, conversion to number never makes sense and should be warned about. Second, converting a symbol to a string is indeed useful for diagnostic output. But it also makes sense to warn about accidentally turning a symbol into a string (which is a different kind of property key):

const obj = {};
const sym = Symbol();
assert.throws(
  () => { obj['__'+sym+'__'] = true },
  { message: 'Cannot convert a Symbol value to a string' });

The downside is that the exceptions make working with symbols more complicated. You have to explicitly convert symbols when assembling strings via the plus operator:

> const mySymbol = Symbol('mySymbol');
> 'Symbol I used: ' + mySymbol
TypeError: Cannot convert a Symbol value to a string
> 'Symbol I used: ' + String(mySymbol)
'Symbol I used: Symbol(mySymbol)'

  Quiz

See quiz app.

23 Control flow statements

23.1 Controlling loops: break and continue

23.1.1 break

23.1.2 break plus label: leaving any labeled statement

23.1.3 continue

23.2 Conditions of control flow statements

23.3 if statements [ES1]

23.3.1 The syntax of if statements

23.4 switch statements [ES3]

23.4.1 A first example of a switch statement

23.4.2 Don’t forget to return or break!

23.4.3 Empty case clauses

23.4.4 Checking for illegal values via a default clause

23.5 while loops [ES1]

23.5.1 Examples of while loops

23.6 do-while loops [ES3]

23.7 for loops [ES1]

23.7.1 Examples of for loops

23.8 for-of loops [ES6]

23.8.1 const: for-of vs. for

23.8.2 Iterating over iterables

23.8.3 Iterating over [index, element] pairs of Arrays

23.9 for-await-of loops [ES2018]

23.10 for-in loops (avoid) [ES1]

23.11 Recomendations for looping

This chapter covers the following control flow statements:

if statement [ES1]

switch statement [ES3]

while loop [ES1]

do-while loop [ES3]

for loop [ES1]

for-of loop [ES6]

for-await-of loop [ES2018]

for-in loop [ES1]

23.1 Controlling loops: break and continue

The two operators break and continue can be used to control loops and other statements while we are inside them.

23.1.1 break

There are two versions of break: one with an operand and one without an operand. The latter version works inside the following statements: while, do-while, for, for-of, for-await-of, for-in and switch. It immediately leaves the current statement:

for (const x of ['a', 'b', 'c']) {
  console.log(x);
  if (x === 'b') break;
  console.log('---')
}

// Output:
// 'a'
// '---'
// 'b'

23.1.2 break plus label: leaving any labeled statement

break with an operand works everywhere. Its operand is a label. Labels can be put in front of any statement, including blocks. break my_label leaves the statement whose label is my_label:

my_label: { // label
  if (condition) break my_label; // labeled break
  // ···
}

In the following example, the search can either:

Fail: The loop finishes without finding a result. That is handled directly after the loop (line B).

Succeed: While looping, we find a result. Then we use break plus label (line A) to skip the code that handles failure.

function findSuffix(stringArray, suffix) {
  let result;
  search_block: {
    for (const str of stringArray) {
      if (str.endsWith(suffix)) {
        // Success:
        result = str;
        break search_block; // (A)
      }
    } // for
    // Failure:
    result = '(Untitled)'; // (B)
  } // search_block

  return { suffix, result };
    // Same as: {suffix: suffix, result: result}
}
assert.deepEqual(
  findSuffix(['notes.txt', 'index.html'], '.html'),
  { suffix: '.html', result: 'index.html' }
);
assert.deepEqual(
  findSuffix(['notes.txt', 'index.html'], '.mjs'),
  { suffix: '.mjs', result: '(Untitled)' }
);

23.1.3 continue

continue only works inside while, do-while, for, for-of, for-await-of, and for-in. It immediately leaves the current loop iteration and continues with the next one – for example:

const lines = [
  'Normal line',
  '# Comment',
  'Another normal line',
];
for (const line of lines) {
  if (line.startsWith('#')) continue;
  console.log(line);
}
// Output:
// 'Normal line'
// 'Another normal line'

23.2 Conditions of control flow statements

if, while, and do-while have conditions that are, in principle, boolean. However, a condition only has to be truthy (true if coerced to boolean) in order to be accepted. In other words, the following two control flow statements are equivalent:

if (value) {}
if (Boolean(value) === true) {}

This is a list of all falsy values:

undefined, null

false

0, NaN

0n

''

All other values are truthy. For more information, see §15.2 “Falsy and truthy values”.

23.3 if statements [ES1]

These are two simple if statements: one with just a “then” branch and one with both a “then” branch and an “else” branch:

if (cond) {
  // then branch
}

if (cond) {
  // then branch
} else {
  // else branch
}

Instead of the block, else can also be followed by another if statement:

if (cond1) {
  // ···
} else if (cond2) {
  // ···
}

if (cond1) {
  // ···
} else if (cond2) {
  // ···
} else {
  // ···
}

You can continue this chain with more else ifs.

23.3.1 The syntax of if statements

The general syntax of if statements is:

if (cond) «then_statement»
else «else_statement»

So far, the then_statement has always been a block, but we can use any statement. That statement must be terminated with a semicolon:

if (true) console.log('Yes'); else console.log('No');

That means that else if is not its own construct; it’s simply an if statement whose else_statement is another if statement.

23.4 switch statements [ES3]

A switch statement looks as follows:

switch («switch_expression») {
  «switch_body»
}

The body of switch consists of zero or more case clauses:

case «case_expression»:
  «statements»

And, optionally, a default clause:

default:
  «statements»

A switch is executed as follows:

It evaluates the switch expression.

It jumps to the first case clause whose expression has the same result as the switch expression.

Otherwise, if there is no such clause, it jumps to the default clause.

Otherwise, if there is no default clause, it does nothing.

23.4.1 A first example of a switch statement

Let’s look at an example: The following function converts a number from 1–7 to the name of a weekday.

function dayOfTheWeek(num) {
  switch (num) {
    case 1:
      return 'Monday';
    case 2:
      return 'Tuesday';
    case 3:
      return 'Wednesday';
    case 4:
      return 'Thursday';
    case 5:
      return 'Friday';
    case 6:
      return 'Saturday';
    case 7:
      return 'Sunday';
  }
}
assert.equal(dayOfTheWeek(5), 'Friday');

23.4.2 Don’t forget to return or break!

At the end of a case clause, execution continues with the next case clause, unless we return or break – for example:

function englishToFrench(english) {
  let french;
  switch (english) {
    case 'hello':
      french = 'bonjour';
    case 'goodbye':
      french = 'au revoir';
  }
  return french;
}
// The result should be 'bonjour'!
assert.equal(englishToFrench('hello'), 'au revoir');

That is, our implementation of dayOfTheWeek() only worked because we used return. We can fix englishToFrench() by using break:

function englishToFrench(english) {
  let french;
  switch (english) {
    case 'hello':
      french = 'bonjour';
      break;
    case 'goodbye':
      french = 'au revoir';
      break;
  }
  return french;
}
assert.equal(englishToFrench('hello'), 'bonjour'); // ok

23.4.3 Empty case clauses

The statements of a case clause can be omitted, which effectively gives us multiple case expressions per case clause:

function isWeekDay(name) {
  switch (name) {
    case 'Monday':
    case 'Tuesday':
    case 'Wednesday':
    case 'Thursday':
    case 'Friday':
      return true;
    case 'Saturday':
    case 'Sunday':
      return false;
  }
}
assert.equal(isWeekDay('Wednesday'), true);
assert.equal(isWeekDay('Sunday'), false);

23.4.4 Checking for illegal values via a default clause

A default clause is jumped to if the switch expression has no other match. That makes it useful for error checking:

function isWeekDay(name) {
  switch (name) {
    case 'Monday':
    case 'Tuesday':
    case 'Wednesday':
    case 'Thursday':
    case 'Friday':
      return true;
    case 'Saturday':
    case 'Sunday':
      return false;
    default:
      throw new Error('Illegal value: '+name);
  }
}
assert.throws(
  () => isWeekDay('January'),
  {message: 'Illegal value: January'});

  Exercises: switch

exercises/control-flow/number_to_month_test.mjs

Bonus: exercises/control-flow/is_object_via_switch_test.mjs

23.5 while loops [ES1]

A while loop has the following syntax:

while («condition») {
  «statements»
}

Before each loop iteration, while evaluates condition:

If the result is falsy, the loop is finished.

If the result is truthy, the while body is executed one more time.

23.5.1 Examples of while loops

The following code uses a while loop. In each loop iteration, it removes the first element of arr via .shift() and logs it.

const arr = ['a', 'b', 'c'];
while (arr.length > 0) {
  const elem = arr.shift(); // remove first element
  console.log(elem);
}
// Output:
// 'a'
// 'b'
// 'c'

If the condition always evaluates to true, then while is an infinite loop:

while (true) {
  if (Math.random() === 0) break;
}

23.6 do-while loops [ES3]

The do-while loop works much like while, but it checks its condition after each loop iteration, not before.

let input;
do {
  input = prompt('Enter text:');
  console.log(input);
} while (input !== ':q');

prompt() is a global function that is available in web browsers. It prompts the user to input text and returns it.

23.7 for loops [ES1]

A for loop has the following syntax:

for («initialization»; «condition»; «post_iteration») {
  «statements»
}

The first line is the head of the loop and controls how often the body (the remainder of the loop) is executed. It has three parts and each of them is optional:

initialization: sets up variables, etc. for the loop. Variables declared here via let or const only exist inside the loop.

condition: This condition is checked before each loop iteration. If it is falsy, the loop stops.

post_iteration: This code is executed after each loop iteration.

A for loop is therefore roughly equivalent to the following while loop:

«initialization»
while («condition») {
  «statements»
  «post_iteration»
}

23.7.1 Examples of for loops

As an example, this is how to count from zero to two via a for loop:

for (let i=0; i<3; i++) {
  console.log(i);
}

// Output:
// 0
// 1
// 2

This is how to log the contents of an Array via a for loop:

const arr = ['a', 'b', 'c'];
for (let i=0; i<arr.length; i++) {
  console.log(arr[i]);
}

// Output:
// 'a'
// 'b'
// 'c'

If we omit all three parts of the head, we get an infinite loop:

for (;;) {
  if (Math.random() === 0) break;
}

23.8 for-of loops [ES6]

A for-of loop iterates over any iterable – a data container that supports the iteration protocol. Each iterated value is stored in a variable, as specified in the head:

for («iteration_variable» of «iterable») {
  «statements»
}

The iteration variable is usually created via a variable declaration:

const iterable = ['hello', 'world'];
for (const elem of iterable) {
  console.log(elem);
}
// Output:
// 'hello'
// 'world'

But we can also use a (mutable) variable that already exists:

const iterable = ['hello', 'world'];
let elem;
for (elem of iterable) {
  console.log(elem);
}

23.8.1 const: for-of vs. for

Note that in for-of loops we can use const. The iteration variable can still be different for each iteration (it just can’t change during the iteration). Think of it as a new const declaration being executed each time in a fresh scope.

In contrast, in for loops we must declare variables via let or var if their values change.

23.8.2 Iterating over iterables

As mentioned before, for-of works with any iterable object, not just with Arrays – for example, with Sets:

const set = new Set(['hello', 'world']);
for (const elem of set) {
  console.log(elem);
}

23.8.3 Iterating over [index, element] pairs of Arrays

Lastly, we can also use for-of to iterate over the [index, element] entries of Arrays:

const arr = ['a', 'b', 'c'];
for (const [index, elem] of arr.entries()) {
  console.log(`${index} -> ${elem}`);
}
// Output:
// '0 -> a'
// '1 -> b'
// '2 -> c'

With [index, element], we are using destructuring to access Array elements.

  Exercise: for-of

exercises/control-flow/array_to_string_test.mjs

23.9 for-await-of loops [ES2018]

for-await-of is like for-of, but it works with asynchronous iterables instead of synchronous ones. And it can only be used inside async functions and async generators.

for await (const item of asyncIterable) {
  // ···
}

for-await-of is described in detail in the chapter on asynchronous iteration.

23.10 for-in loops (avoid) [ES1]

The for-in loop visits all (own and inherited) enumerable property keys of an object. When looping over an Array, it is rarely a good choice:

It visits property keys, not values.

As property keys, the indices of Array elements are strings, not numbers (more information on how Array elements work).

It visits all enumerable property keys (both own and inherited ones), not just those of Array elements.

The following code demonstrates these points:

const arr = ['a', 'b', 'c'];
arr.propKey = 'property value';

for (const key in arr) {
  console.log(key);
}

// Output:
// '0'
// '1'
// '2'
// 'propKey'

23.11 Recomendations for looping

If you want to loop over an asynchronous iterable (in ES2018+), you must use for-await-of.

For looping over a synchronous iterable (in ES6+), you must use for-of. Note that Arrays are iterables.

For looping over an Array in ES5+, you can use the Array method .forEach().

Before ES5, you can use a plain for loop to loop over an Array.

Don’t use for-in to loop over an Array.

  Quiz

See quiz app.

24 Exception handling

24.1 Motivation: throwing and catching exceptions

24.2 throw

24.2.1 Options for creating error objects

24.3 The try statement

24.3.1 The try block

24.3.2 The catch clause

24.3.3 The finally clause

24.4 Error classes

24.4.1 Properties of error objects

This chapter covers how JavaScript handles exceptions.

  Why doesn’t JavaScript throw exceptions more often?

JavaScript didn’t support exceptions until ES3. That explains why they are used sparingly by the language and its standard library.

24.1 Motivation: throwing and catching exceptions

Consider the following code. It reads profiles stored in files into an Array with instances of class Profile:

function readProfiles(filePaths) {
  const profiles = [];
  for (const filePath of filePaths) {
    try {
      const profile = readOneProfile(filePath);
      profiles.push(profile);
    } catch (err) { // (A)
      console.log('Error in: '+filePath, err);
    }
  }
}
function readOneProfile(filePath) {
  const profile = new Profile();
  const file = openFile(filePath);
  // ··· (Read the data in `file` into `profile`)
  return profile;
}
function openFile(filePath) {
  if (!fs.existsSync(filePath)) {
    throw new Error('Could not find file '+filePath); // (B)
  }
  // ··· (Open the file whose path is `filePath`)
}

Let’s examine what happens in line B: An error occurred, but the best place to handle the problem is not the current location, it’s line A. There, we can skip the current file and move on to the next one.

Therefore:

In line B, we use a throw statement to indicate that there was a problem.

In line A, we use a try-catch statement to handle the problem.

When we throw, the following constructs are active:

readProfiles(···)
  for (const filePath of filePaths)
    try
      readOneProfile(···)
        openFile(···)
          if (!fs.existsSync(filePath))
            throw

One by one, throw exits the nested constructs, until it encounters a try statement. Execution continues in the catch clause of that try statement.

24.2 throw

This is the syntax of the throw statement:

throw «value»;

Any value can be thrown, but it’s best to throw an instance of Error or its subclasses.

throw new Error('Problem!');

24.2.1 Options for creating error objects

Using class Error. That is less limiting in JavaScript than in a more static language because we can add our own properties to instances:

const err = new Error('Could not find the file');
err.filePath = filePath;
throw err;

Use one of JavaScript’s subclasses of Error (which are listed later).

Subclassing Error:

class MyError extends Error {
}
function func() {
  throw new MyError('Problem!');
}
assert.throws(
  () => func(),
  MyError);

24.3 The try statement

The maximal version of the try statement looks as follows:

try {
  «try_statements»
} catch (error) {
  «catch_statements»
} finally {
  «finally_statements»
}

We can combine these clauses as follows:

try-catch

try-finally

try-catch-finally

24.3.1 The try block

The try block can be considered the body of the statement. This is where we execute the regular code.

24.3.2 The catch clause

If an exception reaches the try block, then it is assigned to the parameter of the catch clause and the code in that clause is executed. Next, execution normally continues after the try statement. That may change if:

There is a return, break, or throw inside the catch block.

There is a finally clause (which is always executed before the try statement ends).

The following code demonstrates that the value that is thrown in line A is indeed caught in line B.

const errorObject = new Error();
function func() {
  throw errorObject; // (A)
}

try {
  func();
} catch (err) { // (B)
  assert.equal(err, errorObject);
}

24.3.2.1 Omitting the catch binding

Since ECMAScript 2019, we can omit the catch parameter if we are not interested in the value that was thrown:

try {
  // ···
} catch {
  // ···
}

That may be occasionally useful. For example, Node.js has the API function assert.throws(func) that checks whether an error is thrown inside func. It could be implemented as follows.

function throws(func) {
  try {
    func();
  } catch {
    return; // everything OK
  }
  throw new Error('Function didn’t throw an exception!');
}

However, a more complete implementation of this function would have a catch parameter and would, for example, check that its type is as expected.

24.3.3 The finally clause

The code inside the finally clause is always executed at the end of a try statement – no matter what happens in the try block or the catch clause.

