Airbnb JavaScript Style Guide() {

A mostly reasonable approach to JavaScript

Note: this guide assumes you are using Babel, and requires that you use babel-preset-airbnb or the equivalent. It also assumes you are installing shims/polyfills in your app, with airbnb-browser-shims or the equivalent.

This guide is available in other languages too. See Translation

Other Style Guides

• ES5 (Deprecated)
• React
• CSS-in-JavaScript
• CSS & Sass
• Ruby

Table of Contents

1. Types
2. References
3. Objects
4. Arrays
5. Destructuring
6. Strings
7. Functions
8. Arrow Functions
9. Classes & Constructors
10. Modules
11. Iterators and Generators
12. Properties
13. Variables
14. Hoisting
15. Comparison Operators & Equality
16. Blocks
17. Control Statements
18. Comments
19. Whitespace
20. Commas
21. Semicolons
22. Type Casting & Coercion
23. Naming Conventions
24. Accessors
25. Events
26. jQuery
27. ECMAScript 5 Compatibility
28. ECMAScript 6+ (ES 2015+) Styles
29. Standard Library
30. Testing
31. Performance
32. Resources
33. In the Wild
34. Translation
35. The JavaScript Style Guide Guide
36. Chat With Us About JavaScript
37. Contributors
38. License
39. Amendments

Types

• 1.1 Primitives: When you access a primitive type you work directly on its value.

  • string
  • number
  • boolean
  • null
  • undefined
  • symbol
  • bigint
  
  

  1const foo = 1;
  2let bar = foo;
  3
  4bar = 9;
  5
  6console.log(foo, bar); // => 1, 9

  • Symbols and BigInts cannot be faithfully polyfilled, so they should not be used when targeting browsers/environments that don’t support them natively.

• 1.2 Complex: When you access a complex type you work on a reference to its value.

  • object
  • array
  • function
  
  

  1const foo = [1, 2];
  2const bar = foo;
  3
  4bar[0] = 9;
  5
  6console.log(foo[0], bar[0]); // => 9, 9

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References

• 2.1 Use const for all of your references; avoid using var. eslint: prefer-const, no-const-assign

  Why? This ensures that you can’t reassign your references, which can lead to bugs and difficult to comprehend code.

  1// bad
  2var a = 1;
  3var b = 2;
  4
  5// good
  6const a = 1;
  7const b = 2;

• 2.2 If you must reassign references, use let instead of var. eslint: no-var

  Why? let is block-scoped rather than function-scoped like var.

  1// bad
  2var count = 1;
  3if (true) {
  4  count += 1;
  5}
  6
  7// good, use the let.
  8let count = 1;
  9if (true) {
  10  count += 1;
  11}

• 2.3 Note that both let and const are block-scoped, whereas var is function-scoped.

  1// const and let only exist in the blocks they are defined in.
  2{
  3  let a = 1;
  4  const b = 1;
  5  var c = 1;
  6}
  7console.log(a); // ReferenceError
  8console.log(b); // ReferenceError
  9console.log(c); // Prints 1

  In the above code, you can see that referencing a and b will produce a ReferenceError, while c contains the number. This is because a and b are block scoped, while c is scoped to the containing function.

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Objects

• 3.1 Use the literal syntax for object creation. eslint: no-new-object

  1// bad
  2const item = new Object();
  3
  4// good
  5const item = {};

• 3.2 Use computed property names when creating objects with dynamic property names.

  Why? They allow you to define all the properties of an object in one place.

  1
  2function getKey(k) {
  3  return `a key named ${k}`;
  4}
  5
  6// bad
  7const obj = {
  8  id: 5,
  9  name: 'San Francisco',
  10};
  11obj[getKey('enabled')] = true;
  12
  13// good
  14const obj = {
  15  id: 5,
  16  name: 'San Francisco',
  17  [getKey('enabled')]: true,
  18};

• 3.3 Use object method shorthand. eslint: object-shorthand

  1// bad
  2const atom = {
  3  value: 1,
  4
  5  addValue: function (value) {
  6    return atom.value + value;
  7  },
  8};
  9
  10// good
  11const atom = {
  12  value: 1,
  13
  14  addValue(value) {
  15    return atom.value + value;
  16  },
  17};

• 3.4 Use property value shorthand. eslint: object-shorthand

  Why? It is shorter and descriptive.

  1const lukeSkywalker = 'Luke Skywalker';
  2
  3// bad
  4const obj = {
  5  lukeSkywalker: lukeSkywalker,
  6};
  7
  8// good
  9const obj = {
  10  lukeSkywalker,
  11};

• 3.5 Group your shorthand properties at the beginning of your object declaration.

  Why? It’s easier to tell which properties are using the shorthand.

  1const anakinSkywalker = 'Anakin Skywalker';
  2const lukeSkywalker = 'Luke Skywalker';
  3
  4// bad
  5const obj = {
  6  episodeOne: 1,
  7  twoJediWalkIntoACantina: 2,
  8  lukeSkywalker,
  9  episodeThree: 3,
  10  mayTheFourth: 4,
  11  anakinSkywalker,
  12};
  13
  14// good
  15const obj = {
  16  lukeSkywalker,
  17  anakinSkywalker,
  18  episodeOne: 1,
  19  twoJediWalkIntoACantina: 2,
  20  episodeThree: 3,
  21  mayTheFourth: 4,
  22};

• 3.6 Only quote properties that are invalid identifiers. eslint: quote-props

  Why? In general we consider it subjectively easier to read. It improves syntax highlighting, and is also more easily optimized by many JS engines.

  1// bad
  2const bad = {
  3  'foo': 3,
  4  'bar': 4,
  5  'data-blah': 5,
  6};
  7
  8// good
  9const good = {
  10  foo: 3,
  11  bar: 4,
  12  'data-blah': 5,
  13};

• 3.7 Do not call Object.prototype methods directly, such as hasOwnProperty, propertyIsEnumerable, and isPrototypeOf. eslint: no-prototype-builtins

  Why? These methods may be shadowed by properties on the object in question - consider { hasOwnProperty: false } - or, the object may be a null object (Object.create(null)). In modern browsers that support ES2022, or with a polyfill such as https://npmjs.com/object.hasown, Object.hasOwn can also be used as an alternative to Object.prototype.hasOwnProperty.call.

