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 them in parentheses. The only exception is the standard arithmetic operators: +, -, and ** since their precedence is broadly understood. We recommend enclosing / and * in parentheses because their precedence can be ambiguous when they are mixed. eslint: no-mixed-operators

  Why? This improves readability and clarifies the developer’s intention.

  1// bad
  2const foo = a && b < 0 || c > 0 || d + 1 === 0;
  3
  4// bad
  5const bar = a ** b - 5 % d;
  6
  7// bad
  8// one may be confused into thinking (a || b) && c
  9if (a || b && c) {
  10  return d;
  11}
  12
  13// bad
  14const bar = a + b / c * d;
  15
  16// good
  17const foo = (a && b < 0) || c > 0 || (d + 1 === 0);
  18
  19// good
  20const bar = a ** b - (5 % d);
  21
  22// good
  23if (a || (b && c)) {
  24  return d;
  25}
  26
  27// good
  28const bar = a + (b / c) * d;

• 15.9 The nullish coalescing operator (??) is a logical operator that returns its right-hand side operand when its left-hand side operand is null or undefined. Otherwise, it returns the left-hand side operand.

  Why? It provides precision by distinguishing null/undefined from other falsy values, enhancing code clarity and predictability.

  1// bad
  2const value = 0 ?? 'default';
  3// returns 0, not 'default'
  4
  5// bad
  6const value = '' ?? 'default';
  7// returns '', not 'default'
  8
  9// good
  10const value = null ?? 'default';
  11// returns 'default'
  12
  13// good
  14const user = {
  15  name: 'John',
  16  age: null
  17};
  18const age = user.age ?? 18;
  19// returns 18

⬆ back to top

Blocks

• 16.1 Use braces with all multiline blocks. eslint: nonblock-statement-body-position

  1// bad
  2if (test)
  3  return false;
  4
  5// good
  6if (test) return false;
  7
  8// good
  9if (test) {
  10  return false;
  11}
  12
  13// bad
  14function foo() { return false; }
  15
  16// good
  17function bar() {
  18  return false;
  19}

• 16.2 If you’re using multiline blocks with if and else, put else on the same line as your if block’s closing brace. eslint: brace-style

  1// bad
  2if (test) {
  3  thing1();
  4  thing2();
  5}
  6else {
  7  thing3();
  8}
  9
  10// good
  11if (test) {
  12  thing1();
  13  thing2();
  14} else {
  15  thing3();
  16}

• 16.3 If an if block always executes a return statement, the subsequent else block is unnecessary. A return in an else if block following an if block that contains a return can be separated into multiple if blocks. eslint: no-else-return

  1// bad
  2function foo() {
  3  if (x) {
  4    return x;
  5  } else {
  6    return y;
  7  }
  8}
  9
  10// bad
  11function cats() {
  12  if (x) {
  13    return x;
  14  } else if (y) {
  15    return y;
  16  }
  17}
  18
  19// bad
  20function dogs() {
  21  if (x) {
  22    return x;
  23  } else {
  24    if (y) {
  25      return y;
  26    }
  27  }
  28}
  29
  30// good
  31function foo() {
  32  if (x) {
  33    return x;
  34  }
  35
  36  return y;
  37}
  38
  39// good
  40function cats() {
  41  if (x) {
  42    return x;
  43  }
  44
  45  if (y) {
  46    return y;
  47  }
  48}
  49
  50// good
  51function dogs(x) {
  52  if (x) {
  53    if (z) {
  54      return y;
  55    }
  56  } else {
  57    return z;
  58  }
  59}

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Control Statements

• 17.1 In case your control statement (if, while etc.) gets too long or exceeds the maximum line length, each (grouped) condition could be put into a new line. The logical operator should begin the line.

  Why? Requiring operators at the beginning of the line keeps the operators aligned and follows a pattern similar to method chaining. This also improves readability by making it easier to visually follow complex logic.

  1// bad
  2if ((foo === 123 || bar === 'abc') && doesItLookGoodWhenItBecomesThatLong() && isThisReallyHappening()) {
  3  thing1();
  4}
  5
  6// bad
  7if (foo === 123 &&
  8  bar === 'abc') {
  9  thing1();
  10}
  11
  12// bad
  13if (foo === 123
  14  && bar === 'abc') {
  15  thing1();
  16}
  17
  18// bad
  19if (
  20  foo === 123 &&
  21  bar === 'abc'
  22) {
  23  thing1();
  24}
  25
  26// good
  27if (
  28  foo === 123
  29  && bar === 'abc'
  30) {
  31  thing1();
  32}
  33
  34// good
  35if (
  36  (foo === 123 || bar === 'abc')
  37  && doesItLookGoodWhenItBecomesThatLong()
  38  && isThisReallyHappening()
  39) {
  40  thing1();
  41}
  42
  43// good
  44if (foo === 123 && bar === 'abc') {
  45  thing1();
  46}

• 17.2 Don't use selection operators in place of control statements.