Let’s look at a common use case for finally: We have created a resource and want to always destroy it when we are done with it, no matter what happens while working with it. We would implement that as follows:

const resource = createResource();
try {
  // Work with `resource`. Errors may be thrown.
} finally {
  resource.destroy();
}

24.3.3.1 finally is always executed

The finally clause is always executed, even if an error is thrown (line A):

let finallyWasExecuted = false;
assert.throws(
  () => {
    try {
      throw new Error(); // (A)
    } finally {
      finallyWasExecuted = true;
    }
  },
  Error
);
assert.equal(finallyWasExecuted, true);

And even if there is a return statement (line A):

let finallyWasExecuted = false;
function func() {
  try {
    return; // (A)
  } finally {
    finallyWasExecuted = true;
  }
}
func();
assert.equal(finallyWasExecuted, true);

24.4 Error classes

Error is the common superclass of all built-in error classes. It has the following subclasses (I’m quoting the ECMAScript specification):

RangeError: Indicates a value that is not in the set or range of allowable values.

ReferenceError: Indicate that an invalid reference value has been detected.

SyntaxError: Indicates that a parsing error has occurred.

TypeError: is used to indicate an unsuccessful operation when none of the other NativeError objects are an appropriate indication of the failure cause.

URIError: Indicates that one of the global URI handling functions was used in a way that is incompatible with its definition.

24.4.1 Properties of error objects

Consider err, an instance of Error:

const err = new Error('Hello!');
assert.equal(String(err), 'Error: Hello!');

Two properties of err are especially useful:

.message: contains just the error message.

assert.equal(err.message, 'Hello!');

.stack: contains a stack trace. It is supported by all mainstream browsers.

assert.equal(
err.stack,
`
Error: Hello!
    at file://ch_exception-handling.mjs:1:13
`.trim());

  Exercise: Exception handling

exercises/exception-handling/call_function_test.mjs

  Quiz

See quiz app.

25 Callable values

25.1 Kinds of functions

25.2 Ordinary functions

25.2.1 Named function expressions (advanced)

25.2.2 Terminology: function definitions and function expressions

25.2.3 Parts of a function declaration

25.2.4 Roles played by ordinary functions

25.2.5 Terminology: entity vs. syntax vs. role (advanced)

25.3 Specialized functions

25.3.1 Specialized functions are still functions

25.3.2 Arrow functions

25.3.3 The special variable this in methods, ordinary functions and arrow functions

25.3.4 Recommendation: prefer specialized functions over ordinary functions

25.4 Summary: kinds of callable values

25.5 Returning values from functions and methods

25.6 Parameter handling

25.6.1 Terminology: parameters vs. arguments

25.6.2 Terminology: callback

25.6.3 Too many or not enough arguments

25.6.4 Parameter default values

25.6.5 Rest parameters

25.6.6 Named parameters

25.6.7 Simulating named parameters

25.6.8 Spreading (...) into function calls

25.7 Methods of functions: .call(), .apply(), .bind()

25.7.1 The function method .call()

25.7.2 The function method .apply()

25.7.3 The function method .bind()

In this chapter, we look at JavaScript values that can be invoked: functions, methods, and classes.

25.1 Kinds of functions

JavaScript has two categories of functions:

An ordinary function can play several roles:

Real function

Method

Constructor function

A specialized function can only play one of those roles – for example:

An arrow function can only be a real function.

A method can only be a method.

A class can only be a constructor function.

Specialized functions were added to the language in ECMAScript 6.

Read on to find out what all of those things mean.

25.2 Ordinary functions

The following code shows two ways of doing (roughly) the same thing: creating an ordinary function.

// Function declaration (a statement)
function ordinary1(a, b, c) {
  // ···
}

// const plus anonymous (nameless) function expression
const ordinary2 = function (a, b, c) {
  // ···
};

Inside a scope, function declarations are activated early (see §11.8 “Declarations: scope and activation”) and can be called before they are declared. That is occasionally useful.

Variable declarations, such as the one for ordinary2, are not activated early.

25.2.1 Named function expressions (advanced)

So far, we have only seen anonymous function expressions – which don’t have names:

const anonFuncExpr = function (a, b, c) {
  // ···
};

But there are also named function expressions:

const namedFuncExpr = function myName(a, b, c) {
  // `myName` is only accessible in here
};

myName is only accessible inside the body of the function. The function can use it to refer to itself (for self-recursion, etc.) – independently of which variable it is assigned to:

const func = function funcExpr() { return funcExpr };
assert.equal(func(), func);

// The name `funcExpr` only exists inside the function body:
assert.throws(() => funcExpr(), ReferenceError);

Even if they are not assigned to variables, named function expressions have names (line A):

function getNameOfCallback(callback) {
  return callback.name;
}

assert.equal(
  getNameOfCallback(function () {}), ''); // anonymous

assert.equal(
  getNameOfCallback(function named() {}), 'named'); // (A)

Note that functions created via function declarations or variable declarations always have names:

function funcDecl() {}
assert.equal(
  getNameOfCallback(funcDecl), 'funcDecl');

const funcExpr = function () {};
assert.equal(
  getNameOfCallback(funcExpr), 'funcExpr');

One benefit of functions having names is that those names show up in exception stack traces.

25.2.2 Terminology: function definitions and function expressions

A function definition is syntax that creates functions:

A function declaration (a statement)

A function expression

Function declarations always produce ordinary functions. Function expressions produce either ordinary functions or specialized functions:

Ordinary function expressions (which we have already encountered):

Anonymous function expressions

Named function expressions

Specialized function expressions (which we’ll look at later):

Arrow functions (which are always expressions)

While function declarations are still popular in JavaScript, function expressions are almost always arrow functions in modern code.

25.2.3 Parts of a function declaration

Let’s examine the parts of a function declaration via the following example. Most of the terms also apply to function expressions.

function add(x, y) {
  return x + y;
}

add is the name of the function declaration.

add(x, y) is the head of the function declaration.

x and y are the parameters.

The curly braces ({ and }) and everything between them are the body of the function declaration.

The return statement explicitly returns a value from the function.

25.2.3.1 Trailing commas in parameter lists

JavaScript has always allowed and ignored trailing commas in Array literals. Since ES5, they are also allowed in object literals. Since ES2017, we can add trailing commas to parameter lists (declarations and invocations):

// Declaration
function retrieveData(
  contentText,
  keyword,
  {unique, ignoreCase, pageSize}, // trailing comma
) {
  // ···
}

// Invocation
retrieveData(
  '',
  null,
  {ignoreCase: true, pageSize: 10}, // trailing comma
);

25.2.4 Roles played by ordinary functions

Consider the following function declaration from the previous section:

function add(x, y) {
  return x + y;
}

This function declaration creates an ordinary function whose name is add. As an ordinary function, add() can play three roles:

Real function: invoked via a function call.

assert.equal(add(2, 1), 3);

Method: stored in a property, invoked via a method call.

const obj = { addAsMethod: add };
assert.equal(obj.addAsMethod(2, 4), 6); // (A)

In line A, obj is called the receiver of the method call.

Constructor function: invoked via new.

const inst = new add();
assert.equal(inst instanceof add, true);

As an aside, the names of constructor functions (incl. classes) normally start with capital letters.

25.2.5 Terminology: entity vs. syntax vs. role (advanced)

The distinction between the concepts syntax, entity, and role is subtle and often doesn’t matter. But I’d like to sharpen your eye for it:

An entity is a JavaScript feature as it “lives” in RAM. An ordinary function is an entity.

Entities include: ordinary functions, arrow functions, methods, and classes.

Syntax is the code that we use to create entities. Function declarations and anonymous function expressions are syntax. They both create entities that are called ordinary functions.

Syntax includes: function declarations and anonymous function expressions. The syntax that produces arrow functions is also called arrow functions. The same is true for methods and classes.

A role describes how we use entities. The entity ordinary function can play the role real function, or the role method, or the role class. The entity arrow function can also play the role real function.

The roles of functions are: real function, method, and constructor function.

Many other programming languages only have a single entity that plays the role real function. Then they can use the name function for both role and entity.

25.3 Specialized functions

Specialized functions are single-purpose versions of ordinary functions. Each one of them specializes in a single role:

The purpose of an arrow function is to be a real function:

const arrow = () => {
  return 123;
};
assert.equal(arrow(), 123);

The purpose of a method is to be a method:

const obj = {
  myMethod() {
    return 'abc';
  }
};
assert.equal(obj.myMethod(), 'abc');

The purpose of a class is to be a constructor function:

class MyClass {
  /* ··· */
}
const inst = new MyClass();

Apart from nicer syntax, each kind of specialized function also supports new features, making them better at their jobs than ordinary functions.

Arrow functions are explained soon.

Methods are explained in the chapter on single objects.

Classes are explained in the chapter on classes.

Tbl. 16 lists the capabilities of ordinary and specialized functions.

Table 16: Capabilities of four kinds of functions. If a cell value is in parentheses, that implies some kind of limitation. The special variable this is explained in §25.3.3 “The special variable this in methods, ordinary functions and arrow functions”.

| Function call | Method call | Constructor call
Ordinary function | (this === undefined) | ✔ | ✔
Arrow function | ✔ | (lexical this) | ✘
Method | (this === undefined) | ✔ | ✘
Class | ✘ | ✘ | ✔

25.3.1 Specialized functions are still functions

It’s important to note that arrow functions, methods, and classes are still categorized as functions:

> (() => {}) instanceof Function
true
> ({ method() {} }.method) instanceof Function
true
> (class SomeClass {}) instanceof Function
true

25.3.2 Arrow functions

Arrow functions were added to JavaScript for two reasons:

To provide a more concise way for creating functions.

They work better as real functions inside methods: Methods can refer to the object that received a method call via the special variable this. Arrow functions can access the this of a surrounding method, ordinary functions can’t (because they have their own this).

We’ll first examine the syntax of arrow functions and then how this works in various functions.

25.3.2.1 The syntax of arrow functions

Let’s review the syntax of an anonymous function expression:

const f = function (x, y, z) { return 123 };

The (roughly) equivalent arrow function looks as follows. Arrow functions are expressions.

const f = (x, y, z) => { return 123 };

Here, the body of the arrow function is a block. But it can also be an expression. The following arrow function works exactly like the previous one.

const f = (x, y, z) => 123;

If an arrow function has only a single parameter and that parameter is an identifier (not a destructuring pattern) then you can omit the parentheses around the parameter:

const id = x => x;

That is convenient when passing arrow functions as parameters to other functions or methods:

> [1,2,3].map(x => x+1)
[ 2, 3, 4 ]

This previous example demonstrates one benefit of arrow functions – conciseness. If we perform the same task with a function expression, our code is more verbose:

[1,2,3].map(function (x) { return x+1 });

25.3.2.2 Syntax pitfall: returning an object literal from an arrow function

If you want the expression body of an arrow function to be an object literal, you must put the literal in parentheses:

const func1 = () => ({a: 1});
assert.deepEqual(func1(), { a: 1 });

If you don’t, JavaScript thinks, the arrow function has a block body (that doesn’t return anything):

const func2 = () => {a: 1};
assert.deepEqual(func2(), undefined);

{a: 1} is interpreted as a block with the label a: and the expression statement 1. Without an explicit return statement, the block body returns undefined.

This pitfall is caused by syntactic ambiguity: object literals and code blocks have the same syntax. We use the parentheses to tell JavaScript that the body is an expression (an object literal) and not a statement (a block).

25.3.3 The special variable this in methods, ordinary functions and arrow functions

  The special variable this is an object-oriented feature

We are taking a quick look at the special variable this here, in order to understand why arrow functions are better real functions than ordinary functions.

But this feature only matters in object-oriented programming and is covered in more depth in §28.4 “Methods and the special variable this”. Therefore, don’t worry if you don’t fully understand it yet.

Inside methods, the special variable this lets us access the receiver – the object which received the method call:

const obj = {
  myMethod() {
    assert.equal(this, obj);
  }
};
obj.myMethod();

Ordinary functions can be methods and therefore also have the implicit parameter this:

const obj = {
  myMethod: function () {
    assert.equal(this, obj);
  }
};
obj.myMethod();

this is even an implicit parameter when we use an ordinary function as a real function. Then its value is undefined (if strict mode is active, which it almost always is):

function ordinaryFunc() {
  assert.equal(this, undefined);
}
ordinaryFunc();

That means that an ordinary function, used as a real function, can’t access the this of a surrounding method (line A). In contrast, arrow functions don’t have this as an implicit parameter. They treat it like any other variable and can therefore access the this of a surrounding method (line B):

const jill = {
  name: 'Jill',
  someMethod() {
    function ordinaryFunc() {
      assert.throws(
        () => this.name, // (A)
        /^TypeError: Cannot read property 'name' of undefined$/);
    }
    ordinaryFunc();

    const arrowFunc = () => {
      assert.equal(this.name, 'Jill'); // (B)
    };
    arrowFunc();
  },
};
jill.someMethod();

In this code, we can observe two ways of handling this:

Dynamic this: In line A, we try to access the this of .someMethod() from an ordinary function. There, it is shadowed by the function’s own this, which is undefined (as filled in by the function call). Given that ordinary functions receive their this via (dynamic) function or method calls, their this is called dynamic.

Lexical this: In line B, we again try to access the this of .someMethod(). This time, we succeed because the arrow function does not have its own this. this is resolved lexically, just like any other variable. That’s why the this of arrow functions is called lexical.

25.3.4 Recommendation: prefer specialized functions over ordinary functions

Normally, you should prefer specialized functions over ordinary functions, especially classes and methods.

When it comes to real functions, the choice between an arrow function and an ordinary function is less clear-cut, though:

For anonymous inline function expressions, arrow functions are clear winners, due to their compact syntax and them not having this as an implicit parameter:

const twiceOrdinary = [1, 2, 3].map(function (x) {return x * 2});
const twiceArrow = [1, 2, 3].map(x => x * 2);

For stand-alone named function declarations, arrow functions still benefit from lexical this. But function declarations (which produce ordinary functions) have nice syntax and early activation is also occasionally useful (see §11.8 “Declarations: scope and activation”). If this doesn’t appear in the body of an ordinary function, there is no downside to using it as a real function. The static checking tool ESLint can warn us during development when we do this wrong via a built-in rule.

function timesOrdinary(x, y) {
  return x * y;
}
const timesArrow = (x, y) => {
  return x * y;
};

25.4 Summary: kinds of callable values

  This section refers to upcoming content

This section mainly serves as a reference for the current and upcoming chapters. Don’t worry if you don’t understand everything.

So far, all (real) functions and methods, that we have seen, were:

Single-result

Synchronous

Later chapters will cover other modes of programming:

Iteration treats objects as containers of data (so-called iterables) and provides a standardized way for retrieving what is inside them. If a function or a method returns an iterable, it returns multiple values.

Asynchronous programming deals with handling a long-running computation. You are notified when the computation is finished and can do something else in between. The standard pattern for asynchronously delivering single results is called Promise.

These modes can be combined – for example, there are synchronous iterables and asynchronous iterables.

Several new kinds of functions and methods help with some of the mode combinations:

Async functions help implement functions that return Promises. There are also async methods.

Synchronous generator functions help implement functions that return synchronous iterables. There are also synchronous generator methods.

Asynchronous generator functions help implement functions that return asynchronous iterables. There are also asynchronous generator methods.

That leaves us with 4 kinds (2 × 2) of functions and methods:

Synchronous vs. asynchronous

Generator vs. single-result

Tbl. 17 gives an overview of the syntax for creating these 4 kinds of functions and methods.

Table 17: Syntax for creating functions and methods. The last column specifies how many values are produced by an entity.

|  | Result | #
Sync function | Sync method |  |
function f() {} | { m() {} } | value | 1
f = function () {} |  |  |
f = () => {} |  |  |
Sync generator function | Sync gen. method |  |
function* f() {} | { * m() {} } | iterable | 0+
f = function* () {} |  |  |
Async function | Async method |  |
async function f() {} | { async m() {} } | Promise | 1
f = async function () {} |  |  |
f = async () => {} |  |  |
Async generator function | Async gen. method |  |
async function* f() {} | { async * m() {} } | async iterable | 0+
f = async function* () {} |  |  |

25.5 Returning values from functions and methods

(Everything mentioned in this section applies to both functions and methods.)

The return statement explicitly returns a value from a function:

function func() {
  return 123;
}
assert.equal(func(), 123);

Another example:

function boolToYesNo(bool) {
  if (bool) {
    return 'Yes';
  } else {
    return 'No';
  }
}
assert.equal(boolToYesNo(true), 'Yes');
assert.equal(boolToYesNo(false), 'No');

If, at the end of a function, you haven’t returned anything explicitly, JavaScript returns undefined for you:

function noReturn() {
  // No explicit return
}
assert.equal(noReturn(), undefined);

25.6 Parameter handling

Once again, I am only mentioning functions in this section, but everything also applies to methods.