  1// bad
  2console.log(object.hasOwnProperty(key));
  3
  4// good
  5console.log(Object.prototype.hasOwnProperty.call(object, key));
  6
  7// better
  8const has = Object.prototype.hasOwnProperty; // cache the lookup once, in module scope.
  9console.log(has.call(object, key));
  10
  11// best
  12console.log(Object.hasOwn(object, key)); // only supported in browsers that support ES2022
  13
  14/* or */
  15import has from 'has'; // https://www.npmjs.com/package/has
  16console.log(has(object, key));
  17/* or */
  18console.log(Object.hasOwn(object, key)); // https://www.npmjs.com/package/object.hasown

• 3.8 Prefer the object spread syntax over Object.assign to shallow-copy objects. Use the object rest parameter syntax to get a new object with certain properties omitted. eslint: prefer-object-spread

  1// very bad
  2const original = { a: 1, b: 2 };
  3const copy = Object.assign(original, { c: 3 }); // this mutates `original` ಠ_ಠ
  4delete copy.a; // so does this
  5
  6// bad
  7const original = { a: 1, b: 2 };
  8const copy = Object.assign({}, original, { c: 3 }); // copy => { a: 1, b: 2, c: 3 }
  9
  10// good
  11const original = { a: 1, b: 2 };
  12const copy = { ...original, c: 3 }; // copy => { a: 1, b: 2, c: 3 }
  13
  14const { a, ...noA } = copy; // noA => { b: 2, c: 3 }

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Arrays

• 4.1 Use the literal syntax for array creation. eslint: no-array-constructor

  1// bad
  2const items = new Array();
  3
  4// good
  5const items = [];

• 4.2 Use Array#push instead of direct assignment to add items to an array.

  1const someStack = [];
  2
  3// bad
  4someStack[someStack.length] = 'abracadabra';
  5
  6// good
  7someStack.push('abracadabra');

• 4.3 Use array spreads ... to copy arrays.

  1// bad
  2const len = items.length;
  3const itemsCopy = [];
  4let i;
  5
  6for (i = 0; i < len; i += 1) {
  7  itemsCopy[i] = items[i];
  8}
  9
  10// good
  11const itemsCopy = [...items];

• 4.4 To convert an iterable object to an array, use spreads ... instead of Array.from

  1const foo = document.querySelectorAll('.foo');
  2
  3// good
  4const nodes = Array.from(foo);
  5
  6// best
  7const nodes = [...foo];

• 4.5 Use Array.from for converting an array-like object to an array.

  1const arrLike = { 0: 'foo', 1: 'bar', 2: 'baz', length: 3 };
  2
  3// bad
  4const arr = Array.prototype.slice.call(arrLike);
  5
  6// good
  7const arr = Array.from(arrLike);

• 4.6 Use Array.from instead of spread ... for mapping over iterables, because it avoids creating an intermediate array.

  1// bad
  2const baz = [...foo].map(bar);
  3
  4// good
  5const baz = Array.from(foo, bar);

• 4.7 Use return statements in array method callbacks. It’s ok to omit the return if the function body consists of a single statement returning an expression without side effects, following 8.2. eslint: array-callback-return

  1// good
  2[1, 2, 3].map((x) => {
  3  const y = x + 1;
  4  return x * y;
  5});
  6
  7// good
  8[1, 2, 3].map((x) => x + 1);
  9
  10// bad - no returned value means `acc` becomes undefined after the first iteration
  11[[0, 1], [2, 3], [4, 5]].reduce((acc, item, index) => {
  12  const flatten = acc.concat(item);
  13});
  14
  15// good
  16[[0, 1], [2, 3], [4, 5]].reduce((acc, item, index) => {
  17  const flatten = acc.concat(item);
  18  return flatten;
  19});
  20
  21// bad
  22inbox.filter((msg) => {
  23  const { subject, author } = msg;
  24  if (subject === 'Mockingbird') {
  25    return author === 'Harper Lee';
  26  } else {
  27    return false;
  28  }
  29});
  30
  31// good
  32inbox.filter((msg) => {
  33  const { subject, author } = msg;
  34  if (subject === 'Mockingbird') {
  35    return author === 'Harper Lee';
  36  }
  37
  38  return false;
  39});

• 4.8 Use line breaks after opening array brackets and before closing array brackets, if an array has multiple lines

  1// bad
  2const arr = [
  3  [0, 1], [2, 3], [4, 5],
  4];
  5
  6const objectInArray = [{
  7  id: 1,
  8}, {
  9  id: 2,
  10}];
  11
  12const numberInArray = [
  13  1, 2,
  14];
  15
  16// good
  17const arr = [[0, 1], [2, 3], [4, 5]];
  18
  19const objectInArray = [
  20  {
  21    id: 1,
  22  },
  23  {
  24    id: 2,
  25  },
  26];
  27
  28const numberInArray = [
  29  1,
  30  2,
  31];

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Destructuring

• 5.1 Use object destructuring when accessing and using multiple properties of an object. eslint: prefer-destructuring

  Why? Destructuring saves you from creating temporary references for those properties, and from repetitive access of the object. Repeating object access creates more repetitive code, requires more reading, and creates more opportunities for mistakes. Destructuring objects also provides a single site of definition of the object structure that is used in the block, rather than requiring reading the entire block to determine what is used.

  1// bad
  2function getFullName(user) {
  3  const firstName = user.firstName;
  4  const lastName = user.lastName;
  5
  6  return `${firstName} ${lastName}`;
  7}
  8
  9// good
  10function getFullName(user) {
  11  const { firstName, lastName } = user;
  12  return `${firstName} ${lastName}`;
  13}
  14
  15// best
  16function getFullName({ firstName, lastName }) {
  17  return `${firstName} ${lastName}`;
  18}

• 5.2 Use array destructuring. eslint: prefer-destructuring

  1const arr = [1, 2, 3, 4];
  2
  3// bad
  4const first = arr[0];
  5const second = arr[1];
  6
  7// good
  8const [first, second] = arr;

• 5.3 Use object destructuring for multiple return values, not array destructuring.

  Why? You can add new properties over time or change the order of things without breaking call sites.