  1// bad
  2!isRunning && startRunning();
  3
  4// good
  5if (!isRunning) {
  6  startRunning();
  7}

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Comments

• 18.1 Use /** ... */ for multiline comments.

  1// bad
  2// make() returns a new element
  3// based on the passed in tag name
  4//
  5// @param {String} tag
  6// @return {Element} element
  7function make(tag) {
  8
  9  // ...
  10
  11  return element;
  12}
  13
  14// good
  15/**
  16 * make() returns a new element
  17 * based on the passed-in tag name
  18 */
  19function make(tag) {
  20
  21  // ...
  22
  23  return element;
  24}

• 18.2 Use // for single line comments. Place single line comments on a newline above the subject of the comment. Put an empty line before the comment unless it’s on the first line of a block.

  1// bad
  2const active = true;  // is current tab
  3
  4// good
  5// is current tab
  6const active = true;
  7
  8// bad
  9function getType() {
  10  console.log('fetching type...');
  11  // set the default type to 'no type'
  12  const type = this.type || 'no type';
  13
  14  return type;
  15}
  16
  17// good
  18function getType() {
  19  console.log('fetching type...');
  20
  21  // set the default type to 'no type'
  22  const type = this.type || 'no type';
  23
  24  return type;
  25}
  26
  27// also good
  28function getType() {
  29  // set the default type to 'no type'
  30  const type = this.type || 'no type';
  31
  32  return type;
  33}

• 18.3 Start all comments with a space to make it easier to read. eslint: spaced-comment

  1// bad
  2//is current tab
  3const active = true;
  4
  5// good
  6// is current tab
  7const active = true;
  8
  9// bad
  10/**
  11 *make() returns a new element
  12 *based on the passed-in tag name
  13 */
  14function make(tag) {
  15
  16  // ...
  17
  18  return element;
  19}
  20
  21// good
  22/**
  23 * make() returns a new element
  24 * based on the passed-in tag name
  25 */
  26function make(tag) {
  27
  28  // ...
  29
  30  return element;
  31}

• 18.4 Prefixing your comments with FIXME or TODO helps other developers quickly understand if you’re pointing out a problem that needs to be revisited, or if you’re suggesting a solution to the problem that needs to be implemented. These are different than regular comments because they are actionable. The actions are FIXME: -- need to figure this out or TODO: -- need to implement.

• 18.5 Use // FIXME: to annotate problems.

  1class Calculator extends Abacus {
  2  constructor() {
  3    super();
  4
  5    // FIXME: shouldn’t use a global here
  6    total = 0;
  7  }
  8}

• 18.6 Use // TODO: to annotate solutions to problems.

  1class Calculator extends Abacus {
  2  constructor() {
  3    super();
  4
  5    // TODO: total should be configurable by an options param
  6    this.total = 0;
  7  }
  8}

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Whitespace

• 19.1 Use soft tabs (space character) set to 2 spaces. eslint: indent

  1// bad
  2function foo() {
  3∙∙∙∙let name;
  4}
  5
  6// bad
  7function bar() {
  8∙let name;
  9}
  10
  11// good
  12function baz() {
  13∙∙let name;
  14}

• 19.2 Place 1 space before the leading brace. eslint: space-before-blocks

  1// bad
  2function test(){
  3  console.log('test');
  4}
  5
  6// good
  7function test() {
  8  console.log('test');
  9}
  10
  11// bad
  12dog.set('attr',{
  13  age: '1 year',
  14  breed: 'Bernese Mountain Dog',
  15});
  16
  17// good
  18dog.set('attr', {
  19  age: '1 year',
  20  breed: 'Bernese Mountain Dog',
  21});

• 19.3 Place 1 space before the opening parenthesis in control statements (if, while etc.). Place no space between the argument list and the function name in function calls and declarations. eslint: keyword-spacing

  1// bad
  2if(isJedi) {
  3  fight ();
  4}
  5
  6// good
  7if (isJedi) {
  8  fight();
  9}
  10
  11// bad
  12function fight () {
  13  console.log ('Swooosh!');
  14}
  15
  16// good
  17function fight() {
  18  console.log('Swooosh!');
  19}