25.6.1 Terminology: parameters vs. arguments

The term parameter and the term argument basically mean the same thing. If you want to, you can make the following distinction:

Parameters are part of a function definition. They are also called formal parameters and formal arguments.

Arguments are part of a function call. They are also called actual parameters and actual arguments.

25.6.2 Terminology: callback

A callback or callback function is a function that is an argument of a function or method call.

The following is an example of a callback:

const myArray = ['a', 'b'];
const callback = (x) => console.log(x);
myArray.forEach(callback);

// Output:
// 'a'
// 'b'

25.6.3 Too many or not enough arguments

JavaScript does not complain if a function call provides a different number of arguments than expected by the function definition:

Extra arguments are ignored.

Missing parameters are set to undefined.

For example:

function foo(x, y) {
  return [x, y];
}

// Too many arguments:
assert.deepEqual(foo('a', 'b', 'c'), ['a', 'b']);

// The expected number of arguments:
assert.deepEqual(foo('a', 'b'), ['a', 'b']);

// Not enough arguments:
assert.deepEqual(foo('a'), ['a', undefined]);

25.6.4 Parameter default values

Parameter default values specify the value to use if a parameter has not been provided – for example:

function f(x, y=0) {
  return [x, y];
}

assert.deepEqual(f(1), [1, 0]);
assert.deepEqual(f(), [undefined, 0]);

undefined also triggers the default value:

assert.deepEqual(
  f(undefined, undefined),
  [undefined, 0]);

25.6.5 Rest parameters

A rest parameter is declared by prefixing an identifier with three dots (...). During a function or method call, it receives an Array with all remaining arguments. If there are no extra arguments at the end, it is an empty Array – for example:

function f(x, ...y) {
  return [x, y];
}
assert.deepEqual(
  f('a', 'b', 'c'),
  ['a', ['b', 'c']]);
assert.deepEqual(
  f(),
  [undefined, []]);

25.6.5.1 Enforcing a certain number of arguments via a rest parameter

You can use a rest parameter to enforce a certain number of arguments. Take, for example, the following function:

function createPoint(x, y) {
  return {x, y};
    // same as {x: x, y: y}
}

This is how we force callers to always provide two arguments:

function createPoint(...args) {
  if (args.length !== 2) {
    throw new Error('Please provide exactly 2 arguments!');
  }
  const [x, y] = args; // (A)
  return {x, y};
}

In line A, we access the elements of args via destructuring.

25.6.6 Named parameters

When someone calls a function, the arguments provided by the caller are assigned to the parameters received by the callee. Two common ways of performing the mapping are:

Positional parameters: An argument is assigned to a parameter if they have the same position. A function call with only positional arguments looks as follows.

selectEntries(3, 20, 2)

Named parameters: An argument is assigned to a parameter if they have the same name. JavaScript doesn’t have named parameters, but you can simulate them. For example, this is a function call with only (simulated) named arguments:

selectEntries({start: 3, end: 20, step: 2})

Named parameters have several benefits:

They lead to more self-explanatory code because each argument has a descriptive label. Just compare the two versions of selectEntries(): with the second one, it is much easier to see what happens.

The order of the arguments doesn’t matter (as long as the names are correct).

Handling more than one optional parameter is more convenient: callers can easily provide any subset of all optional parameters and don’t have to be aware of the ones they omit (with positional parameters, you have to fill in preceding optional parameters, with undefined).

25.6.7 Simulating named parameters

JavaScript doesn’t have real named parameters. The official way of simulating them is via object literals:

function selectEntries({start=0, end=-1, step=1}) {
  return {start, end, step};
}

This function uses destructuring to access the properties of its single parameter. The pattern it uses is an abbreviation for the following pattern:

{start: start=0, end: end=-1, step: step=1}

This destructuring pattern works for empty object literals:

> selectEntries({})
{ start: 0, end: -1, step: 1 }

But it does not work if you call the function without any parameters:

> selectEntries()
TypeError: Cannot read property 'start' of undefined

You can fix this by providing a default value for the whole pattern. This default value works the same as default values for simpler parameter definitions: if the parameter is missing, the default is used.

function selectEntries({start=0, end=-1, step=1} = {}) {
  return {start, end, step};
}
assert.deepEqual(
  selectEntries(),
  { start: 0, end: -1, step: 1 });

25.6.8 Spreading (...) into function calls

If you put three dots (...) in front of the argument of a function call, then you spread it. That means that the argument must be an iterable object and the iterated values all become arguments. In other words, a single argument is expanded into multiple arguments – for example:

function func(x, y) {
  console.log(x);
  console.log(y);
}
const someIterable = ['a', 'b'];
func(...someIterable);
  // same as func('a', 'b')

// Output:
// 'a'
// 'b'

Spreading and rest parameters use the same syntax (...), but they serve opposite purposes:

Rest parameters are used when defining functions or methods. They collect arguments into Arrays.

Spread arguments are used when calling functions or methods. They turn iterable objects into arguments.

25.6.8.1 Example: spreading into Math.max()

Math.max() returns the largest one of its zero or more arguments. Alas, it can’t be used for Arrays, but spreading gives us a way out:

> Math.max(-1, 5, 11, 3)
11
> Math.max(...[-1, 5, 11, 3])
11
> Math.max(-1, ...[-5, 11], 3)
11

25.6.8.2 Example: spreading into Array.prototype.push()

Similarly, the Array method .push() destructively adds its zero or more parameters to the end of its Array. JavaScript has no method for destructively appending an Array to another one. Once again, we are saved by spreading:

const arr1 = ['a', 'b'];
const arr2 = ['c', 'd'];

arr1.push(...arr2);
assert.deepEqual(arr1, ['a', 'b', 'c', 'd']);

  Exercises: Parameter handling

Positional parameters: exercises/callables/positional_parameters_test.mjs

Named parameters: exercises/callables/named_parameters_test.mjs

25.7 Methods of functions: .call(), .apply(), .bind()

Functions are objects and have methods. In this section, we look at three of those methods: .call(), .apply(), and .bind().

25.7.1 The function method .call()

Each function someFunc has the following method:

someFunc.call(thisValue, arg1, arg2, arg3);

This method invocation is loosely equivalent to the following function call:

someFunc(arg1, arg2, arg3);

However, with .call(), we can also specify a value for the implicit parameter this. In other words: .call() makes the implicit parameter this explicit.

The following code demonstrates the use of .call():

function func(x, y) {
  return [this, x, y];
}

assert.deepEqual(
  func.call('hello', 'a', 'b'),
  ['hello', 'a', 'b']);

As we have seen before, if we function-call an ordinary function, its this is undefined:

assert.deepEqual(
  func('a', 'b'),
  [undefined, 'a', 'b']);

Therefore, the previous function call is equivalent to:

assert.deepEqual(
  func.call(undefined, 'a', 'b'),
  [undefined, 'a', 'b']);

In arrow functions, the value for this provided via .call() (or other means) is ignored.

25.7.2 The function method .apply()

Each function someFunc has the following method:

someFunc.apply(thisValue, [arg1, arg2, arg3]);

This method invocation is loosely equivalent to the following function call (which uses spreading):

someFunc(...[arg1, arg2, arg3]);

However, with .apply(), we can also specify a value for the implicit parameter this.

The following code demonstrates the use of .apply():

function func(x, y) {
  return [this, x, y];
}

const args = ['a', 'b'];
assert.deepEqual(
  func.apply('hello', args),
  ['hello', 'a', 'b']);

25.7.3 The function method .bind()

.bind() is another method of function objects. This method is invoked as follows:

const boundFunc = someFunc.bind(thisValue, arg1, arg2);

.bind() returns a new function boundFunc(). Calling that function invokes someFunc() with this set to thisValue and these parameters: arg1, arg2, followed by the parameters of boundFunc().

That is, the following two function calls are equivalent:

boundFunc('a', 'b')
someFunc.call(thisValue, arg1, arg2, 'a', 'b')

25.7.3.1 An alternative to .bind()

Another way of pre-filling this and parameters is via an arrow function:

const boundFunc2 = (...args) =>
  someFunc.call(thisValue, arg1, arg2, ...args);

25.7.3.2 An implementation of .bind()

Considering the previous section, .bind() can be implemented as a real function as follows:

function bind(func, thisValue, ...boundArgs) {
  return (...args) =>
    func.call(thisValue, ...boundArgs, ...args);
}

25.7.3.3 Example: binding a real function

Using .bind() for real functions is somewhat unintuitive because we have to provide a value for this. Given that it is undefined during function calls, it is usually set to undefined or null.

In the following example, we create add8(), a function that has one parameter, by binding the first parameter of add() to 8.

function add(x, y) {
  return x + y;
}

const add8 = add.bind(undefined, 8);
assert.equal(add8(1), 9);

  Quiz

See quiz app.

26 Evaluating code dynamically: eval(), new Function() (advanced)

26.1 eval()

26.2 new Function()

26.3 Recommendations

In this chapter, we’ll look at two ways of evaluating code dynamically: eval() and new Function().

26.1 eval()

Given a string str with JavaScript code, eval(str) evaluates that code and returns the result:

> eval('2 ** 4')
16

There are two ways of invoking eval():

Directly, via a function call. Then the code in its argument is evaluated inside the current scope.

Indirectly, not via a function call. Then it evaluates its code in global scope.

“Not via a function call” means “anything that looks different than eval(···)”:

eval.call(undefined, '···') (uses method .call() of functions)

eval?.() () (uses optional chaining)

(0, eval)('···') (uses the comma operator)

globalThis.eval('···')

const e = eval; e('···')

Etc.

The following code illustrates the difference:

globalThis.myVariable = 'global';
function func() {
  const myVariable = 'local';

  // Direct eval
  assert.equal(eval('myVariable'), 'local');

  // Indirect eval
  assert.equal(eval.call(undefined, 'myVariable'), 'global');
}

Evaluating code in global context is safer because the code has access to fewer internals.

26.2 new Function()

new Function() creates a function object and is invoked as follows:

const func = new Function('«param_1»', ···, '«param_n»', '«func_body»');

The previous statement is equivalent to the next statement. Note that «param_1», etc., are not inside string literals, anymore.

const func = function («param_1», ···, «param_n») {
  «func_body»
};

In the next example, we create the same function twice, first via new Function(), then via a function expression:

const times1 = new Function('a', 'b', 'return a * b');
const times2 = function (a, b) { return a * b };

  new Function() creates non-strict mode functions

By default, functions created via new Function() are sloppy. If we want the function body to be in strict mode, we have to switch it on manually.

26.3 Recommendations

Avoid dynamic evaluation of code as much as you can:

It’s a security risk because it may enable an attacker to execute arbitrary code with the privileges of your code.

It may be switched off – for example, in browsers, via a Content Security Policy.

Very often, JavaScript is dynamic enough so that you don’t need eval() or similar. In the following example, what we are doing with eval() (line A) can be achieved just as well without it (line B).

const obj = {a: 1, b: 2};
const propKey = 'b';

assert.equal(eval('obj.' + propKey), 2); // (A)
assert.equal(obj[propKey], 2); // (B)

If you have to dynamically evaluate code:

Prefer new Function() over eval(): it always executes its code in global context and a function provides a clean interface to the evaluated code.

Prefer indirect eval over direct eval: evaluating code in global context is safer.

27 Modules

27.1 Overview: syntax of ECMAScript modules

27.1.1 Exporting

27.1.2 Importing

27.2 JavaScript source code formats

27.2.1 Code before built-in modules was written in ECMAScript 5

27.3 Before we had modules, we had scripts

27.4 Module systems created prior to ES6

27.4.1 Server side: CommonJS modules

27.4.2 Client side: AMD (Asynchronous Module Definition) modules

27.4.3 Characteristics of JavaScript modules

27.5 ECMAScript modules

27.5.1 ES modules: syntax, semantics, loader API

27.6 Named exports and imports

27.6.1 Named exports

27.6.2 Named imports

27.6.3 Namespace imports

27.6.4 Named exporting styles: inline versus clause (advanced)

27.7 Default exports and imports

27.7.1 The two styles of default-exporting

27.7.2 The default export as a named export (advanced)

27.8 More details on exporting and importing

27.8.1 Imports are read-only views on exports

27.8.2 ESM’s transparent support for cyclic imports (advanced)

27.9 npm packages

27.9.1 Packages are installed inside a directory node_modules/

27.9.2 Why can npm be used to install frontend libraries?

27.10 Naming modules

27.11 Module specifiers

27.11.1 Categories of module specifiers

27.11.2 ES module specifiers in browsers

27.11.3 ES module specifiers on Node.js

27.12 Loading modules dynamically via import() [ES2020]

27.12.1 Example: loading a module dynamically

27.12.2 Use cases for import()

27.13 import.meta – metadata for the current module [ES2020]

27.13.1 import.meta.url

27.13.2 import.meta.url and class URL

27.13.3 import.meta.url on Node.js

27.14 Polyfills: emulating native web platform features (advanced)

27.14.1 Sources of this section

27.1 Overview: syntax of ECMAScript modules

27.1.1 Exporting

// Named exports
export function f() {}
export const one = 1;
export {foo, b as bar};

// Default exports
export default function f() {} // declaration with optional name
// Replacement for `const` (there must be exactly one value)
export default 123;

// Re-exporting from another module
export {foo, b as bar} from './some-module.mjs';
export * from './some-module.mjs';
export * as ns from './some-module.mjs'; // ES2020

27.1.2 Importing

// Named imports
import {foo, bar as b} from './some-module.mjs';
// Namespace import
import * as someModule from './some-module.mjs';
// Default import
import someModule from './some-module.mjs';

// Combinations:
import someModule, * as someModule from './some-module.mjs';
import someModule, {foo, bar as b} from './some-module.mjs';

// Empty import (for modules with side effects)
import './some-module.mjs';

27.2 JavaScript source code formats

The current landscape of JavaScript modules is quite diverse: ES6 brought built-in modules, but the source code formats that came before them, are still around, too. Understanding the latter helps understand the former, so let’s investigate. The next sections describe the following ways of delivering JavaScript source code:

Scripts are code fragments that browsers run in global scope. They are precursors of modules.

CommonJS modules are a module format that is mainly used on servers (e.g., via Node.js).

AMD modules are a module format that is mainly used in browsers.

ECMAScript modules are JavaScript’s built-in module format. It supersedes all previous formats.

Tbl. 18 gives an overview of these code formats. Note that for CommonJS modules and ECMAScript modules, two filename extensions are commonly used. Which one is appropriate depends on how you want to use a file. Details are given later in this chapter.

Table 18: Ways of delivering JavaScript source code.

| Runs on | Loaded | Filename ext.
Script | browsers | async | .js
CommonJS module | servers | sync | .js .cjs
AMD module | browsers | async | .js
ECMAScript module | browsers and servers | async | .js .mjs

27.2.1 Code before built-in modules was written in ECMAScript 5

Before we get to built-in modules (which were introduced with ES6), all code that you’ll see, will be written in ES5. Among other things:

ES5 did not have const and let, only var.

ES5 did not have arrow functions, only function expressions.

27.3 Before we had modules, we had scripts

Initially, browsers only had scripts – pieces of code that were executed in global scope. As an example, consider an HTML file that loads script files via the following HTML:

<script src="other-module1.js"></script>
<script src="other-module2.js"></script>
<script src="my-module.js"></script>

The main file is my-module.js, where we simulate a module:

var myModule = (function () { // Open IIFE
  // Imports (via global variables)
  var importedFunc1 = otherModule1.importedFunc1;
  var importedFunc2 = otherModule2.importedFunc2;

  // Body
  function internalFunc() {
    // ···
  }
  function exportedFunc() {
    importedFunc1();
    importedFunc2();
    internalFunc();
  }

  // Exports (assigned to global variable `myModule`)
  return {
    exportedFunc: exportedFunc,
  };
})(); // Close IIFE

myModule is a global variable that is assigned the result of immediately invoking a function expression. The function expression starts in the first line. It is invoked in the last line.

This way of wrapping a code fragment is called immediately invoked function expression (IIFE, coined by Ben Alman). What do we gain from an IIFE? var is not block-scoped (like const and let), it is function-scoped: the only way to create new scopes for var-declared variables is via functions or methods (with const and let, you can use either functions, methods, or blocks {}). Therefore, the IIFE in the example hides all of the following variables from global scope and minimizes name clashes: importedFunc1, importedFunc2, internalFunc, exportedFunc.

Note that we are using an IIFE in a particular manner: at the end, we pick what we want to export and return it via an object literal. That is called the revealing module pattern (coined by Christian Heilmann).

This way of simulating modules, has several issues:

Libraries in script files export and import functionality via global variables, which risks name clashes.