  1// bad
  2function processInput(input) {
  3  // then a miracle occurs
  4  return [left, right, top, bottom];
  5}
  6
  7// the caller needs to think about the order of return data
  8const [left, __, top] = processInput(input);
  9
  10// good
  11function processInput(input) {
  12  // then a miracle occurs
  13  return { left, right, top, bottom };
  14}
  15
  16// the caller selects only the data they need
  17const { left, top } = processInput(input);

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Strings

• 6.1 Use single quotes '' for strings. eslint: quotes

  1// bad
  2const name = "Capt. Janeway";
  3
  4// bad - template literals should contain interpolation or newlines
  5const name = `Capt. Janeway`;
  6
  7// good
  8const name = 'Capt. Janeway';

• 6.2 Strings that cause the line to go over 100 characters should not be written across multiple lines using string concatenation.

  Why? Broken strings are painful to work with and make code less searchable.

  1// bad
  2const errorMessage = 'This is a super long error that was thrown because \
  3of Batman. When you stop to think about how Batman had anything to do \
  4with this, you would get nowhere \
  5fast.';
  6
  7// bad
  8const errorMessage = 'This is a super long error that was thrown because ' +
  9  'of Batman. When you stop to think about how Batman had anything to do ' +
  10  'with this, you would get nowhere fast.';
  11
  12// good
  13const errorMessage = 'This is a super long error that was thrown because of Batman. When you stop to think about how Batman had anything to do with this, you would get nowhere fast.';

• 6.3 When programmatically building up strings, use template strings instead of concatenation. eslint: prefer-template template-curly-spacing

  Why? Template strings give you a readable, concise syntax with proper newlines and string interpolation features.

  1// bad
  2function sayHi(name) {
  3  return 'How are you, ' + name + '?';
  4}
  5
  6// bad
  7function sayHi(name) {
  8  return ['How are you, ', name, '?'].join();
  9}
  10
  11// bad
  12function sayHi(name) {
  13  return `How are you, ${ name }?`;
  14}
  15
  16// good
  17function sayHi(name) {
  18  return `How are you, ${name}?`;
  19}

• 6.4 Never use eval() on a string; it opens too many vulnerabilities. eslint: no-eval

• 6.5 Do not unnecessarily escape characters in strings. eslint: no-useless-escape

  Why? Backslashes harm readability, thus they should only be present when necessary.

  1// bad
  2const foo = '\'this\' \i\s \"quoted\"';
  3
  4// good
  5const foo = '\'this\' is "quoted"';
  6const foo = `my name is '${name}'`;

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Functions

• 7.1 Use named function expressions instead of function declarations. eslint: func-style, func-names

  Why? Function declarations are hoisted, which means that it’s easy - too easy - to reference the function before it is defined in the file. This harms readability and maintainability. If you find that a function’s definition is large or complex enough that it is interfering with understanding the rest of the file, then perhaps it’s time to extract it to its own module! Don’t forget to explicitly name the expression, regardless of whether or not the name is inferred from the containing variable (which is often the case in modern browsers or when using compilers such as Babel). This eliminates any assumptions made about the Error’s call stack. (Discussion)

  1// bad
  2function foo() {
  3  // ...
  4}
  5
  6// bad
  7const foo = function () {
  8  // ...
  9};
  10
  11// good
  12// lexical name distinguished from the variable-referenced invocation(s)
  13const short = function longUniqueMoreDescriptiveLexicalFoo() {
  14  // ...
  15};

• 7.2 Wrap immediately invoked function expressions in parentheses. eslint: wrap-iife

  Why? An immediately invoked function expression is a single unit - wrapping both it, and its invocation parens, in parens, cleanly expresses this. Note that in a world with modules everywhere, you almost never need an IIFE.

  1// immediately-invoked function expression (IIFE)
  2(function () {
  3  console.log('Welcome to the Internet. Please follow me.');
  4}());

• 7.3 Never declare a function in a non-function block (if, while, etc). Assign the function to a variable instead. Browsers will allow you to do it, but they all interpret it differently, which is bad news bears. eslint: no-loop-func

• 7.4 Note: ECMA-262 defines a block as a list of statements. A function declaration is not a statement.

  1// bad
  2if (currentUser) {
  3  function test() {
  4    console.log('Nope.');
  5  }
  6}
  7
  8// good
  9let test;
  10if (currentUser) {
  11  test = () => {
  12    console.log('Yup.');
  13  };
  14}

• 7.5 Never name a parameter arguments. This will take precedence over the arguments object that is given to every function scope.

  1// bad
  2function foo(name, options, arguments) {
  3  // ...
  4}
  5
  6// good
  7function foo(name, options, args) {
  8  // ...
  9}

• 7.6 Never use arguments, opt to use rest syntax ... instead. eslint: prefer-rest-params

  Why? ... is explicit about which arguments you want pulled. Plus, rest arguments are a real Array, and not merely Array-like like arguments.

  1// bad
  2function concatenateAll() {
  3  const args = Array.prototype.slice.call(arguments);
  4  return args.join('');
  5}
  6
  7// good
  8function concatenateAll(...args) {
  9  return args.join('');
  10}

• 7.7 Use default parameter syntax rather than mutating function arguments.

  1// really bad
  2function handleThings(opts) {
  3  // No! We shouldn’t mutate function arguments.
  4  // Double bad: if opts is falsy it'll be set to an object which may
  5  // be what you want but it can introduce subtle bugs.
  6  opts = opts || {};
  7  // ...
  8}
  9
  10// still bad
  11function handleThings(opts) {
  12  if (opts === void 0) {
  13    opts = {};
  14  }
  15  // ...
  16}
  17
  18// good
  19function handleThings(opts = {}) {
  20  // ...
  21}

• 7.8 Avoid side effects with default parameters.

  Why? They are confusing to reason about.

  1let b = 1;
  2// bad
  3function count(a = b++) {
  4  console.log(a);
  5}
  6count();  // 1
  7count();  // 2
  8count(3); // 3
  9count();  // 3

• 7.9 Always put default parameters last. eslint: default-param-last

  1// bad
  2function handleThings(opts = {}, name) {
  3  // ...
  4}
  5
  6// good
  7function handleThings(name, opts = {}) {
  8  // ...
  9}

• 7.10 Never use the Function constructor to create a new function. eslint: no-new-func

  Why? Creating a function in this way evaluates a string similarly to eval(), which opens vulnerabilities.

  1// bad
  2const add = new Function('a', 'b', 'return a + b');
  3
  4// still bad
  5const subtract = Function('a', 'b', 'return a - b');

• 7.11 Spacing in a function signature. eslint: space-before-function-paren space-before-blocks

  Why? Consistency is good, and you shouldn’t have to add or remove a space when adding or removing a name.

  1// bad
  2const f = function(){};
  3const g = function (){};
  4const h = function() {};
  5
  6// good
  7const x = function () {};
  8const y = function a() {};

• 7.12 Never mutate parameters. eslint: no-param-reassign

  Why? Manipulating objects passed in as parameters can cause unwanted variable side effects in the original caller.

  1// bad
  2function f1(obj) {
  3  obj.key = 1;
  4}
  5
  6// good
  7function f2(obj) {
  8  const key = Object.prototype.hasOwnProperty.call(obj, 'key') ? obj.key : 1;
  9}

• 7.13 Never reassign parameters. eslint: no-param-reassign

  Why? Reassigning parameters can lead to unexpected behavior, especially when accessing the arguments object. It can also cause optimization issues, especially in V8.

  1// bad
  2function f1(a) {
  3  a = 1;
  4  // ...
  5}
  6
  7function f2(a) {
  8  if (!a) { a = 1; }
  9  // ...
  10}
  11
  12// good
  13function f3(a) {
  14  const b = a || 1;
  15  // ...
  16}
  17
  18function f4(a = 1) {
  19  // ...
  20}

• 7.14 Prefer the use of the spread syntax ... to call variadic functions. eslint: prefer-spread

  Why? It’s cleaner, you don’t need to supply a context, and you can not easily compose new with apply.

  1// bad
  2const x = [1, 2, 3, 4, 5];
  3console.log.apply(console, x);
  4
  5// good
  6const x = [1, 2, 3, 4, 5];
  7console.log(...x);
  8
  9// bad
  10new (Function.prototype.bind.apply(Date, [null, 2016, 8, 5]));
  11
  12// good
  13new Date(...[2016, 8, 5]);

• 7.15 Functions with multiline signatures, or invocations, should be indented just like every other multiline list in this guide: with each item on a line by itself, with a trailing comma on the last item. eslint: function-paren-newline

  1// bad
  2function foo(bar,
  3             baz,
  4             quux) {
  5  // ...
  6}
  7
  8// good
  9function foo(
  10  bar,
  11  baz,
  12  quux,
  13) {
  14  // ...
  15}
  16
  17// bad
  18console.log(foo,
  19  bar,
  20  baz);
  21
  22// good
  23console.log(
  24  foo,
  25  bar,
  26  baz,
  27);

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Arrow Functions

• 8.1 When you must use an anonymous function (as when passing an inline callback), use arrow function notation. eslint: prefer-arrow-callback, arrow-spacing

  Why? It creates a version of the function that executes in the context of this, which is usually what you want, and is a more concise syntax.

  Why not? If you have a fairly complicated function, you might move that logic out into its own named function expression.

  1// bad
  2[1, 2, 3].map(function (x) {
  3  const y = x + 1;
  4  return x * y;
  5});
  6
  7// good
  8[1, 2, 3].map((x) => {
  9  const y = x + 1;
  10  return x * y;
  11});

• 8.2 If the function body consists of a single statement returning an expression without side effects, omit the braces and use the implicit return. Otherwise, keep the braces and use a return statement. eslint: arrow-parens, arrow-body-style

  Why? Syntactic sugar. It reads well when multiple functions are chained together.

  1// bad
  2[1, 2, 3].map((number) => {
  3  const nextNumber = number + 1;
  4  `A string containing the ${nextNumber}.`;
  5});
  6
  7// good
  8[1, 2, 3].map((number) => `A string containing the ${number + 1}.`);
  9
  10// good
  11[1, 2, 3].map((number) => {
  12  const nextNumber = number + 1;
  13  return `A string containing the ${nextNumber}.`;
  14});
  15
  16// good
  17[1, 2, 3].map((number, index) => ({
  18  [index]: number,
  19}));
  20
  21// No implicit return with side effects
  22function foo(callback) {
  23  const val = callback();
  24  if (val === true) {
  25    // Do something if callback returns true
  26  }
  27}
  28
  29let bool = false;
  30
  31// bad
  32foo(() => bool = true);
  33
  34// good
  35foo(() => {
  36  bool = true;
  37});

• 8.3 In case the expression spans over multiple lines, wrap it in parentheses for better readability.

  Why? It shows clearly where the function starts and ends.

  1// bad
  2['get', 'post', 'put'].map((httpMethod) => Object.prototype.hasOwnProperty.call(
  3    httpMagicObjectWithAVeryLongName,
  4    httpMethod,
  5  )
  6);
  7
  8// good
  9['get', 'post', 'put'].map((httpMethod) => (
  10  Object.prototype.hasOwnProperty.call(
  11    httpMagicObjectWithAVeryLongName,
  12    httpMethod,
  13  )
  14));

• 8.4 Always include parentheses around arguments for clarity and consistency. eslint: arrow-parens

  Why? Minimizes diff churn when adding or removing arguments.

  1// bad
  2[1, 2, 3].map(x => x * x);
  3
  4// good
  5[1, 2, 3].map((x) => x * x);
  6
  7// bad
  8[1, 2, 3].map(number => (
  9  `A long string with the ${number}. It’s so long that we don’t want it to take up space on the .map line!`
  10));
  11
  12// good
  13[1, 2, 3].map((number) => (
  14  `A long string with the ${number}. It’s so long that we don’t want it to take up space on the .map line!`
  15));
  16
  17// bad
  18[1, 2, 3].map(x => {
  19  const y = x + 1;
  20  return x * y;
  21});
  22
  23// good
  24[1, 2, 3].map((x) => {
  25  const y = x + 1;
  26  return x * y;
  27});

• 8.5 Avoid confusing arrow function syntax (=>) with comparison operators (<=, >=). eslint: no-confusing-arrow

  1// bad
  2const itemHeight = (item) => item.height <= 256 ? item.largeSize : item.smallSize;
  3
  4// bad
  5const itemHeight = (item) => item.height >= 256 ? item.largeSize : item.smallSize;
  6
  7// good
  8const itemHeight = (item) => (item.height <= 256 ? item.largeSize : item.smallSize);
  9
  10// good
  11const itemHeight = (item) => {
  12  const { height, largeSize, smallSize } = item;
  13  return height <= 256 ? largeSize : smallSize;
  14};

• 8.6 Enforce the location of arrow function bodies with implicit returns. eslint: implicit-arrow-linebreak

  1// bad
  2(foo) =>
  3  bar;
  4
  5(foo) =>
  6  (bar);
  7
  8// good
  9(foo) => bar;
  10(foo) => (bar);
  11(foo) => (
  12   bar
  13)

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Classes & Constructors

• 9.1 Always use class. Avoid manipulating prototype directly.