Dependencies are not stated explicitly, and there is no built-in way for a script to load the scripts it depends on. Therefore, the web page has to load not just the scripts that are needed by the page but also the dependencies of those scripts, the dependencies’ dependencies, etc. And it has to do so in the right order!

27.4 Module systems created prior to ES6

Prior to ECMAScript 6, JavaScript did not have built-in modules. Therefore, the flexible syntax of the language was used to implement custom module systems within the language. Two popular ones are:

CommonJS (targeting the server side)

AMD (Asynchronous Module Definition, targeting the client side)

27.4.1 Server side: CommonJS modules

The original CommonJS standard for modules was created for server and desktop platforms. It was the foundation of the original Node.js module system, where it achieved enormous popularity. Contributing to that popularity were the npm package manager for Node and tools that enabled using Node modules on the client side (browserify, webpack, and others).

From now on, CommonJS module means the Node.js version of this standard (which has a few additional features). This is an example of a CommonJS module:

// Imports
var importedFunc1 = require('./other-module1.js').importedFunc1;
var importedFunc2 = require('./other-module2.js').importedFunc2;

// Body
function internalFunc() {
  // ···
}
function exportedFunc() {
  importedFunc1();
  importedFunc2();
  internalFunc();
}

// Exports
module.exports = {
  exportedFunc: exportedFunc,
};

CommonJS can be characterized as follows:

Designed for servers.

Modules are meant to be loaded synchronously (the importer waits while the imported module is loaded and executed).

Compact syntax.

27.4.2 Client side: AMD (Asynchronous Module Definition) modules

The AMD module format was created to be easier to use in browsers than the CommonJS format. Its most popular implementation is RequireJS. The following is an example of an AMD module.

define(['./other-module1.js', './other-module2.js'],
  function (otherModule1, otherModule2) {
    var importedFunc1 = otherModule1.importedFunc1;
    var importedFunc2 = otherModule2.importedFunc2;

    function internalFunc() {
      // ···
    }
    function exportedFunc() {
      importedFunc1();
      importedFunc2();
      internalFunc();
    }

    return {
      exportedFunc: exportedFunc,
    };
  });

AMD can be characterized as follows:

Designed for browsers.

Modules are meant to be loaded asynchronously. That’s a crucial requirement for browsers, where code can’t wait until a module has finished downloading. It has to be notified once the module is available.

The syntax is slightly more complicated.

On the plus side, AMD modules can be executed directly. In contrast, CommonJS modules must either be compiled before deployment or custom source code must be generated and evaluated dynamically (think eval()). That isn’t always permitted on the web.

27.4.3 Characteristics of JavaScript modules

Looking at CommonJS and AMD, similarities between JavaScript module systems emerge:

There is one module per file.

Such a file is basically a piece of code that is executed:

Local scope: The code is executed in a local “module scope”. Therefore, by default, all of the variables, functions, and classes declared in it are internal and not global.

Exports: If you want any declared entity to be exported, you must explicitly mark it as an export.

Imports: Each module can import exported entities from other modules. Those other modules are identified via module specifiers (usually paths, occasionally full URLs).

Modules are singletons: Even if a module is imported multiple times, only a single “instance” of it exists.

No global variables are used. Instead, module specifiers serve as global IDs.

27.5 ECMAScript modules

ECMAScript modules (ES modules or ESM) were introduced with ES6. They continue the tradition of JavaScript modules and have all of their aforementioned characteristics. Additionally:

With CommonJS, ES modules share the compact syntax and support for cyclic dependencies.

With AMD, ES modules share being designed for asynchronous loading.

ES modules also have new benefits:

The syntax is even more compact than CommonJS’s.

Modules have static structures (which can’t be changed at runtime). That helps with static checking, optimized access of imports, dead code elimination, and more.

Support for cyclic imports is completely transparent.

This is an example of ES module syntax:

import {importedFunc1} from './other-module1.mjs';
import {importedFunc2} from './other-module2.mjs';

function internalFunc() {
  ···
}

export function exportedFunc() {
  importedFunc1();
  importedFunc2();
  internalFunc();
}

From now on, “module” means “ECMAScript module”.

27.5.1 ES modules: syntax, semantics, loader API

The full standard of ES modules comprises the following parts:

Syntax (how code is written): What is a module? How are imports and exports declared? Etc.

Semantics (how code is executed): How are variable bindings exported? How are imports connected with exports? Etc.

A programmatic loader API for configuring module loading.

Parts 1 and 2 were introduced with ES6. Work on part 3 is ongoing.

27.6 Named exports and imports

27.6.1 Named exports

Each module can have zero or more named exports.

As an example, consider the following two files:

lib/my-math.mjs
main.mjs

Module my-math.mjs has two named exports: square and LIGHTSPEED.

// Not exported, private to module
function times(a, b) {
  return a * b;
}
export function square(x) {
  return times(x, x);
}
export const LIGHTSPEED = 299792458;

To export something, we put the keyword export in front of a declaration. Entities that are not exported are private to a module and can’t be accessed from outside.

27.6.2 Named imports

Module main.mjs has a single named import, square:

import {square} from './lib/my-math.mjs';
assert.equal(square(3), 9);

It can also rename its import:

import {square as sq} from './lib/my-math.mjs';
assert.equal(sq(3), 9);

27.6.2.1 Syntactic pitfall: named importing is not destructuring

Both named importing and destructuring look similar:

import {foo} from './bar.mjs'; // import
const {foo} = require('./bar.mjs'); // destructuring

But they are quite different:

Imports remain connected with their exports.

You can destructure again inside a destructuring pattern, but the {} in an import statement can’t be nested.

The syntax for renaming is different:

import {foo as f} from './bar.mjs'; // importing
const {foo: f} = require('./bar.mjs'); // destructuring

Rationale: Destructuring is reminiscent of an object literal (including nesting), while importing evokes the idea of renaming.

  Exercise: Named exports

exercises/modules/export_named_test.mjs

27.6.3 Namespace imports

Namespace imports are an alternative to named imports. If we namespace-import a module, it becomes an object whose properties are the named exports. This is what main.mjs looks like if we use a namespace import:

import * as myMath from './lib/my-math.mjs';
assert.equal(myMath.square(3), 9);

assert.deepEqual(
  Object.keys(myMath), ['LIGHTSPEED', 'square']);

27.6.4 Named exporting styles: inline versus clause (advanced)

The named export style we have seen so far was inline: We exported entities by prefixing them with the keyword export.

But we can also use separate export clauses. For example, this is what lib/my-math.mjs looks like with an export clause:

function times(a, b) {
  return a * b;
}
function square(x) {
  return times(x, x);
}
const LIGHTSPEED = 299792458;

export { square, LIGHTSPEED }; // semicolon!

With an export clause, we can rename before exporting and use different names internally:

function times(a, b) {
  return a * b;
}
function sq(x) {
  return times(x, x);
}
const LS = 299792458;

export {
  sq as square,
  LS as LIGHTSPEED, // trailing comma is optional
};

27.7 Default exports and imports

Each module can have at most one default export. The idea is that the module is the default-exported value.

  Avoid mixing named exports and default exports

A module can have both named exports and a default export, but it’s usually better to stick to one export style per module.

As an example for default exports, consider the following two files:

my-func.mjs
main.mjs

Module my-func.mjs has a default export:

const GREETING = 'Hello!';
export default function () {
  return GREETING;
}

Module main.mjs default-imports the exported function:

import myFunc from './my-func.mjs';
assert.equal(myFunc(), 'Hello!');

Note the syntactic difference: the curly braces around named imports indicate that we are reaching into the module, while a default import is the module.

  What are use cases for default exports?

The most common use case for a default export is a module that contains a single function or a single class.

27.7.1 The two styles of default-exporting

There are two styles of doing default exports.

First, you can label existing declarations with export default:

export default function foo() {} // no semicolon!
export default class Bar {} // no semicolon!

Second, you can directly default-export values. In that style, export default is itself much like a declaration.

export default 'abc';
export default foo();
export default /^xyz$/;
export default 5 * 7;
export default { no: false, yes: true };

27.7.1.1 Why are there two default export styles?

The reason is that export default can’t be used to label const: const may define multiple values, but export default needs exactly one value. Consider the following hypothetical code:

// Not legal JavaScript!
export default const foo = 1, bar = 2, baz = 3;

With this code, you don’t know which one of the three values is the default export.

  Exercise: Default exports

exercises/modules/export_default_test.mjs

27.7.2 The default export as a named export (advanced)

Internally, a default export is simply a named export whose name is default. As an example, consider the previous module my-func.mjs with a default export:

const GREETING = 'Hello!';
export default function () {
  return GREETING;
}

The following module my-func2.mjs is equivalent to that module:

const GREETING = 'Hello!';
function greet() {
  return GREETING;
}

export {
  greet as default,
};

For importing, we can use a normal default import:

import myFunc from './my-func2.mjs';
assert.equal(myFunc(), 'Hello!');

Or we can use a named import:

import {default as myFunc} from './my-func2.mjs';
assert.equal(myFunc(), 'Hello!');

The default export is also available via property .default of namespace imports:

import * as mf from './my-func2.mjs';
assert.equal(mf.default(), 'Hello!');

  Isn’t default illegal as a variable name?

default can’t be a variable name, but it can be an export name and it can be a property name:

const obj = {
  default: 123,
};
assert.equal(obj.default, 123);

27.8 More details on exporting and importing

27.8.1 Imports are read-only views on exports

So far, we have used imports and exports intuitively, and everything seems to have worked as expected. But now it is time to take a closer look at how imports and exports are really related.

Consider the following two modules:

counter.mjs
main.mjs

counter.mjs exports a (mutable!) variable and a function:

export let counter = 3;
export function incCounter() {
  counter++;
}

main.mjs name-imports both exports. When we use incCounter(), we discover that the connection to counter is live – we can always access the live state of that variable:

import { counter, incCounter } from './counter.mjs';

// The imported value `counter` is live
assert.equal(counter, 3);
incCounter();
assert.equal(counter, 4);

Note that while the connection is live and we can read counter, we cannot change this variable (e.g., via counter++).

There are two benefits to handling imports this way:

It is easier to split modules because previously shared variables can become exports.

This behavior is crucial for supporting transparent cyclic imports. Read on for more information.

27.8.2 ESM’s transparent support for cyclic imports (advanced)

ESM supports cyclic imports transparently. To understand how that is achieved, consider the following example: fig. 7 shows a directed graph of modules importing other modules. P importing M is the cycle in this case.

Figure 7: A directed graph of modules importing modules: M imports N and O, N imports P and Q, etc.

After parsing, these modules are set up in two phases:

Instantiation: Every module is visited and its imports are connected to its exports. Before a parent can be instantiated, all of its children must be instantiated.

Evaluation: The bodies of the modules are executed. Once again, children are evaluated before parents.

This approach handles cyclic imports correctly, due to two features of ES modules:

Due to the static structure of ES modules, the exports are already known after parsing. That makes it possible to instantiate P before its child M: P can already look up M’s exports.

When P is evaluated, M hasn’t been evaluated, yet. However, entities in P can already mention imports from M. They just can’t use them, yet, because the imported values are filled in later. For example, a function in P can access an import from M. The only limitation is that we must wait until after the evaluation of M, before calling that function.

Imports being filled in later is enabled by them being “live immutable views” on exports.

27.9 npm packages

The npm software registry is the dominant way of distributing JavaScript libraries and apps for Node.js and web browsers. It is managed via the npm package manager (short: npm). Software is distributed as so-called packages. A package is a directory containing arbitrary files and a file package.json at the top level that describes the package. For example, when npm creates an empty package inside a directory my-package/, you get this package.json:

{
  "name": "my-package",
  "version": "1.0.0",
  "description": "",
  "main": "index.js",
  "scripts": {
    "test": "echo \"Error: no test specified\" && exit 1"
  },
  "keywords": [],
  "author": "",
  "license": "ISC"
}

Some of these properties contain simple metadata:

name specifies the name of this package. Once it is uploaded to the npm registry, it can be installed via npm install my-package.

version is used for version management and follows semantic versioning, with three numbers:

Major version: is incremented when incompatible API changes are made.

Minor version: is incremented when functionality is added in a backward compatible manner.

Patch version: is incremented when backward compatible changes are made.

description, keywords, author make it easier to find packages.

license clarifies how you can use this package.

Other properties enable advanced configuration:

main: specifies the module that “is” the package (explained later in this chapter).

scripts: are commands that you can execute via npm run. For example, the script test can be executed via npm run test.

For more information on package.json, consult the npm documentation.

27.9.1 Packages are installed inside a directory node_modules/

npm always installs packages inside a directory node_modules. There are usually many of these directories. Which one npm uses, depends on the directory where one currently is. For example, if we are inside a directory /tmp/a/b/, npm tries to find a node_modules in the current directory, its parent directory, the parent directory of the parent, etc. In other words, it searches the following chain of locations:

/tmp/a/b/node_modules

/tmp/a/node_modules

/tmp/node_modules

When installing a package some-pkg, npm uses the closest node_modules. If, for example, we are inside /tmp/a/b/ and there is a node_modules in that directory, then npm puts the package inside the directory:

/tmp/a/b/node_modules/some-pkg/

When importing a module, we can use a special module specifier to tell Node.js that we want to import it from an installed package. How exactly that works, is explained later. For now, consider the following example:

// /home/jane/proj/main.mjs
import * as theModule from 'the-package/the-module.mjs';

To find the-module.mjs (Node.js prefers the filename extension .mjs for ES modules), Node.js walks up the node_module chain and searches the following locations:

/home/jane/proj/node_modules/the-package/the-module.mjs

/home/jane/node_modules/the-package/the-module.mjs

/home/node_modules/the-package/the-module.mjs

27.9.2 Why can npm be used to install frontend libraries?

Finding installed modules in node_modules directories is only supported on Node.js. So why can we also use npm to install libraries for browsers?

That is enabled via bundling tools, such as webpack, that compile and optimize code before it is deployed online. During this compilation process, the code in npm packages is adapted so that it works in browsers.

27.10 Naming modules

There are no established best practices for naming module files and the variables they are imported into.

In this chapter, I’m using the following naming style:

The names of module files are dash-cased and start with lowercase letters:

./my-module.mjs
./some-func.mjs

The names of namespace imports are lowercased and camel-cased:

import * as myModule from './my-module.mjs';

The names of default imports are lowercased and camel-cased:

import someFunc from './some-func.mjs';

What are the rationales behind this style?

npm doesn’t allow uppercase letters in package names (source). Thus, we avoid camel case, so that “local” files have names that are consistent with those of npm packages. Using only lowercase letters also minimizes conflicts between file systems that are case-sensitive and file systems that aren’t: the former distinguish files whose names have the same letters, but with different cases; the latter don’t.

There are clear rules for translating dash-cased file names to camel-cased JavaScript variable names. Due to how we name namespace imports, these rules work for both namespace imports and default imports.

I also like underscore-cased module file names because you can directly use these names for namespace imports (without any translation):

import * as my_module from './my_module.mjs';

But that style does not work for default imports: I like underscore-casing for namespace objects, but it is not a good choice for functions, etc.

27.11 Module specifiers

Module specifiers are the strings that identify modules. They work slightly differently in browsers and Node.js. Before we can look at the differences, we need to learn about the different categories of module specifiers.

27.11.1 Categories of module specifiers

In ES modules, we distinguish the following categories of specifiers. These categories originated with CommonJS modules.

Relative path: starts with a dot. Examples:

'./some/other/module.mjs'
'../../lib/counter.mjs'

Absolute path: starts with a slash. Example:

'/home/jane/file-tools.mjs'

URL: includes a protocol (technically, paths are URLs, too). Examples:

'https://example.com/some-module.mjs'
'file:///home/john/tmp/main.mjs'

Bare path: does not start with a dot, a slash or a protocol, and consists of a single filename without an extension. Examples:

'lodash'
'the-package'

Deep import path: starts with a bare path and has at least one slash. Example:

'the-package/dist/the-module.mjs'

27.11.2 ES module specifiers in browsers

Browsers handle module specifiers as follows:

Relative paths, absolute paths, and URLs work as expected. They all must point to real files (in contrast to CommonJS, which lets you omit filename extensions and more).

The file name extensions of modules don’t matter, as long as they are served with the content type text/javascript.

How bare paths will end up being handled is not yet clear. You will probably eventually be able to map them to other specifiers via lookup tables.

Note that bundling tools such as webpack, which combine modules into fewer files, are often less strict with specifiers than browsers. That’s because they operate at build/compile time (not at runtime) and can search for files by traversing the file system.

27.11.3 ES module specifiers on Node.js

Node.js handles module specifiers as follows:

Relative paths are resolved as they are in web browsers – relative to the path of the current module.

Absolute paths are currently not supported. As a workaround, you can use URLs that start with file:///. You can create such URLs via url.pathToFileURL().

Only file: is supported as a protocol for URL specifiers.

A bare path is interpreted as a package name and resolved relative to the closest node_modules directory. What module should be loaded, is determined by looking at property "main" of the package’s package.json (similarly to CommonJS).

Deep import paths are also resolved relatively to the closest node_modules directory. They contain file names, so it is always clear which module is meant.

All specifiers, except bare paths, must refer to actual files. That is, ESM does not support the following CommonJS features:

CommonJS automatically adds missing filename extensions.

CommonJS can import a directory dir if there is a dir/package.json with a "main" property.

CommonJS can import a directory dir if there is a module dir/index.js.

All built-in Node.js modules are available via bare paths and have named ESM exports – for example:

import * as assert from 'assert/strict';
import * as path from 'path';

assert.equal(
  path.join('a/b/c', '../d'), 'a/b/d');

27.11.3.1 Filename extensions on Node.js

Node.js supports the following default filename extensions:

.mjs for ES modules

.cjs for CommonJS modules

The filename extension .js stands for either ESM or CommonJS. Which one it is is configured via the “closest” package.json (in the current directory, the parent directory, etc.). Using package.json in this manner is independent of packages.

In that package.json, there is a property "type", which has two settings:

"commonjs" (the default): files with the extension .js or without an extension are interpreted as CommonJS modules.

"module": files with the extension .js or without an extension are interpreted as ESM modules.

27.11.3.2 Interpreting non-file source code as either CommonJS or ESM

Not all source code executed by Node.js comes from files. You can also send it code via stdin, --eval, and --print. The command line option --input-type lets you specify how such code is interpreted:

As CommonJS (the default): --input-type=commonjs

As ESM: --input-type=module

27.12 Loading modules dynamically via import() [ES2020]

So far, the only way to import a module has been via an import statement. That statement has several limitations:

You must use it at the top level of a module. That is, you can’t, for example, import something when you are inside a block.

The module specifier is always fixed. That is, you can’t change what you import depending on a condition. And you can’t assemble a specifier dynamically.

The import() operator changes that. Let’s look at an example of it being used.

27.12.1 Example: loading a module dynamically

Consider the following files:

lib/my-math.mjs
main1.mjs
main2.mjs

We have already seen module my-math.mjs:

// Not exported, private to module
function times(a, b) {
  return a * b;
}
export function square(x) {
  return times(x, x);
}
export const LIGHTSPEED = 299792458;

This is what using import() looks like in main1.mjs:

const dir = './lib/';
const moduleSpecifier = dir + 'my-math.mjs';

function loadConstant() {
  return import(moduleSpecifier)
  .then(myMath => {
    const result = myMath.LIGHTSPEED;
    assert.equal(result, 299792458);
    return result;
  });
}

Method .then() is part of Promises, a mechanism for handling asynchronous results, which is covered later in this book.

Two things in this code weren’t possible before:

We are importing inside a function (not at the top level).

The module specifier comes from a variable.

Next, we’ll implement the exact same functionality in main2.mjs but via a so-called async function, which provides nicer syntax for Promises.

const dir = './lib/';
const moduleSpecifier = dir + 'my-math.mjs';

async function loadConstant() {
  const myMath = await import(moduleSpecifier);
  const result = myMath.LIGHTSPEED;
  assert.equal(result, 299792458);
  return result;
}

  Why is import() an operator and not a function?

Even though it works much like a function, import() is an operator: in order to resolve module specifiers relatively to the current module, it needs to know from which module it is invoked. A normal function cannot receive this information as implicitly as an operator can. It would need, for example, a parameter.

27.12.2 Use cases for import()

27.12.2.1 Loading code on demand

Some functionality of web apps doesn’t have to be present when they start, it can be loaded on demand. Then import() helps because you can put such functionality into modules – for example:

button.addEventListener('click', event => {
  import('./dialogBox.mjs')
    .then(dialogBox => {
      dialogBox.open();
    })
    .catch(error => {
      /* Error handling */
    })
});

27.12.2.2 Conditional loading of modules

We may want to load a module depending on whether a condition is true. For example, a module with a polyfill that makes a new feature available on legacy platforms:

if (isLegacyPlatform()) {
  import('./my-polyfill.mjs')
    .then(···);
}

27.12.2.3 Computed module specifiers

For applications such as internationalization, it helps if you can dynamically compute module specifiers:

import(`messages_${getLocale()}.mjs`)
  .then(···);

27.13 import.meta – metadata for the current module [ES2020]

The object import.meta holds metadata for the current module.

27.13.1 import.meta.url

The most important property of import.meta is .url which contains a string with the URL of the current module file – for example:

'https://example.com/code/main.mjs'

27.13.2 import.meta.url and class URL

Class URL is available via a global variable in browsers and on Node.js. You can look up its full functionality in the Node.js documentation. When working with import.meta.url, its constructor is especially useful:

new URL(input: string, base?: string|URL)

Parameter input contains the URL to be parsed. It can be relative if the second parameter, base, is provided.

In other words, this constructor lets us resolve a relative path against a base URL:

> new URL('other.mjs', 'https://example.com/code/main.mjs').href
'https://example.com/code/other.mjs'
> new URL('../other.mjs', 'https://example.com/code/main.mjs').href
'https://example.com/other.mjs'

This is how we get a URL instance that points to a file data.txt that sits next to the current module:

const urlOfData = new URL('data.txt', import.meta.url);

27.13.3 import.meta.url on Node.js

On Node.js, import.meta.url is always a string with a file: URL – for example:

'file:///Users/rauschma/my-module.mjs'

27.13.3.1 Example: reading a sibling file of a module

Many Node.js file system operations accept either strings with paths or instances of URL. That enables us to read a sibling file data.txt of the current module:

import * as fs from 'fs';
function readData() {
  // data.txt sits next to current module
  const urlOfData = new URL('data.txt', import.meta.url);
  return fs.readFileSync(urlOfData, {encoding: 'UTF-8'});
}

27.13.3.2 Module fs and URLs

For most functions of the module fs, we can refer to files via:

Paths – in strings or instances of Buffer.

URLs – in instances of URL (with the protocol file:)

For more information on this topic, see the Node.js API documentation.

27.13.3.3 Converting between file: URLs and paths

The Node.js module url has two functions for converting between file: URLs and paths:

fileURLToPath(url: URL|string): string
Converts a file: URL to a path.

pathToFileURL(path: string): URL
Converts a path to a file: URL.

If you need a path that can be used in the local file system, then property .pathname of URL instances does not always work:

assert.equal(
  new URL('file:///tmp/with%20space.txt').pathname,
  '/tmp/with%20space.txt');

Therefore, it is better to use fileURLToPath():

import * as url from 'url';
assert.equal(
  url.fileURLToPath('file:///tmp/with%20space.txt'),
  '/tmp/with space.txt'); // result on Unix

Similarly, pathToFileURL() does more than just prepend 'file://' to an absolute path.

27.14 Polyfills: emulating native web platform features (advanced)

  Backends have polyfills, too

This section is about frontend development and web browsers, but similar ideas apply to backend development.

Polyfills help with a conflict that we are facing when developing a web application in JavaScript:

On one hand, we want to use modern web platform features that make the app better and/or development easier.

On the other hand, the app should run on as many browsers as possible.

Given a web platform feature X:

A polyfill for X is a piece of code. If it is executed on a platform that already has built-in support for X, it does nothing. Otherwise, it makes the feature available on the platform. In the latter case, the polyfilled feature is (mostly) indistinguishable from a native implementation. In order to achieve that, the polyfill usually makes global changes. For example, it may modify global data or configure a global module loader. Polyfills are often packaged as modules.

The term polyfill was coined by Remy Sharp.

A speculative polyfill is a polyfill for a proposed web platform feature (that is not standardized, yet).

Alternative term: prollyfill

A replica of X is a library that reproduces the API and functionality of X locally. Such a library exists independently of a native (and global) implementation of X.

Replica is a new term introduced in this section. Alternative term: ponyfill

There is also the term shim, but it doesn’t have a universally agreed upon definition. It often means roughly the same as polyfill.

Every time our web applications starts, it must first execute all polyfills for features that may not be available everywhere. Afterwards, we can be sure that those features are available natively.

27.14.1 Sources of this section

“What is a Polyfill?” by Remy Sharp

Inspiration for the term replica: The Eiffel Tower in Las Vegas

Useful clarification of “polyfill” and related terms: “Polyfills and the evolution of the Web”. Edited by Andrew Betts.

  Quiz

See quiz app.

28 Single objects

28.1 What is an object?

28.1.1 Roles of objects: record vs. dictionary

28.2 Objects as records

28.2.1 Object literals: properties

28.2.2 Object literals: property value shorthands

28.2.3 Getting properties

28.2.4 Setting properties

28.2.5 Object literals: methods

28.2.6 Object literals: accessors

28.3 Spreading into object literals (...) [ES2018]

28.3.1 Use case for spreading: copying objects

28.3.2 Use case for spreading: default values for missing properties

28.3.3 Use case for spreading: non-destructively changing properties

28.4 Methods and the special variable this

28.4.1 Methods are properties whose values are functions

28.4.2 The special variable this

28.4.3 Methods and .call()

28.4.4 Methods and .bind()

28.4.5 this pitfall: extracting methods

28.4.6 this pitfall: accidentally shadowing this

28.4.7 The value of this in various contexts (advanced)

28.5 Optional chaining for property accesses and method calls [ES2020] (advanced)

28.5.1 Example: optional static property access

28.5.2 The operators in more detail (advanced)

28.5.3 Short-circuiting (advanced)

28.5.4 Frequently asked questions

28.6 Objects as dictionaries (advanced)

28.6.1 Arbitrary fixed strings as property keys

28.6.2 Computed property keys

28.6.3 The in operator: is there a property with a given key?

28.6.4 Deleting properties

28.6.5 Listing property keys

28.6.6 Listing property values via Object.values()

28.6.7 Listing property entries via Object.entries() [ES2017]

28.6.8 Properties are listed deterministically

28.6.9 Assembling objects via Object.fromEntries() [ES2019]

28.6.10 The pitfalls of using an object as a dictionary

28.7 Standard methods (advanced)

28.7.1 .toString()

28.7.2 .valueOf()

28.8 Advanced topics

28.8.1 Object.assign() [ES6]

28.8.2 Freezing objects [ES5]

28.8.3 Property attributes and property descriptors [ES5]

In this book, JavaScript’s style of object-oriented programming (OOP) is introduced in four steps. This chapter covers step 1; the next chapter covers steps 2–4. The steps are (fig. 8):

Single objects (this chapter): How do objects, JavaScript’s basic OOP building blocks, work in isolation?

Prototype chains (next chapter): Each object has a chain of zero or more prototype objects. Prototypes are JavaScript’s core inheritance mechanism.

Classes (next chapter): JavaScript’s classes are factories for objects. The relationship between a class and its instances is based on prototypal inheritance.

Subclassing (next chapter): The relationship between a subclass and its superclass is also based on prototypal inheritance.

Figure 8: This book introduces object-oriented programming in JavaScript in four steps.

28.1 What is an object?

In JavaScript:

An object is a set of properties (key-value entries).

A property key can only be a string or a symbol.

28.1.1 Roles of objects: record vs. dictionary

Objects play two roles in JavaScript:

Records: Objects-as-records have a fixed number of properties, whose keys are known at development time. Their values can have different types.

Dictionaries: Objects-as-dictionaries have a variable number of properties, whose keys are not known at development time. All of their values have the same type.

These roles influence how objects are explained in this chapter:

First, we’ll explore objects-as-records. Even though property keys are strings or symbols under the hood, they will appear as fixed identifiers to us, in this part of the chapter.

Later, we’ll explore objects-as-dictionaries. Note that Maps are usually better dictionaries than objects. However, some of the operations that we’ll encounter, can also be useful for objects-as-records.

28.2 Objects as records

Let’s first explore the role record of objects.

28.2.1 Object literals: properties

Object literals are one way of creating objects-as-records. They are a stand-out feature of JavaScript: we can directly create objects – no need for classes! This is an example:

const jane = {
  first: 'Jane',
  last: 'Doe', // optional trailing comma
};

In the example, we created an object via an object literal, which starts and ends with curly braces {}. Inside it, we defined two properties (key-value entries):

The first property has the key first and the value 'Jane'.

The second property has the key last and the value 'Doe'.

Since ES5, trailing commas are allowed in object literals.

We will later see other ways of specifying property keys, but with this way of specifying them, they must follow the rules of JavaScript variable names. For example, we can use first_name as a property key, but not first-name). However, reserved words are allowed:

const obj = {
  if: true,
  const: true,
};

In order to check the effects of various operations on objects, we’ll occasionally use Object.keys() in this part of the chapter. It lists property keys:

> Object.keys({a:1, b:2})
[ 'a', 'b' ]

28.2.2 Object literals: property value shorthands

Whenever the value of a property is defined via a variable name and that name is the same as the key, we can omit the key.

function createPoint(x, y) {
  return {x, y};
}
assert.deepEqual(
  createPoint(9, 2),
  { x: 9, y: 2 }
);

28.2.3 Getting properties

This is how we get (read) a property (line A):

const jane = {
  first: 'Jane',
  last: 'Doe',
};

// Get property .first
assert.equal(jane.first, 'Jane'); // (A)

Getting an unknown property produces undefined:

assert.equal(jane.unknownProperty, undefined);

28.2.4 Setting properties

This is how we set (write to) a property:

const obj = {
  prop: 1,
};
assert.equal(obj.prop, 1);
obj.prop = 2; // (A)
assert.equal(obj.prop, 2);

We just changed an existing property via setting. If we set an unknown property, we create a new entry:

const obj = {}; // empty object
assert.deepEqual(
  Object.keys(obj), []);

obj.unknownProperty = 'abc';
assert.deepEqual(
  Object.keys(obj), ['unknownProperty']);

28.2.5 Object literals: methods

The following code shows how to create the method .says() via an object literal:

const jane = {
  first: 'Jane', // data property
  says(text) {   // method
    return `${this.first} says “${text}”`; // (A)
  }, // comma as separator (optional at end)
};
assert.equal(jane.says('hello'), 'Jane says “hello”');

During the method call jane.says('hello'), jane is called the receiver of the method call and assigned to the special variable this (more on this in §28.4 “Methods and the special variable this”). That enables method .says() to access the sibling property .first in line A.

28.2.6 Object literals: accessors

There are two kinds of accessors in JavaScript:

A getter is a method-like entity that is invoked by getting a property.

A setter is a method-like entity that is invoked by setting a property.

28.2.6.1 Getters

A getter is created by prefixing a method definition with the modifier get:

const jane = {
  first: 'Jane',
  last: 'Doe',
  get full() {
    return `${this.first} ${this.last}`;
  },
};

assert.equal(jane.full, 'Jane Doe');
jane.first = 'John';
assert.equal(jane.full, 'John Doe');

28.2.6.2 Setters

A setter is created by prefixing a method definition with the modifier set:

const jane = {
  first: 'Jane',
  last: 'Doe',
  set full(fullName) {
    const parts = fullName.split(' ');
    this.first = parts[0];
    this.last = parts[1];
  },
};

jane.full = 'Richard Roe';
assert.equal(jane.first, 'Richard');
assert.equal(jane.last, 'Roe');

  Exercise: Creating an object via an object literal

exercises/single-objects/color_point_object_test.mjs

28.3 Spreading into object literals (...) [ES2018]

Inside a function call, spreading (...) turns the iterated values of an iterable object into arguments.

Inside an object literal, a spread property adds the properties of another object to the current one:

> const obj = {foo: 1, bar: 2};
> {...obj, baz: 3}
{ foo: 1, bar: 2, baz: 3 }

If property keys clash, the property that is mentioned last “wins”:

> const obj = {foo: 1, bar: 2, baz: 3};
> {...obj, foo: true}
{ foo: true, bar: 2, baz: 3 }
> {foo: true, ...obj}
{ foo: 1, bar: 2, baz: 3 }

All values are spreadable, even undefined and null:

> {...undefined}
{}
> {...null}
{}
> {...123}
{}
> {...'abc'}
{ '0': 'a', '1': 'b', '2': 'c' }
> {...['a', 'b']}
{ '0': 'a', '1': 'b' }

Property .length of strings and of Arrays is hidden from this kind of operation (it is not enumerable; see §28.8.3 “Property attributes and property descriptors [ES5]” for more information).

28.3.1 Use case for spreading: copying objects

We can use spreading to create a copy of an object original:

const copy = {...original};

Caveat – copying is shallow: copy is a fresh object with duplicates of all properties (key-value entries) of original. But if property values are objects, then those are not copied themselves; they are shared between original and copy. Let’s look at an example:

const original = { a: 1, b: {foo: true} };
const copy = {...original};

The first level of copy is really a copy: If we change any properties at that level, it does not affect the original:

copy.a = 2;
assert.deepEqual(
  original, { a: 1, b: {foo: true} }); // no change

However, deeper levels are not copied. For example, the value of .b is shared between original and copy. Changing .b in the copy also changes it in the original.

copy.b.foo = false;
assert.deepEqual(
  original, { a: 1, b: {foo: false} });

  JavaScript doesn’t have built-in support for deep copying

Deep copies of objects (where all levels are copied) are notoriously difficult to do generically. Therefore, JavaScript does not have a built-in operation for them (for now). If we need such an operation, we have to implement it ourselves.