  Why? class syntax is more concise and easier to reason about.

  1// bad
  2function Queue(contents = []) {
  3  this.queue = [...contents];
  4}
  5Queue.prototype.pop = function () {
  6  const value = this.queue[0];
  7  this.queue.splice(0, 1);
  8  return value;
  9};
  10
  11// good
  12class Queue {
  13  constructor(contents = []) {
  14    this.queue = [...contents];
  15  }
  16  pop() {
  17    const value = this.queue[0];
  18    this.queue.splice(0, 1);
  19    return value;
  20  }
  21}

• 9.2 Use extends for inheritance.

  Why? It is a built-in way to inherit prototype functionality without breaking instanceof.

  1// bad
  2const inherits = require('inherits');
  3function PeekableQueue(contents) {
  4  Queue.apply(this, contents);
  5}
  6inherits(PeekableQueue, Queue);
  7PeekableQueue.prototype.peek = function () {
  8  return this.queue[0];
  9};
  10
  11// good
  12class PeekableQueue extends Queue {
  13  peek() {
  14    return this.queue[0];
  15  }
  16}

• 9.3 Methods can return this to help with method chaining.

  1// bad
  2Jedi.prototype.jump = function () {
  3  this.jumping = true;
  4  return true;
  5};
  6
  7Jedi.prototype.setHeight = function (height) {
  8  this.height = height;
  9};
  10
  11const luke = new Jedi();
  12luke.jump(); // => true
  13luke.setHeight(20); // => undefined
  14
  15// good
  16class Jedi {
  17  jump() {
  18    this.jumping = true;
  19    return this;
  20  }
  21
  22  setHeight(height) {
  23    this.height = height;
  24    return this;
  25  }
  26}
  27
  28const luke = new Jedi();
  29
  30luke.jump()
  31  .setHeight(20);

• 9.4 It’s okay to write a custom toString() method, just make sure it works successfully and causes no side effects.

  1class Jedi {
  2  constructor(options = {}) {
  3    this.name = options.name || 'no name';
  4  }
  5
  6  getName() {
  7    return this.name;
  8  }
  9
  10  toString() {
  11    return `Jedi - ${this.getName()}`;
  12  }
  13}

• 9.5 Classes have a default constructor if one is not specified. An empty constructor function or one that just delegates to a parent class is unnecessary. eslint: no-useless-constructor

  1// bad
  2class Jedi {
  3  constructor() {}
  4
  5  getName() {
  6    return this.name;
  7  }
  8}
  9
  10// bad
  11class Rey extends Jedi {
  12  constructor(...args) {
  13    super(...args);
  14  }
  15}
  16
  17// good
  18class Rey extends Jedi {
  19  constructor(...args) {
  20    super(...args);
  21    this.name = 'Rey';
  22  }
  23}

• 9.6 Avoid duplicate class members. eslint: no-dupe-class-members

  Why? Duplicate class member declarations will silently prefer the last one - having duplicates is almost certainly a bug.

  1// bad
  2class Foo {
  3  bar() { return 1; }
  4  bar() { return 2; }
  5}
  6
  7// good
  8class Foo {
  9  bar() { return 1; }
  10}
  11
  12// good
  13class Foo {
  14  bar() { return 2; }
  15}

• 9.7 Class methods should use this or be made into a static method unless an external library or framework requires using specific non-static methods. Being an instance method should indicate that it behaves differently based on properties of the receiver. eslint: class-methods-use-this

  1// bad
  2class Foo {
  3  bar() {
  4    console.log('bar');
  5  }
  6}
  7
  8// good - this is used
  9class Foo {
  10  bar() {
  11    console.log(this.bar);
  12  }
  13}
  14
  15// good - constructor is exempt
  16class Foo {
  17  constructor() {
  18    // ...
  19  }
  20}
  21
  22// good - static methods aren't expected to use this
  23class Foo {
  24  static bar() {
  25    console.log('bar');
  26  }
  27}

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Modules

• 10.1 Always use modules (import/export) over a non-standard module system. You can always transpile to your preferred module system.

  Why? Modules are the future, let’s start using the future now.

  1// bad
  2const AirbnbStyleGuide = require('./AirbnbStyleGuide');
  3module.exports = AirbnbStyleGuide.es6;
  4
  5// ok
  6import AirbnbStyleGuide from './AirbnbStyleGuide';
  7export default AirbnbStyleGuide.es6;
  8
  9// best
  10import { es6 } from './AirbnbStyleGuide';
  11export default es6;

• 10.2 Do not use wildcard imports.

  Why? This makes sure you have a single default export.

  1// bad
  2import * as AirbnbStyleGuide from './AirbnbStyleGuide';
  3
  4// good
  5import AirbnbStyleGuide from './AirbnbStyleGuide';

• 10.3 And do not export directly from an import.

  Why? Although the one-liner is concise, having one clear way to import and one clear way to export makes things consistent.

  1// bad
  2// filename es6.js
  3export { es6 as default } from './AirbnbStyleGuide';
  4
  5// good
  6// filename es6.js
  7import { es6 } from './AirbnbStyleGuide';
  8export default es6;

• 10.4 Only import from a path in one place. eslint: no-duplicate-imports

  Why? Having multiple lines that import from the same path can make code harder to maintain.