28.3.2 Use case for spreading: default values for missing properties

If one of the inputs of our code is an object with data, we can make properties optional by specifying default values that are used if those properties are missing. One technique for doing so is via an object whose properties contain the default values. In the following example, that object is DEFAULTS:

const DEFAULTS = {foo: 'a', bar: 'b'};
const providedData = {foo: 1};

const allData = {...DEFAULTS, ...providedData};
assert.deepEqual(allData, {foo: 1, bar: 'b'});

The result, the object allData, is created by copying DEFAULTS and overriding its properties with those of providedData.

But we don’t need an object to specify the default values; we can also specify them inside the object literal, individually:

const providedData = {foo: 1};

const allData = {foo: 'a', bar: 'b', ...providedData};
assert.deepEqual(allData, {foo: 1, bar: 'b'});

28.3.3 Use case for spreading: non-destructively changing properties

So far, we have encountered one way of changing a property .foo of an object: We set it (line A) and mutate the object. That is, this way of changing a property is destructive.

const obj = {foo: 'a', bar: 'b'};
obj.foo = 1; // (A)
assert.deepEqual(obj, {foo: 1, bar: 'b'});

With spreading, we can change .foo non-destructively – we make a copy of obj where .foo has a different value:

const obj = {foo: 'a', bar: 'b'};
const updatedObj = {...obj, foo: 1};
assert.deepEqual(updatedObj, {foo: 1, bar: 'b'});

  Exercise: Non-destructively updating a property via spreading (fixed key)

exercises/single-objects/update_name_test.mjs

28.4 Methods and the special variable this

28.4.1 Methods are properties whose values are functions

Let’s revisit the example that was used to introduce methods:

const jane = {
  first: 'Jane',
  says(text) {
    return `${this.first} says “${text}”`;
  },
};

Somewhat surprisingly, methods are functions:

assert.equal(typeof jane.says, 'function');

Why is that? We learned in the chapter on callable values, that ordinary functions play several roles. Method is one of those roles. Therefore, under the hood, jane roughly looks as follows.

const jane = {
  first: 'Jane',
  says: function (text) {
    return `${this.first} says “${text}”`;
  },
};

28.4.2 The special variable this

Consider the following code:

const obj = {
  someMethod(x, y) {
    assert.equal(this, obj); // (A)
    assert.equal(x, 'a');
    assert.equal(y, 'b');
  }
};
obj.someMethod('a', 'b'); // (B)

In line B, obj is the receiver of a method call. It is passed to the function stored in obj.someMethod via an implicit (hidden) parameter whose name is this (line A).

This is an important point: The best way to understand this is as an implicit parameter of ordinary functions (and therefore methods, too).

28.4.3 Methods and .call()

Methods are functions and in §25.7 “Methods of functions: .call(), .apply(), .bind()”, we saw that functions have methods themselves. One of those methods is .call(). Let’s look at an example to understand how this method works.

In the previous section, there was this method invocation:

obj.someMethod('a', 'b')

This invocation is equivalent to:

obj.someMethod.call(obj, 'a', 'b');

Which is also equivalent to:

const func = obj.someMethod;
func.call(obj, 'a', 'b');

.call() makes the normally implicit parameter this explicit: When invoking a function via .call(), the first parameter is this, followed by the regular (explicit) function parameters.

As an aside, this means that there are actually two different dot operators:

One for accessing properties: obj.prop

Another one for calling methods: obj.prop()

They are different in that (2) is not just (1) followed by the function call operator (). Instead, (2) additionally provides a value for this.

28.4.4 Methods and .bind()

.bind() is another method of function objects. In the following code, we use .bind() to turn method .says() into the stand-alone function func():

const jane = {
  first: 'Jane',
  says(text) {
    return `${this.first} says “${text}”`; // (A)
  },
};

const func = jane.says.bind(jane, 'hello');
assert.equal(func(), 'Jane says “hello”');

Setting this to jane via .bind() is crucial here. Otherwise, func() wouldn’t work properly because this is used in line A. In the next section, we’ll explore why that is.

28.4.5 this pitfall: extracting methods

We now know quite a bit about functions and methods and are ready to take a look at the biggest pitfall involving methods and this: function-calling a method extracted from an object can fail if we are not careful.

In the following example, we fail when we extract method jane.says(), store it in the variable func, and function-call func().

const jane = {
  first: 'Jane',
  says(text) {
    return `${this.first} says “${text}”`;
  },
};
const func = jane.says; // extract the method
assert.throws(
  () => func('hello'), // (A)
  {
    name: 'TypeError',
    message: "Cannot read property 'first' of undefined",
  });

In line A, we are making a normal function call. And in normal function calls, this is undefined (if strict mode is active, which it almost always is). Line A is therefore equivalent to:

assert.throws(
  () => jane.says.call(undefined, 'hello'), // `this` is undefined!
  {
    name: 'TypeError',
    message: "Cannot read property 'first' of undefined",
  });

How do we fix this? We need to use .bind() to extract method .says():

const func2 = jane.says.bind(jane);
assert.equal(func2('hello'), 'Jane says “hello”');

The .bind() ensures that this is always jane when we call func().

We can also use arrow functions to extract methods:

const func3 = text => jane.says(text);
assert.equal(func3('hello'), 'Jane says “hello”');

28.4.5.1 Example: extracting a method

The following is a simplified version of code that we may see in actual web development:

class ClickHandler {
  constructor(id, elem) {
    this.id = id;
    elem.addEventListener('click', this.handleClick); // (A)
  }
  handleClick(event) {
    alert('Clicked ' + this.id);
  }
}

In line A, we don’t extract the method .handleClick() properly. Instead, we should do:

elem.addEventListener('click', this.handleClick.bind(this));

28.4.5.2 How to avoid the pitfall of extracting methods

Alas, there is no simple way around the pitfall of extracting methods: Whenever we extract a method, we have to be careful and do it properly – for example, by binding this or by using an arrow function.

  Exercise: Extracting a method

exercises/single-objects/method_extraction_exrc.mjs

28.4.6 this pitfall: accidentally shadowing this

  Accidentally shadowing this is only an issue with ordinary functions

Arrow functions don’t shadow this.

Consider the following problem: when we are inside an ordinary function, we can’t access the this of the surrounding scope because the ordinary function has its own this. In other words, a variable in an inner scope hides a variable in an outer scope. That is called shadowing. The following code is an example:

const prefixer = {
  prefix: '==> ',
  prefixStringArray(stringArray) {
    return stringArray.map(
      function (x) {
        return this.prefix + x; // (A)
      });
  },
};
assert.throws(
  () => prefixer.prefixStringArray(['a', 'b']),
  /^TypeError: Cannot read property 'prefix' of undefined$/);

In line A, we want to access the this of .prefixStringArray(). But we can’t since the surrounding ordinary function has its own this that shadows (blocks access to) the this of the method. The value of the former this is undefined due to the callback being function-called. That explains the error message.

The simplest way to fix this problem is via an arrow function, which doesn’t have its own this and therefore doesn’t shadow anything:

const prefixer = {
  prefix: '==> ',
  prefixStringArray(stringArray) {
    return stringArray.map(
      (x) => {
        return this.prefix + x;
      });
  },
};
assert.deepEqual(
  prefixer.prefixStringArray(['a', 'b']),
  ['==> a', '==> b']);

We can also store this in a different variable (line A), so that it doesn’t get shadowed:

prefixStringArray(stringArray) {
  const that = this; // (A)
  return stringArray.map(
    function (x) {
      return that.prefix + x;
    });
},

Another option is to specify a fixed this for the callback via .bind() (line A):

prefixStringArray(stringArray) {
  return stringArray.map(
    function (x) {
      return this.prefix + x;
    }.bind(this)); // (A)
},

Lastly, .map() lets us specify a value for this (line A) that it uses when invoking the callback:

prefixStringArray(stringArray) {
  return stringArray.map(
    function (x) {
      return this.prefix + x;
    },
    this); // (A)
},

28.4.6.1 Avoiding the pitfall of accidentally shadowing this

If you follow the advice in §25.3.4 “Recommendation: prefer specialized functions over ordinary functions”, you can avoid the pitfall of accidentally shadowing this. This is a summary:

Use arrow functions as anonymous inline functions. They don’t have this as an implicit parameter and don’t shadow it.

For named stand-alone function declarations you can either use arrow functions or function declarations. If you do the latter, make sure this isn’t mentioned in their bodies.

28.4.7 The value of this in various contexts (advanced)

What is the value of this in various contexts?

Inside a callable entity, the value of this depends on how the callable entity is invoked and what kind of callable entity it is:

Function call:

Ordinary functions: this === undefined (in strict mode)

Arrow functions: this is same as in surrounding scope (lexical this)

Method call: this is receiver of call

new: this refers to newly created instance

We can also access this in all common top-level scopes:

<script> element: this === globalThis

ECMAScript modules: this === undefined

CommonJS modules: this === module.exports

  Tip: pretend that this doesn’t exist in top-level scopes

I like to do that because top-level this is confusing and rarely useful.

28.5 Optional chaining for property accesses and method calls [ES2020] (advanced)

The following kinds of optional chaining operations exist:

obj?.prop     // optional static property access
obj?.[«expr»] // optional dynamic property access
func?.(«arg0», «arg1») // optional function or method call

The rough idea is:

If the value before the question mark is neither undefined nor null, then perform the operation after the question mark.

Otherwise, return undefined.

28.5.1 Example: optional static property access

Consider the following data:

const persons = [
  {
    surname: 'Zoe',
    address: {
      street: {
        name: 'Sesame Street',
        number: '123',
      },
    },
  },
  {
    surname: 'Mariner',
  },
  {
    surname: 'Carmen',
    address: {
    },
  },
];

We can use optional chaining to safely extract street names:

const streetNames = persons.map(
  p => p.address?.street?.name);
assert.deepEqual(
  streetNames, ['Sesame Street', undefined, undefined]
);

28.5.1.1 Handling defaults via nullish coalescing

The nullish coalescing operator allows us to use the default value '(no street)' instead of undefined:

const streetNames = persons.map(
  p => p.address?.street?.name ?? '(no name)');
assert.deepEqual(
  streetNames, ['Sesame Street', '(no name)', '(no name)']
);

28.5.2 The operators in more detail (advanced)

28.5.2.1 Optional static property access

The following two expressions are equivalent:

o?.prop
(o !== undefined && o !== null) ? o.prop : undefined

Examples:

assert.equal(undefined?.prop, undefined);
assert.equal(null?.prop,      undefined);
assert.equal({prop:1}?.prop,  1);

28.5.2.2 Optional dynamic property access

The following two expressions are equivalent:

o?.[«expr»]
(o !== undefined && o !== null) ? o[«expr»] : undefined

Examples:

const key = 'prop';
assert.equal(undefined?.[key], undefined);
assert.equal(null?.[key], undefined);
assert.equal({prop:1}?.[key], 1);

28.5.2.3 Optional function or method call

The following two expressions are equivalent:

f?.(arg0, arg1)
(f !== undefined && f !== null) ? f(arg0, arg1) : undefined

Examples:

assert.equal(undefined?.(123), undefined);
assert.equal(null?.(123), undefined);
assert.equal(String?.(123), '123');

Note that this operator produces an error if its left-hand side is not callable:

assert.throws(
  () => true?.(123),
  TypeError);

Why? The idea is that the operator only tolerates deliberate omissions. An uncallable value (other than undefined and null) is probably an error and should be reported, rather than worked around.

28.5.3 Short-circuiting (advanced)

In a chain of property accesses and function/method invocations, evaluation stops once the first optional operator encounters undefined or null at its left-hand side:

function isInvoked(obj) {
  let invoked = false;
  obj?.a.b.m(invoked = true);
  return invoked;
}

assert.equal(
  isInvoked({a: {b: {m() {}}}}), true);

// The left-hand side of ?. is undefined
// and the assignment is not executed
assert.equal(
  isInvoked(undefined), false);

This behavior differs from a normal operator/function where JavaScript always evaluates all operands/arguments before evaluating the operator/function. It is called short-circuiting. Other short-circuiting operators:

a && b

a || b

c ? t : e

28.5.4 Frequently asked questions

28.5.4.1 Why are there dots in o?.[x] and f?.()?

The syntaxes of the following two optional operator are not ideal:

obj?.[«expr»]          // better: obj?[«expr»]
func?.(«arg0», «arg1») // better: func?(«arg0», «arg1»)

Alas, the less elegant syntax is necessary, because distinguishing the ideal syntax (first expression) from the conditional operator (second expression) is too complicated:

obj?['a', 'b', 'c'].map(x => x+x)
obj ? ['a', 'b', 'c'].map(x => x+x) : []

28.5.4.2 Why does null?.prop evaluate to undefined and not null?

The operator ?. is mainly about its right-hand side: Does property .prop exist? If not, stop early. Therefore, keeping information about its left-hand side is rarely useful. However, only having a single “early termination” value does simplify things.

28.6 Objects as dictionaries (advanced)

Objects work best as records. But before ES6, JavaScript did not have a data structure for dictionaries (ES6 brought Maps). Therefore, objects had to be used as dictionaries, which imposed a signficant constraint: keys had to be strings (symbols were also introduced with ES6).

We first look at features of objects that are related to dictionaries but also useful for objects-as-records. This section concludes with tips for actually using objects as dictionaries (spoiler: use Maps if you can).

28.6.1 Arbitrary fixed strings as property keys

So far, we have always used objects as records. Property keys were fixed tokens that had to be valid identifiers and internally became strings:

const obj = {
  mustBeAnIdentifier: 123,
};

// Get property
assert.equal(obj.mustBeAnIdentifier, 123);

// Set property
obj.mustBeAnIdentifier = 'abc';
assert.equal(obj.mustBeAnIdentifier, 'abc');

As a next step, we’ll go beyond this limitation for property keys: In this section, we’ll use arbitrary fixed strings as keys. In the next subsection, we’ll dynamically compute keys.

Two techniques allow us to use arbitrary strings as property keys.

First, when creating property keys via object literals, we can quote property keys (with single or double quotes):

const obj = {
  'Can be any string!': 123,
};

Second, when getting or setting properties, we can use square brackets with strings inside them:

// Get property
assert.equal(obj['Can be any string!'], 123);

// Set property
obj['Can be any string!'] = 'abc';
assert.equal(obj['Can be any string!'], 'abc');

We can also use these techniques for methods:

const obj = {
  'A nice method'() {
    return 'Yes!';
  },
};

assert.equal(obj['A nice method'](), 'Yes!');

28.6.2 Computed property keys

So far, property keys were always fixed strings inside object literals. In this section we learn how to dynamically compute property keys. That enables us to use either arbitrary strings or symbols.

The syntax of dynamically computed property keys in object literals is inspired by dynamically accessing properties. That is, we can use square brackets to wrap expressions:

const obj = {
  ['Hello world!']: true,
  ['f'+'o'+'o']: 123,
  [Symbol.toStringTag]: 'Goodbye', // (A)
};

assert.equal(obj['Hello world!'], true);
assert.equal(obj.foo, 123);
assert.equal(obj[Symbol.toStringTag], 'Goodbye');

The main use case for computed keys is having symbols as property keys (line A).

Note that the square brackets operator for getting and setting properties works with arbitrary expressions:

assert.equal(obj['f'+'o'+'o'], 123);
assert.equal(obj['==> foo'.slice(-3)], 123);

Methods can have computed property keys, too:

const methodKey = Symbol();
const obj = {
  [methodKey]() {
    return 'Yes!';
  },
};

assert.equal(obj[methodKey](), 'Yes!');

For the remainder of this chapter, we’ll mostly use fixed property keys again (because they are syntactically more convenient). But all features are also available for arbitrary strings and symbols.

  Exercise: Non-destructively updating a property via spreading (computed key)

exercises/single-objects/update_property_test.mjs

28.6.3 The in operator: is there a property with a given key?

The in operator checks if an object has a property with a given key:

const obj = {
  foo: 'abc',
  bar: false,
};

assert.equal('foo' in obj, true);
assert.equal('unknownKey' in obj, false);

28.6.3.1 Checking if a property exists via truthiness

We can also use a truthiness check to determine if a property exists:

assert.equal(
  obj.foo ? 'exists' : 'does not exist',
  'exists');
assert.equal(
  obj.unknownKey ? 'exists' : 'does not exist',
  'does not exist');

The previous checks work because obj.foo is truthy and because reading a missing property returns undefined (which is falsy).

There is, however, one important caveat: truthiness checks fail if the property exists, but has a falsy value (undefined, null, false, 0, "", etc.):

assert.equal(
  obj.bar ? 'exists' : 'does not exist',
  'does not exist'); // should be: 'exists'

28.6.4 Deleting properties

We can delete properties via the delete operator:

const obj = {
  foo: 123,
};
assert.deepEqual(Object.keys(obj), ['foo']);

delete obj.foo;

assert.deepEqual(Object.keys(obj), []);

28.6.5 Listing property keys

Table 19: Standard library methods for listing own (non-inherited) property keys. All of them return Arrays with strings and/or symbols.

| enumerable | non-e. | string | symbol
Object.keys() | ✔ |  | ✔ |
Object.getOwnPropertyNames() | ✔ | ✔ | ✔ |
Object.getOwnPropertySymbols() | ✔ | ✔ |  | ✔
Reflect.ownKeys() | ✔ | ✔ | ✔ | ✔

Each of the methods in tbl. 19 returns an Array with the own property keys of the parameter. In the names of the methods, we can see that the following distinction is made:

A property key can be either a string or a symbol.

A property name is a property key whose value is a string.

A property symbol is a property key whose value is a symbol.

The next section describes the term enumerable and demonstrates each of the methods.

28.6.5.1 Enumerability

Enumerability is an attribute of a property. Non-enumerable properties are ignored by some operations – for example, by Object.keys() (see tbl. 19) and by spread properties. By default, most properties are enumerable. The next example shows how to change that. It also demonstrates the various ways of listing property keys.

const enumerableSymbolKey = Symbol('enumerableSymbolKey');
const nonEnumSymbolKey = Symbol('nonEnumSymbolKey');

// We create enumerable properties via an object literal
const obj = {
  enumerableStringKey: 1,
  [enumerableSymbolKey]: 2,
}

// For non-enumerable properties, we need a more powerful tool
Object.defineProperties(obj, {
  nonEnumStringKey: {
    value: 3,
    enumerable: false,
  },
  [nonEnumSymbolKey]: {
    value: 4,
    enumerable: false,
  },
});

assert.deepEqual(
  Object.keys(obj),
  [ 'enumerableStringKey' ]);
assert.deepEqual(
  Object.getOwnPropertyNames(obj),
  [ 'enumerableStringKey', 'nonEnumStringKey' ]);
assert.deepEqual(
  Object.getOwnPropertySymbols(obj),
  [ enumerableSymbolKey, nonEnumSymbolKey ]);
assert.deepEqual(
  Reflect.ownKeys(obj),
  [
    'enumerableStringKey', 'nonEnumStringKey',
    enumerableSymbolKey, nonEnumSymbolKey,
  ]);

Object.defineProperties() is explained later in this chapter.

28.6.6 Listing property values via Object.values()

Object.values() lists the values of all enumerable properties of an object:

const obj = {foo: 1, bar: 2};
assert.deepEqual(
  Object.values(obj),
  [1, 2]);

28.6.7 Listing property entries via Object.entries() [ES2017]

Object.entries() lists key-value pairs of enumerable properties. Each pair is encoded as a two-element Array:

const obj = {foo: 1, bar: 2};
assert.deepEqual(
  Object.entries(obj),
  [
    ['foo', 1],
    ['bar', 2],
  ]);

  Exercise: Object.entries()

exercises/single-objects/find_key_test.mjs

28.6.8 Properties are listed deterministically

Own (non-inherited) properties of objects are always listed in the following order:

Properties with string keys that contain integer indices (that includes Array indices):
In ascending numeric order

Remaining properties with string keys:
In the order in which they were added

Properties with symbol keys:
In the order in which they were added

The following example demonstrates how property keys are sorted according to these rules:

> Object.keys({b:0,a:0, 10:0,2:0})
[ '2', '10', 'b', 'a' ]

  The order of properties

The ECMAScript specification describes in more detail how properties are ordered.

28.6.9 Assembling objects via Object.fromEntries() [ES2019]

Given an iterable over [key, value] pairs, Object.fromEntries() creates an object:

assert.deepEqual(
  Object.fromEntries([['foo',1], ['bar',2]]),
  {
    foo: 1,
    bar: 2,
  }
);

Object.fromEntries() does the opposite of Object.entries().

To demonstrate both, we’ll use them to implement two tool functions from the library Underscore in the next subsubsections.

28.6.9.1 Example: pick(object, ...keys)

pick returns a copy of object that only has those properties whose keys are mentioned as arguments:

const address = {
  street: 'Evergreen Terrace',
  number: '742',
  city: 'Springfield',
  state: 'NT',
  zip: '49007',
};
assert.deepEqual(
  pick(address, 'street', 'number'),
  {
    street: 'Evergreen Terrace',
    number: '742',
  }
);

We can implement pick() as follows:

function pick(object, ...keys) {
  const filteredEntries = Object.entries(object)
    .filter(([key, _value]) => keys.includes(key));
  return Object.fromEntries(filteredEntries);
}

28.6.9.2 Example: invert(object)

invert returns a copy of object where the keys and values of all properties are swapped:

assert.deepEqual(
  invert({a: 1, b: 2, c: 3}),
  {1: 'a', 2: 'b', 3: 'c'}
);

We can implement invert() like this:

function invert(object) {
  const reversedEntries = Object.entries(object)
    .map(([key, value]) => [value, key]);
  return Object.fromEntries(reversedEntries);
}

28.6.9.3 A simple implementation of Object.fromEntries()

The following function is a simplified version of Object.fromEntries():

function fromEntries(iterable) {
  const result = {};
  for (const [key, value] of iterable) {
    let coercedKey;
    if (typeof key === 'string' || typeof key === 'symbol') {
      coercedKey = key;
    } else {
      coercedKey = String(key);
    }
    result[coercedKey] = value;
  }
  return result;
}

  Exercise: Object.entries() and Object.fromEntries()

exercises/single-objects/omit_properties_test.mjs

28.6.10 The pitfalls of using an object as a dictionary

If we use plain objects (created via object literals) as dictionaries, we have to look out for two pitfalls.

The first pitfall is that the in operator also finds inherited properties:

const dict = {};
assert.equal('toString' in dict, true);

We want dict to be treated as empty, but the in operator detects the properties it inherits from its prototype, Object.prototype.

The second pitfall is that we can’t use the property key __proto__ because it has special powers (it sets the prototype of the object):

const dict = {};

dict['__proto__'] = 123;
// No property was added to dict:
assert.deepEqual(Object.keys(dict), []);

28.6.10.1 Safely using objects as dictionaries

So how do you avoid the two pitfalls?

Whenever you can, use Maps. They are the best solution for dictionaries.

If you can’t, use a library for objects-as-dictionaries that does everything safely.

If you can’t, use an object without a prototype.

The following code demonstrates using objects without prototypes as dictionaries:

const dict = Object.create(null); // no prototype

assert.equal('toString' in dict, false); // (A)

dict['__proto__'] = 123;
assert.deepEqual(Object.keys(dict), ['__proto__']);

We avoided both pitfalls:

First, a property without a prototype does not inherit any properties (line A).

Second, in modern JavaScript, __proto__ is implemented via Object.prototype. That means that it is switched off if Object.prototype is not in the prototype chain.

  Exercise: Using an object as a dictionary

exercises/single-objects/simple_dict_test.mjs

28.7 Standard methods (advanced)

Object.prototype defines several standard methods that can be overridden to configure how an object is treated by the language. Two important ones are:

.toString()

.valueOf()

28.7.1 .toString()

.toString() determines how objects are converted to strings:

> String({toString() { return 'Hello!' }})
'Hello!'
> String({})
'[object Object]'

28.7.2 .valueOf()

.valueOf() determines how objects are converted to numbers:

> Number({valueOf() { return 123 }})
123
> Number({})
NaN

28.8 Advanced topics

The following subsections give brief overviews of a few advanced topics.

28.8.1 Object.assign() [ES6]

Object.assign() is a tool method:

Object.assign(target, source_1, source_2, ···)

This expression assigns all properties of source_1 to target, then all properties of source_2, etc. At the end, it returns target – for example:

const target = { foo: 1 };

const result = Object.assign(
  target,
  {bar: 2},
  {baz: 3, bar: 4});

assert.deepEqual(
  result, { foo: 1, bar: 4, baz: 3 });
// target was modified and returned:
assert.equal(result, target);

The use cases for Object.assign() are similar to those for spread properties. In a way, it spreads destructively.

28.8.2 Freezing objects [ES5]

Object.freeze(obj) makes obj completely immutable: We can’t change properties, add properties, or change its prototype – for example:

const frozen = Object.freeze({ x: 2, y: 5 });
assert.throws(
  () => { frozen.x = 7 },
  {
    name: 'TypeError',
    message: /^Cannot assign to read only property 'x'/,
  });

There is one caveat: Object.freeze(obj) freezes shallowly. That is, only the properties of obj are frozen but not objects stored in properties.

  More information

For more information on freezing and other ways of locking down objects, see Deep JavaScript.

28.8.3 Property attributes and property descriptors [ES5]

Just as objects are composed of properties, properties are composed of attributes. The value of a property is only one of several attributes. Others include:

writable: Is it possible to change the value of the property?

enumerable: Is the property considered by Object.keys(), spreading, etc.?

When we are using one of the operations for handling property attributes, attributes are specified via property descriptors: objects where each property represents one attribute. For example, this is how we read the attributes of a property obj.foo:

const obj = { foo: 123 };
assert.deepEqual(
  Object.getOwnPropertyDescriptor(obj, 'foo'),
  {
    value: 123,
    writable: true,
    enumerable: true,
    configurable: true,
  });

And this is how we set the attributes of a property obj.bar:

const obj = {
  foo: 1,
  bar: 2,
};

assert.deepEqual(Object.keys(obj), ['foo', 'bar']);

// Hide property `bar` from Object.keys()
Object.defineProperty(obj, 'bar', {
  enumerable: false,
});

assert.deepEqual(Object.keys(obj), ['foo']);

Further reading:

Enumerability is covered in greater detail earlier in this chapter.

For more information on property attributes and property descriptors, see Deep JavaScript.

  Quiz

See quiz app.

29 Prototype chains and classes

29.1 Prototype chains

29.1.1 JavaScript’s operations: all properties vs. own properties

29.1.2 Pitfall: only the first member of a prototype chain is mutated

29.1.3 Tips for working with prototypes (advanced)

29.1.4 Sharing data via prototypes

29.2 Classes

29.2.1 A class for persons

29.2.2 Classes under the hood

29.2.3 Class definitions: prototype properties

29.2.4 Class definitions: static properties

29.2.5 The instanceof operator

29.2.6 Why I recommend classes

29.3 Private data for classes

29.3.1 Private data: naming convention

29.3.2 Private data: WeakMaps

29.3.3 More techniques for private data

29.4 Subclassing

29.4.1 Subclasses under the hood (advanced)

29.4.2 instanceof in more detail (advanced)

29.4.3 Prototype chains of built-in objects (advanced)

29.4.4 Dispatched vs. direct method calls (advanced)

29.4.5 Mixin classes (advanced)

29.5 FAQ: objects

29.5.1 Why do objects preserve the insertion order of properties?

In this book, JavaScript’s style of object-oriented programming (OOP) is introduced in four steps. This chapter covers steps 2–4, the previous chapter covers step 1. The steps are (fig. 9):

Single objects (previous chapter): How do objects, JavaScript’s basic OOP building blocks, work in isolation?

Prototype chains (this chapter): Each object has a chain of zero or more prototype objects. Prototypes are JavaScript’s core inheritance mechanism.

Classes (this chapter): JavaScript’s classes are factories for objects. The relationship between a class and its instances is based on prototypal inheritance.

Subclassing (this chapter): The relationship between a subclass and its superclass is also based on prototypal inheritance.

Figure 9: This book introduces object-oriented programming in JavaScript in four steps.

29.1 Prototype chains

Prototypes are JavaScript’s only inheritance mechanism: each object has a prototype that is either null or an object. In the latter case, the object inherits all of the prototype’s properties.

In an object literal, you can set the prototype via the special property __proto__:

const proto = {
  protoProp: 'a',
};
const obj = {
  __proto__: proto,
  objProp: 'b',
};

// obj inherits .protoProp:
assert.equal(obj.protoProp, 'a');
assert.equal('protoProp' in obj, true);

Given that a prototype object can have a prototype itself, we get a chain of objects – the so-called prototype chain. That means that inheritance gives us the impression that we are dealing with single objects, but we are actually dealing with chains of objects.

Fig. 10 shows what the prototype chain of obj looks like.

Figure 10: obj starts a chain of objects that continues with proto and other objects.

Non-inherited properties are called own properties. obj has one own property, .objProp.

29.1.1 JavaScript’s operations: all properties vs. own properties

Some operations consider all properties (own and inherited) – for example, getting properties:

> const obj = { foo: 1 };
> typeof obj.foo // own
'number'
> typeof obj.toString // inherited
'function'

Other operations only consider own properties – for example, Object.keys():

> Object.keys(obj)
[ 'foo' ]

Read on for another operation that also only considers own properties: setting properties.

29.1.2 Pitfall: only the first member of a prototype chain is mutated

One aspect of prototype chains that may be counter-intuitive is that setting any property via an object – even an inherited one – only changes that very object – never one of the prototypes.

Consider the following object obj:

const proto = {
  protoProp: 'a',
};
const obj = {
  __proto__: proto,
  objProp: 'b',
};

In the next code snippet, we set the inherited property obj.protoProp (line A). That “changes” it by creating an own property: When reading obj.protoProp, the own property is found first and its value overrides the value of the inherited property.

// In the beginning, obj has one own property
assert.deepEqual(Object.keys(obj), ['objProp']);

obj.protoProp = 'x'; // (A)

// We created a new own property:
assert.deepEqual(Object.keys(obj), ['objProp', 'protoProp']);

// The inherited property itself is unchanged:
assert.equal(proto.protoProp, 'a');

// The own property overrides the inherited property:
assert.equal(obj.protoProp, 'x');

The prototype chain of obj is depicted in fig. 11.

Figure 11: The own property .protoProp of obj overrides the property inherited from proto.

29.1.3 Tips for working with prototypes (advanced)

29.1.3.1 Best practice: avoid __proto__, except in object literals

I recommend to avoid the pseudo-property __proto__: As we will see later, not all objects have it.

However, __proto__ in object literals is different. There, it is a built-in feature and always available.

The recommended ways of getting and setting prototypes are:

The best way to get a prototype is via the following method:

Object.getPrototypeOf(obj: Object) : Object

The best way to set a prototype is when creating an object – via __proto__ in an object literal or via:

Object.create(proto: Object) : Object

If you have to, you can use Object.setPrototypeOf() to change the prototype of an existing object. But that may affect performance negatively.

This is how these features are used:

const proto1 = {};
const proto2 = {};

const obj = Object.create(proto1);
assert.equal(Object.getPrototypeOf(obj), proto1);

Object.setPrototypeOf(obj, proto2);
assert.equal(Object.getPrototypeOf(obj), proto2);

29.1.3.2 Check: is an object a prototype of another one?

So far, “p is a prototype of o” always meant “p is a direct prototype of o”. But it can also be used more loosely and mean that p is in the prototype chain of o. That looser relationship can be checked via:

p.isPrototypeOf(o)

For example:

const a = {};
const b = {__proto__: a};
const c = {__proto__: b};

assert.equal(a.isPrototypeOf(b), true);
assert.equal(a.isPrototypeOf(c), true);

assert.equal(a.isPrototypeOf(a), false);
assert.equal(c.isPrototypeOf(a), false);

29.1.4 Sharing data via prototypes

Consider the following code:

const jane = {
  name: 'Jane',
  describe() {
    return 'Person named '+this.name;
  },
};
const tarzan = {
  name: 'Tarzan',
  describe() {
    return 'Person named '+this.name;
  },
};

assert.equal(jane.describe(), 'Person named Jane');
assert.equal(tarzan.describe(), 'Person named Tarzan');

We have two objects that are very similar. Both have two properties whose names are .name and .describe. Additionally, method .describe() is the same. How can we avoid duplicating that method?

We can move it to an object PersonProto and make that object a prototype of both jane and tarzan:

const PersonProto = {
  describe() {
    return 'Person named ' + this.name;
  },
};
const jane = {
  __proto__: PersonProto,
  name: 'Jane',
};
const tarzan = {
  __proto__: PersonProto,
  name: 'Tarzan',
};

The name of the prototype reflects that both jane and tarzan are persons.

Figure 12: Objects jane and tarzan share method .describe(), via their common prototype PersonProto.

Fig. 12 illustrates how the three objects are connected: The objects at the bottom now contain the properties that are specific to jane and tarzan. The object at the top contains the properties that are shared between them.