  1// bad
  2import foo from 'foo';
  3// … some other imports … //
  4import { named1, named2 } from 'foo';
  5
  6// good
  7import foo, { named1, named2 } from 'foo';
  8
  9// good
  10import foo, {
  11  named1,
  12  named2,
  13} from 'foo';

• 10.5 Do not export mutable bindings. eslint: import/no-mutable-exports

  Why? Mutation should be avoided in general, but in particular when exporting mutable bindings. While this technique may be needed for some special cases, in general, only constant references should be exported.

  1// bad
  2let foo = 3;
  3export { foo };
  4
  5// good
  6const foo = 3;
  7export { foo };

• 10.6 In modules with a single export, prefer default export over named export. eslint: import/prefer-default-export

  Why? To encourage more files that only ever export one thing, which is better for readability and maintainability.

  1// bad
  2export function foo() {}
  3
  4// good
  5export default function foo() {}

• 10.7 Put all imports above non-import statements. eslint: import/first

  Why? Since imports are hoisted, keeping them all at the top prevents surprising behavior.

  1// bad
  2import foo from 'foo';
  3foo.init();
  4
  5import bar from 'bar';
  6
  7// good
  8import foo from 'foo';
  9import bar from 'bar';
  10
  11foo.init();

• 10.8 Multiline imports should be indented just like multiline array and object literals. eslint: object-curly-newline

  Why? The curly braces follow the same indentation rules as every other curly brace block in the style guide, as do the trailing commas.

  1// bad
  2import {longNameA, longNameB, longNameC, longNameD, longNameE} from 'path';
  3
  4// good
  5import {
  6  longNameA,
  7  longNameB,
  8  longNameC,
  9  longNameD,
  10  longNameE,
  11} from 'path';

• 10.9 Disallow Webpack loader syntax in module import statements. eslint: import/no-webpack-loader-syntax

  Why? Since using Webpack syntax in the imports couples the code to a module bundler. Prefer using the loader syntax in webpack.config.js.

  1// bad
  2import fooSass from 'css!sass!foo.scss';
  3import barCss from 'style!css!bar.css';
  4
  5// good
  6import fooSass from 'foo.scss';
  7import barCss from 'bar.css';

• 10.10 Do not include JavaScript filename extensions eslint: import/extensions

  Why? Including extensions inhibits refactoring, and inappropriately hardcodes implementation details of the module you're importing in every consumer.

  1// bad
  2import foo from './foo.js';
  3import bar from './bar.jsx';
  4import baz from './baz/index.jsx';
  5
  6// good
  7import foo from './foo';
  8import bar from './bar';
  9import baz from './baz';

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Iterators and Generators

• 11.1 Don’t use iterators. Prefer JavaScript’s higher-order functions instead of loops like for-in or for-of. eslint: no-iterator no-restricted-syntax

  Why? This enforces our immutable rule. Dealing with pure functions that return values is easier to reason about than side effects.

  Use map() / every() / filter() / find() / findIndex() / reduce() / some() / ... to iterate over arrays, and Object.keys() / Object.values() / Object.entries() to produce arrays so you can iterate over objects.

  1const numbers = [1, 2, 3, 4, 5];
  2
  3// bad
  4let sum = 0;
  5for (let num of numbers) {
  6  sum += num;
  7}
  8sum === 15;
  9
  10// good
  11let sum = 0;
  12numbers.forEach((num) => {
  13  sum += num;
  14});
  15sum === 15;
  16
  17// best (use the functional force)
  18const sum = numbers.reduce((total, num) => total + num, 0);
  19sum === 15;
  20
  21// bad
  22const increasedByOne = [];
  23for (let i = 0; i < numbers.length; i++) {
  24  increasedByOne.push(numbers[i] + 1);
  25}
  26
  27// good
  28const increasedByOne = [];
  29numbers.forEach((num) => {
  30  increasedByOne.push(num + 1);
  31});
  32
  33// best (keeping it functional)
  34const increasedByOne = numbers.map((num) => num + 1);

• 11.2 Don’t use generators for now.

  Why? They don’t transpile well to ES5.

• 11.3 If you must use generators, or if you disregard our advice, make sure their function signature is spaced properly. eslint: generator-star-spacing

  Why? function and * are part of the same conceptual keyword - * is not a modifier for function, function* is a unique construct, different from function.

  1// bad
  2function * foo() {
  3  // ...
  4}
  5
  6// bad
  7const bar = function * () {
  8  // ...
  9};
  10
  11// bad
  12const baz = function *() {
  13  // ...
  14};
  15
  16// bad
  17const quux = function*() {
  18  // ...
  19};
  20
  21// bad
  22function*foo() {
  23  // ...
  24}
  25
  26// bad
  27function *foo() {
  28  // ...
  29}
  30
  31// very bad
  32function
  33*
  34foo() {
  35  // ...
  36}
  37
  38// very bad
  39const wat = function
  40*
  41() {
  42  // ...
  43};
  44
  45// good
  46function* foo() {
  47  // ...
  48}
  49
  50// good
  51const foo = function* () {
  52  // ...
  53};

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Properties

• 12.1 Use dot notation when accessing properties. eslint: dot-notation

  1const luke = {
  2  jedi: true,
  3  age: 28,
  4};
  5
  6// bad
  7const isJedi = luke['jedi'];
  8
  9// good
  10const isJedi = luke.jedi;

• 12.2 Use bracket notation [] when accessing properties with a variable.

  1const luke = {
  2  jedi: true,
  3  age: 28,
  4};
  5
  6function getProp(prop) {
  7  return luke[prop];
  8}
  9
  10const isJedi = getProp('jedi');

• 12.3 Use exponentiation operator ** when calculating exponentiations. eslint: prefer-exponentiation-operator.

  1// bad
  2const binary = Math.pow(2, 10);
  3
  4// good
  5const binary = 2 ** 10;

⬆ back to top

Variables

• 13.1 Always use const or let to declare variables. Not doing so will result in global variables. We want to avoid polluting the global namespace. Captain Planet warned us of that. eslint: no-undef prefer-const

  1// bad
  2superPower = new SuperPower();
  3
  4// good
  5const superPower = new SuperPower();

• 13.2 Use one const or let declaration per variable or assignment. eslint: one-var

  Why? It’s easier to add new variable declarations this way, and you never have to worry about swapping out a ; for a , or introducing punctuation-only diffs. You can also step through each declaration with the debugger, instead of jumping through all of them at once.

  1// bad
  2const items = getItems(),
  3    goSportsTeam = true,
  4    dragonball = 'z';
  5
  6// bad
  7// (compare to above, and try to spot the mistake)
  8const items = getItems(),
  9    goSportsTeam = true;
  10    dragonball = 'z';
  11
  12// good
  13const items = getItems();
  14const goSportsTeam = true;
  15const dragonball = 'z';

• 13.3 Group all your consts and then group all your lets.

  Why? This is helpful when later on you might need to assign a variable depending on one of the previously assigned variables.

  1// bad
  2let i, len, dragonball,
  3    items = getItems(),
  4    goSportsTeam = true;
  5
  6// bad
  7let i;
  8const items = getItems();
  9let dragonball;
  10const goSportsTeam = true;
  11let len;
  12
  13// good
  14const goSportsTeam = true;
  15const items = getItems();
  16let dragonball;
  17let i;
  18let length;

• 13.4 Assign variables where you need them, but place them in a reasonable place.

  Why? let and const are block scoped and not function scoped.

  1// bad - unnecessary function call
  2function checkName(hasName) {
  3  const name = getName();
  4
  5  if (hasName === 'test') {
  6    return false;
  7  }
  8
  9  if (name === 'test') {
  10    this.setName('');
  11    return false;
  12  }
  13
  14  return name;
  15}
  16
  17// good
  18function checkName(hasName) {
  19  if (hasName === 'test') {
  20    return false;
  21  }
  22
  23  const name = getName();
  24
  25  if (name === 'test') {
  26    this.setName('');
  27    return false;
  28  }
  29
  30  return name;
  31}

• 13.5 Don’t chain variable assignments. eslint: no-multi-assign

  Why? Chaining variable assignments creates implicit global variables.

  1// bad
  2(function example() {
  3  // JavaScript interprets this as
  4  // let a = ( b = ( c = 1 ) );
  5  // The let keyword only applies to variable a; variables b and c become
  6  // global variables.
  7  let a = b = c = 1;
  8}());
  9
  10console.log(a); // throws ReferenceError
  11console.log(b); // 1
  12console.log(c); // 1
  13
  14// good
  15(function example() {
  16  let a = 1;
  17  let b = a;
  18  let c = a;
  19}());
  20
  21console.log(a); // throws ReferenceError
  22console.log(b); // throws ReferenceError
  23console.log(c); // throws ReferenceError
  24
  25// the same applies for `const`

• 13.6 Avoid using unary increments and decrements (++, --). eslint no-plusplus

  Why? Per the eslint documentation, unary increment and decrement statements are subject to automatic semicolon insertion and can cause silent errors with incrementing or decrementing values within an application. It is also more expressive to mutate your values with statements like num += 1 instead of num++ or num ++. Disallowing unary increment and decrement statements also prevents you from pre-incrementing/pre-decrementing values unintentionally which can also cause unexpected behavior in your programs.

  1// bad
  2
  3const array = [1, 2, 3];
  4let num = 1;
  5num++;
  6--num;
  7
  8let sum = 0;
  9let truthyCount = 0;
  10for (let i = 0; i < array.length; i++) {
  11  let value = array[i];
  12  sum += value;
  13  if (value) {
  14    truthyCount++;
  15  }
  16}
  17
  18// good
  19
  20const array = [1, 2, 3];
  21let num = 1;
  22num += 1;
  23num -= 1;
  24
  25const sum = array.reduce((a, b) => a + b, 0);
  26const truthyCount = array.filter(Boolean).length;

• 13.7 Avoid linebreaks before or after = in an assignment. If your assignment violates max-len, surround the value in parens. eslint operator-linebreak.

  Why? Linebreaks surrounding = can obfuscate the value of an assignment.

  1// bad
  2const foo =
  3  superLongLongLongLongLongLongLongLongFunctionName();
  4
  5// bad
  6const foo
  7  = 'superLongLongLongLongLongLongLongLongString';
  8
  9// good
  10const foo = (
  11  superLongLongLongLongLongLongLongLongFunctionName()
  12);
  13
  14// good
  15const foo = 'superLongLongLongLongLongLongLongLongString';

• 13.8 Disallow unused variables. eslint: no-unused-vars

  Why? Variables that are declared and not used anywhere in the code are most likely an error due to incomplete refactoring. Such variables take up space in the code and can lead to confusion by readers.

  1// bad
  2
  3const some_unused_var = 42;
  4
  5// Write-only variables are not considered as used.
  6let y = 10;
  7y = 5;
  8
  9// A read for a modification of itself is not considered as used.
  10let z = 0;
  11z = z + 1;
  12
  13// Unused function arguments.
  14function getX(x, y) {
  15    return x;
  16}
  17
  18// good
  19
  20function getXPlusY(x, y) {
  21  return x + y;
  22}
  23
  24const x = 1;
  25const y = a + 2;
  26
  27alert(getXPlusY(x, y));
  28
  29// 'type' is ignored even if unused because it has a rest property sibling.
  30// This is a form of extracting an object that omits the specified keys.
  31const { type, ...coords } = data;
  32// 'coords' is now the 'data' object without its 'type' property.

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Hoisting

• 14.1 var declarations get hoisted to the top of their closest enclosing function scope, their assignment does not. const and let declarations are blessed with a new concept called Temporal Dead Zones (TDZ). It’s important to know why typeof is no longer safe.

  1// we know this wouldn’t work (assuming there
  2// is no notDefined global variable)
  3function example() {
  4  console.log(notDefined); // => throws a ReferenceError
  5}
  6
  7// creating a variable declaration after you
  8// reference the variable will work due to
  9// variable hoisting. Note: the assignment
  10// value of `true` is not hoisted.
  11function example() {
  12  console.log(declaredButNotAssigned); // => undefined
  13  var declaredButNotAssigned = true;
  14}
  15
  16// the interpreter is hoisting the variable
  17// declaration to the top of the scope,
  18// which means our example could be rewritten as:
  19function example() {
  20  let declaredButNotAssigned;
  21  console.log(declaredButNotAssigned); // => undefined
  22  declaredButNotAssigned = true;
  23}
  24
  25// using const and let
  26function example() {
  27  console.log(declaredButNotAssigned); // => throws a ReferenceError
  28  console.log(typeof declaredButNotAssigned); // => throws a ReferenceError
  29  const declaredButNotAssigned = true;
  30}

• 14.2 Anonymous function expressions hoist their variable name, but not the function assignment.