When you make the method call jane.describe(), this points to the receiver of that method call, jane (in the bottom-left corner of the diagram). That’s why the method still works. tarzan.describe() works similarly.

assert.equal(jane.describe(), 'Person named Jane');
assert.equal(tarzan.describe(), 'Person named Tarzan');

29.2 Classes

We are now ready to take on classes, which are basically a compact syntax for setting up prototype chains. Under the hood, JavaScript’s classes are unconventional. But that is something you rarely see when working with them. They should normally feel familiar to people who have used other object-oriented programming languages.

29.2.1 A class for persons

We have previously worked with jane and tarzan, single objects representing persons. Let’s use a class declaration to implement a factory for person objects:

class Person {
  constructor(name) {
    this.name = name;
  }
  describe() {
    return 'Person named '+this.name;
  }
}

jane and tarzan can now be created via new Person():

const jane = new Person('Jane');
assert.equal(jane.name, 'Jane');
assert.equal(jane.describe(), 'Person named Jane');

const tarzan = new Person('Tarzan');
assert.equal(tarzan.name, 'Tarzan');
assert.equal(tarzan.describe(), 'Person named Tarzan');

Class Person has two methods:

The normal method .describe()

The special method .constructor() which is called directly after a new instance has been created and initializes that instance. It receives the arguments that are passed to the new operator (after the class name). If you don’t need any arguments to set up a new instance, you can omit the constructor.

29.2.1.1 Class expressions

There are two kinds of class definitions (ways of defining classes):

Class declarations, which we have seen in the previous section.

Class expressions, which we’ll see next.

Class expressions can be anonymous and named:

// Anonymous class expression
const Person = class { ··· };

// Named class expression
const Person = class MyClass { ··· };

The name of a named class expression works similarly to the name of a named function expression.

This was a first look at classes. We’ll explore more features soon, but first we need to learn the internals of classes.

29.2.2 Classes under the hood

There is a lot going on under the hood of classes. Let’s look at the diagram for jane (fig. 13).

Figure 13: The class Person has the property .prototype that points to an object that is the prototype of all instances of Person. jane is one such instance.

The main purpose of class Person is to set up the prototype chain on the right (jane, followed by Person.prototype). It is interesting to note that both constructs inside class Person (.constructor and .describe()) created properties for Person.prototype, not for Person.

The reason for this slightly odd approach is backward compatibility: prior to classes, constructor functions (ordinary functions, invoked via the new operator) were often used as factories for objects. Classes are mostly better syntax for constructor functions and therefore remain compatible with old code. That explains why classes are functions:

> typeof Person
'function'

In this book, I use the terms constructor (function) and class interchangeably.

It is easy to confuse .__proto__ and .prototype. Hopefully, fig. 13 makes it clear how they differ:

.__proto__ is a pseudo-property for accessing the prototype of an object.

.prototype is a normal property that is only special due to how the new operator uses it. The name is not ideal: Person.prototype does not point to the prototype of Person, it points to the prototype of all instances of Person.

29.2.2.1 Person.prototype.constructor (advanced)

There is one detail in fig. 13 that we haven’t looked at, yet: Person.prototype.constructor points back to Person:

> Person.prototype.constructor === Person
true

This setup also exists due to backward compatibility. But it has two additional benefits.

First, each instance of a class inherits property .constructor. Therefore, given an instance, you can make “similar” objects using it:

const jane = new Person('Jane');

const cheeta = new jane.constructor('Cheeta');
// cheeta is also an instance of Person
// (the instanceof operator is explained later)
assert.equal(cheeta instanceof Person, true);

Second, you can get the name of the class that created a given instance:

const tarzan = new Person('Tarzan');

assert.equal(tarzan.constructor.name, 'Person');

29.2.3 Class definitions: prototype properties

All constructs in the body of the following class declaration create properties of Foo.prototype.

class Foo {
  constructor(prop) {
    this.prop = prop;
  }
  protoMethod() {
    return 'protoMethod';
  }
  get protoGetter() {
    return 'protoGetter';
  }
}

Let’s examine them in order:

.constructor() is called after creating a new instance of Foo to set up that instance.

.protoMethod() is a normal method. It is stored in Foo.prototype.

.protoGetter is a getter that is stored in Foo.prototype.

The following interaction uses class Foo:

> const foo = new Foo(123);
> foo.prop
123

> foo.protoMethod()
'protoMethod'
> foo.protoGetter
'protoGetter'

29.2.4 Class definitions: static properties

All constructs in the body of the following class declaration create so-called static properties – properties of Bar itself.

class Bar {
  static staticMethod() {
    return 'staticMethod';
  }
  static get staticGetter() {
    return 'staticGetter';
  }
}

The static method and the static getter are used as follows:

> Bar.staticMethod()
'staticMethod'
> Bar.staticGetter
'staticGetter'

29.2.5 The instanceof operator

The instanceof operator tells you if a value is an instance of a given class:

> new Person('Jane') instanceof Person
true
> ({}) instanceof Person
false
> ({}) instanceof Object
true
> [] instanceof Array
true

We’ll explore the instanceof operator in more detail later, after we have looked at subclassing.

29.2.6 Why I recommend classes

I recommend using classes for the following reasons:

Classes are a common standard for object creation and inheritance that is now widely supported across frameworks (React, Angular, Ember, etc.). This is an improvement to how things were before, when almost every framework had its own inheritance library.

They help tools such as IDEs and type checkers with their work and enable new features there.

If you come from another language to JavaScript and are used to classes, then you can get started more quickly.

JavaScript engines optimize them. That is, code that uses classes is almost always faster than code that uses a custom inheritance library.

You can subclass built-in constructor functions such as Error.

That doesn’t mean that classes are perfect:

There is a risk of overdoing inheritance.

There is a risk of putting too much functionality in classes (when some of it is often better put in functions).

How they work superficially and under the hood is quite different. In other words, there is a disconnect between syntax and semantics. Two examples are:

A method definition inside a class C creates a method in the object C.prototype.

Classes are functions.

The motivation for the disconnect is backward compatibility. Thankfully, the disconnect causes few problems in practice; you are usually OK if you go along with what classes pretend to be.

  Exercise: Writing a class

exercises/proto-chains-classes/point_class_test.mjs

29.3 Private data for classes

This section describes techniques for hiding some of the data of an object from the outside. We discuss them in the context of classes, but they also work for objects created directly, e.g., via object literals.

29.3.1 Private data: naming convention

The first technique makes a property private by prefixing its name with an underscore. This doesn’t protect the property in any way; it merely signals to the outside: “You don’t need to know about this property.”

In the following code, the properties ._counter and ._action are private.

class Countdown {
  constructor(counter, action) {
    this._counter = counter;
    this._action = action;
  }
  dec() {
    this._counter--;
    if (this._counter === 0) {
      this._action();
    }
  }
}

// The two properties aren’t really private:
assert.deepEqual(
  Object.keys(new Countdown()),
  ['_counter', '_action']);

With this technique, you don’t get any protection and private names can clash. On the plus side, it is easy to use.

29.3.2 Private data: WeakMaps

Another technique is to use WeakMaps. How exactly that works is explained in the chapter on WeakMaps. This is a preview:

const _counter = new WeakMap();
const _action = new WeakMap();

class Countdown {
  constructor(counter, action) {
    _counter.set(this, counter);
    _action.set(this, action);
  }
  dec() {
    let counter = _counter.get(this);
    counter--;
    _counter.set(this, counter);
    if (counter === 0) {
      _action.get(this)();
    }
  }
}

// The two pseudo-properties are truly private:
assert.deepEqual(
  Object.keys(new Countdown()),
  []);

This technique offers you considerable protection from outside access and there can’t be any name clashes. But it is also more complicated to use.

29.3.3 More techniques for private data

This book explains the most important techniques for private data in classes. There will also probably soon be built-in support for it. Consult the ECMAScript proposal “Class Public Instance Fields & Private Instance Fields” for details.

A few additional techniques are explained in Exploring ES6.

29.4 Subclassing

Classes can also subclass (“extend”) existing classes. As an example, the following class Employee subclasses Person:

class Person {
  constructor(name) {
    this.name = name;
  }
  describe() {
    return `Person named ${this.name}`;
  }
  static logNames(persons) {
    for (const person of persons) {
      console.log(person.name);
    }
  }
}

class Employee extends Person {
  constructor(name, title) {
    super(name);
    this.title = title;
  }
  describe() {
    return super.describe() +
      ` (${this.title})`;
  }
}

const jane = new Employee('Jane', 'CTO');
assert.equal(
  jane.describe(),
  'Person named Jane (CTO)');

Two comments:

Inside a .constructor() method, you must call the super-constructor via super() before you can access this. That’s because this doesn’t exist before the super-constructor is called (this phenomenon is specific to classes).

Static methods are also inherited. For example, Employee inherits the static method .logNames():

> 'logNames' in Employee
true

  Exercise: Subclassing

exercises/proto-chains-classes/color_point_class_test.mjs

29.4.1 Subclasses under the hood (advanced)

Figure 14: These are the objects that make up class Person and its subclass, Employee. The left column is about classes. The right column is about the Employee instance jane and its prototype chain.

The classes Person and Employee from the previous section are made up of several objects (fig. 14). One key insight for understanding how these objects are related is that there are two prototype chains:

The instance prototype chain, on the right.

The class prototype chain, on the left.

29.4.1.1 The instance prototype chain (right column)

The instance prototype chain starts with jane and continues with Employee.prototype and Person.prototype. In principle, the prototype chain ends at this point, but we get one more object: Object.prototype. This prototype provides services to virtually all objects, which is why it is included here, too:

> Object.getPrototypeOf(Person.prototype) === Object.prototype
true

29.4.1.2 The class prototype chain (left column)

In the class prototype chain, Employee comes first, Person next. Afterward, the chain continues with Function.prototype, which is only there because Person is a function and functions need the services of Function.prototype.

> Object.getPrototypeOf(Person) === Function.prototype
true

29.4.2 instanceof in more detail (advanced)

We have not yet seen how instanceof really works. Given the expression:

x instanceof C

How does instanceof determine if x is an instance of C (or a subclass of C)? It does so by checking if C.prototype is in the prototype chain of x. That is, the following expression is equivalent:

C.prototype.isPrototypeOf(x)

If we go back to fig. 14, we can confirm that the prototype chain does lead us to the following correct answers:

> jane instanceof Employee
true
> jane instanceof Person
true
> jane instanceof Object
true

29.4.3 Prototype chains of built-in objects (advanced)

Next, we’ll use our knowledge of subclassing to understand the prototype chains of a few built-in objects. The following tool function p() helps us with our explorations.

const p = Object.getPrototypeOf.bind(Object);

We extracted method .getPrototypeOf() of Object and assigned it to p.

29.4.3.1 The prototype chain of {}

Let’s start by examining plain objects:

> p({}) === Object.prototype
true
> p(p({})) === null
true

Figure 15: The prototype chain of an object created via an object literal starts with that object, continues with Object.prototype, and ends with null.

Fig. 15 shows a diagram for this prototype chain. We can see that {} really is an instance of Object – Object.prototype is in its prototype chain.

29.4.3.2 The prototype chain of []

What does the prototype chain of an Array look like?

> p([]) === Array.prototype
true
> p(p([])) === Object.prototype
true
> p(p(p([]))) === null
true

Figure 16: The prototype chain of an Array has these members: the Array instance, Array.prototype, Object.prototype, null.

This prototype chain (visualized in fig. 16) tells us that an Array object is an instance of Array, which is a subclass of Object.

29.4.3.3 The prototype chain of function () {}

Lastly, the prototype chain of an ordinary function tells us that all functions are objects:

> p(function () {}) === Function.prototype
true
> p(p(function () {})) === Object.prototype
true

29.4.3.4 Objects that aren’t instances of Object

An object is only an instance of Object if Object.prototype is in its prototype chain. Most objects created via various literals are instances of Object:

> ({}) instanceof Object
true
> (() => {}) instanceof Object
true
> /abc/ug instanceof Object
true

Objects that don’t have prototypes are not instances of Object:

> ({ __proto__: null }) instanceof Object
false

Object.prototype ends most prototype chains. Its prototype is null, which means it isn’t an instance of Object either:

> Object.prototype instanceof Object
false

29.4.3.5 How exactly does the pseudo-property .__proto__ work?

The pseudo-property .__proto__ is implemented by class Object via a getter and a setter. It could be implemented like this:

class Object {
  get __proto__() {
    return Object.getPrototypeOf(this);
  }
  set __proto__(other) {
    Object.setPrototypeOf(this, other);
  }
  // ···
}

That means that you can switch .__proto__ off by creating an object that doesn’t have Object.prototype in its prototype chain (see the previous section):

> '__proto__' in {}
true
> '__proto__' in { __proto__: null }
false

29.4.4 Dispatched vs. direct method calls (advanced)

Let’s examine how method calls work with classes. We are revisiting jane from earlier:

class Person {
  constructor(name) {
    this.name = name;
  }
  describe() {
    return 'Person named '+this.name;
  }
}
const jane = new Person('Jane');

Fig. 17 has a diagram with jane’s prototype chain.

Figure 17: The prototype chain of jane starts with jane and continues with Person.prototype.

Normal method calls are dispatched – the method call jane.describe() happens in two steps:

Dispatch: In the prototype chain of jane, find the first property whose key is 'describe' and retrieve its value.

const func = jane.describe;

Call: Call the value, while setting this to jane.

func.call(jane);

This way of dynamically looking for a method and invoking it is called dynamic dispatch.

You can make the same method call directly, without dispatching:

Person.prototype.describe.call(jane)

This time, we directly point to the method via Person.prototype.describe and don’t search for it in the prototype chain. We also specify this differently via .call().

Note that this always points to the beginning of a prototype chain. That enables .describe() to access .name.

29.4.4.1 Borrowing methods

Direct method calls become useful when you are working with methods of Object.prototype. For example, Object.prototype.hasOwnProperty(k) checks if this has a non-inherited property whose key is k:

> const obj = { foo: 123 };
> obj.hasOwnProperty('foo')
true
> obj.hasOwnProperty('bar')
false

However, in the prototype chain of an object, there may be another property with the key 'hasOwnProperty' that overrides the method in Object.prototype. Then a dispatched method call doesn’t work:

> const obj = { hasOwnProperty: true };
> obj.hasOwnProperty('bar')
TypeError: obj.hasOwnProperty is not a function

The workaround is to use a direct method call:

> Object.prototype.hasOwnProperty.call(obj, 'bar')
false
> Object.prototype.hasOwnProperty.call(obj, 'hasOwnProperty')
true

This kind of direct method call is often abbreviated as follows:

> ({}).hasOwnProperty.call(obj, 'bar')
false
> ({}).hasOwnProperty.call(obj, 'hasOwnProperty')
true

This pattern may seem inefficient, but most engines optimize this pattern, so performance should not be an issue.

29.4.5 Mixin classes (advanced)

JavaScript’s class system only supports single inheritance. That is, each class can have at most one superclass. One way around this limitation is via a technique called mixin classes (short: mixins).

The idea is as follows: Let’s say we want a class C to inherit from two superclasses S1 and S2. That would be multiple inheritance, which JavaScript doesn’t support.

Our workaround is to turn S1 and S2 into mixins, factories for subclasses:

const S1 = (Sup) => class extends Sup { /*···*/ };
const S2 = (Sup) => class extends Sup { /*···*/ };

Each of these two functions returns a class that extends a given superclass Sup. We create class C as follows:

class C extends S2(S1(Object)) {
  /*···*/
}

We now have a class C that extends a class S2 that extends a class S1 that extends Object (which most classes do implicitly).

29.4.5.1 Example: a mixin for brand management

We implement a mixin Branded that has helper methods for setting and getting the brand of an object:

const Branded = (Sup) => class extends Sup {
  setBrand(brand) {
    this._brand = brand;
    return this;
  }
  getBrand() {
    return this._brand;
  }
};

We use this mixin to implement brand management for a class Car:

class Car extends Branded(Object) {
  constructor(model) {
    super();
    this._model = model;
  }
  toString() {
    return `${this.getBrand()} ${this._model}`;
  }
}

The following code confirms that the mixin worked: Car has method .setBrand() of Branded.

const modelT = new Car('Model T').setBrand('Ford');
assert.equal(modelT.toString(), 'Ford Model T');

29.4.5.2 The benefits of mixins

Mixins free us from the constraints of single inheritance:

The same class can extend a single superclass and zero or more mixins.

The same mixin can be used by multiple classes.

29.5 FAQ: objects

29.5.1 Why do objects preserve the insertion order of properties?

In principle, objects are unordered. The main reason for ordering properties is so that operations that list entries, keys, or values are deterministic. That helps, e.g., with testing.

  Quiz

See quiz app.

30 Where are the remaining chapters?

You are reading a preview version of this book. You can either read all essential chapters online or you can buy the full version.

You can take a look at the full table of contents, which is also linked to from the book’s homepage.