  1function example() {
  2  console.log(anonymous); // => undefined
  3
  4  anonymous(); // => TypeError anonymous is not a function
  5
  6  var anonymous = function () {
  7    console.log('anonymous function expression');
  8  };
  9}

• 14.3 Named function expressions hoist the variable name, not the function name or the function body.

  1function example() {
  2  console.log(named); // => undefined
  3
  4  named(); // => TypeError named is not a function
  5
  6  superPower(); // => ReferenceError superPower is not defined
  7
  8  var named = function superPower() {
  9    console.log('Flying');
  10  };
  11}
  12
  13// the same is true when the function name
  14// is the same as the variable name.
  15function example() {
  16  console.log(named); // => undefined
  17
  18  named(); // => TypeError named is not a function
  19
  20  var named = function named() {
  21    console.log('named');
  22  };
  23}

• 14.4 Function declarations hoist their name and the function body.

  1function example() {
  2  superPower(); // => Flying
  3
  4  function superPower() {
  5    console.log('Flying');
  6  }
  7}

• 14.5 Variables, classes, and functions should be defined before they can be used. eslint: no-use-before-define

  Why? When variables, classes, or functions are declared after being used, it can harm readability since a reader won't know what a thing that's referenced is. It's much clearer for a reader to first encounter the source of a thing (whether imported from another module, or defined in the file) before encountering a use of the thing.

  1// bad
  2
  3// Variable a is being used before it is being defined.
  4console.log(a); // this will be undefined, since while the declaration is hoisted, the initialization is not
  5var a = 10;
  6
  7// Function fun is being called before being defined.
  8fun();
  9function fun() {}
  10
  11// Class A is being used before being defined.
  12new A(); // ReferenceError: Cannot access 'A' before initialization
  13class A {
  14}
  15
  16// `let` and `const` are hoisted, but they don't have a default initialization.
  17// The variables 'a' and 'b' are in a Temporal Dead Zone where JavaScript
  18// knows they exist (declaration is hoisted) but they are not accessible
  19// (as they are not yet initialized).
  20
  21console.log(a); // ReferenceError: Cannot access 'a' before initialization
  22console.log(b); // ReferenceError: Cannot access 'b' before initialization
  23let a = 10;
  24const b = 5;
  25
  26
  27// good
  28
  29var a = 10;
  30console.log(a); // 10
  31
  32function fun() {}
  33fun();
  34
  35class A {
  36}
  37new A();
  38
  39let a = 10;
  40const b = 5;
  41console.log(a); // 10
  42console.log(b); // 5

• For more information refer to JavaScript Scoping & Hoisting by Ben Cherry.

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Comparison Operators & Equality

• 15.1 Use === and !== over == and !=. eslint: eqeqeq

• 15.2 Conditional statements such as the if statement evaluate their expression using coercion with the ToBoolean abstract method and always follow these simple rules:

  • Objects evaluate to true
  • Undefined evaluates to false
  • Null evaluates to false
  • Booleans evaluate to the value of the boolean
  • Numbers evaluate to false if +0, -0, or NaN, otherwise true
  • Strings evaluate to false if an empty string '', otherwise true

  1if ([0] && []) {
  2  // true
  3  // an array (even an empty one) is an object, objects will evaluate to true
  4}

• 15.3 Use shortcuts for booleans, but explicit comparisons for strings and numbers.

  1// bad
  2if (isValid === true) {
  3  // ...
  4}
  5
  6// good
  7if (isValid) {
  8  // ...
  9}
  10
  11// bad
  12if (name) {
  13  // ...
  14}
  15
  16// good
  17if (name !== '') {
  18  // ...
  19}
  20
  21// bad
  22if (collection.length) {
  23  // ...
  24}
  25
  26// good
  27if (collection.length > 0) {
  28  // ...
  29}

• 15.4 For more information see Truth, Equality, and JavaScript by Angus Croll.

• 15.5 Use braces to create blocks in case and default clauses that contain lexical declarations (e.g. let, const, function, and class). eslint: no-case-declarations

  Why? Lexical declarations are visible in the entire switch block but only get initialized when assigned, which only happens when its case is reached. This causes problems when multiple case clauses attempt to define the same thing.

  1// bad
  2switch (foo) {
  3  case 1:
  4    let x = 1;
  5    break;
  6  case 2:
  7    const y = 2;
  8    break;
  9  case 3:
  10    function f() {
  11      // ...
  12    }
  13    break;
  14  default:
  15    class C {}
  16}
  17
  18// good
  19switch (foo) {
  20  case 1: {
  21    let x = 1;
  22    break;
  23  }
  24  case 2: {
  25    const y = 2;
  26    break;
  27  }
  28  case 3: {
  29    function f() {
  30      // ...
  31    }
  32    break;
  33  }
  34  case 4:
  35    bar();
  36    break;
  37  default: {
  38    class C {}
  39  }
  40}

• 15.6 Ternaries should not be nested and generally be single line expressions. eslint: no-nested-ternary

  1// bad
  2const foo = maybe1 > maybe2
  3  ? "bar"
  4  : value1 > value2 ? "baz" : null;
  5
  6// split into 2 separated ternary expressions
  7const maybeNull = value1 > value2 ? 'baz' : null;
  8
  9// better
  10const foo = maybe1 > maybe2
  11  ? 'bar'
  12  : maybeNull;
  13
  14// best
  15const foo = maybe1 > maybe2 ? 'bar' : maybeNull;

• 15.7 Avoid unneeded ternary statements. eslint: no-unneeded-ternary

  1// bad
  2const foo = a ? a : b;
  3const bar = c ? true : false;
  4const baz = c ? false : true;
  5const quux = a != null ? a : b;
  6
  7// good
  8const foo = a || b;
  9const bar = !!c;
  10const baz = !c;
  11const quux = a ?? b;

• 15.8 When mixing operators, enclose th