• 19.4 Set off operators with spaces. eslint: space-infix-ops

  1// bad
  2const x=y+5;
  3
  4// good
  5const x = y + 5;

• 19.5 End files with a single newline character. eslint: eol-last

  1// bad
  2import { es6 } from './AirbnbStyleGuide';
  3  // ...
  4export default es6;

  1// bad
  2import { es6 } from './AirbnbStyleGuide';
  3  // ...
  4export default es6;↵
  5↵

  1// good
  2import { es6 } from './AirbnbStyleGuide';
  3  // ...
  4export default es6;↵

• 19.6 Use indentation when making long method chains (more than 2 method chains). Use a leading dot, which emphasizes that the line is a method call, not a new statement. eslint: newline-per-chained-call no-whitespace-before-property

  1// bad
  2$('#items').find('.selected').highlight().end().find('.open').updateCount();
  3
  4// bad
  5$('#items').
  6  find('.selected').
  7    highlight().
  8    end().
  9  find('.open').
  10    updateCount();
  11
  12// good
  13$('#items')
  14  .find('.selected')
  15    .highlight()
  16    .end()
  17  .find('.open')
  18    .updateCount();
  19
  20// bad
  21const leds = stage.selectAll('.led').data(data).enter().append('svg:svg').classed('led', true)
  22    .attr('width', (radius + margin) * 2).append('svg:g')
  23    .attr('transform', `translate(${radius + margin}, ${radius + margin})`)
  24    .call(tron.led);
  25
  26// good
  27const leds = stage.selectAll('.led')
  28    .data(data)
  29  .enter().append('svg:svg')
  30    .classed('led', true)
  31    .attr('width', (radius + margin) * 2)
  32  .append('svg:g')
  33    .attr('transform', `translate(${radius + margin}, ${radius + margin})`)
  34    .call(tron.led);
  35
  36// good
  37const leds = stage.selectAll('.led').data(data);
  38const svg = leds.enter().append('svg:svg');
  39svg.classed('led', true).attr('width', (radius + margin) * 2);
  40const g = svg.append('svg:g');
  41g.attr('transform', `translate(${radius + margin}, ${radius + margin})`).call(tron.led);

• 19.7 Leave a blank line after blocks and before the next statement.

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

• 19.8 Do not pad your blocks with blank lines. eslint: padded-blocks

  1// bad
  2function bar() {
  3
  4  console.log(foo);
  5
  6}
  7
  8// bad
  9if (baz) {
  10
  11  console.log(quux);
  12} else {
  13  console.log(foo);
  14
  15}
  16
  17// bad
  18class Foo {
  19
  20  constructor(bar) {
  21    this.bar = bar;
  22  }
  23}
  24
  25// good
  26function bar() {
  27  console.log(foo);
  28}
  29
  30// good
  31if (baz) {
  32  console.log(quux);
  33} else {
  34  console.log(foo);
  35}

• 19.9 Do not use multiple blank lines to pad your code. eslint: no-multiple-empty-lines

  1// bad
  2class Person {
  3  constructor(fullName, email, birthday) {
  4    this.fullName = fullName;
  5
  6
  7    this.email = email;
  8
  9
  10    this.setAge(birthday);
  11  }
  12
  13
  14  setAge(birthday) {
  15    const today = new Date();
  16
  17
  18    const age = this.getAge(today, birthday);
  19
  20
  21    this.age = age;
  22  }
  23
  24
  25  getAge(today, birthday) {
  26    // ..
  27  }
  28}
  29
  30// good
  31class Person {
  32  constructor(fullName, email, birthday) {
  33    this.fullName = fullName;
  34    this.email = email;
  35    this.setAge(birthday);
  36  }
  37
  38  setAge(birthday) {
  39    const today = new Date();
  40    const age = getAge(today, birthday);
  41    this.age = age;
  42  }
  43
  44  getAge(today, birthday) {
  45    // ..
  46  }
  47}

• 19.10 Do not add spaces inside parentheses. eslint: space-in-parens

  1// bad
  2function bar( foo ) {
  3  return foo;
  4}
  5
  6// good
  7function bar(foo) {
  8  return foo;
  9}
  10
  11// bad
  12if ( foo ) {
  13  console.log(foo);
  14}
  15
  16// good
  17if (foo) {
  18  console.log(foo);
  19}

• 19.11 Do not add spaces inside brackets. eslint: array-bracket-spacing

  1// bad
  2const foo = [ 1, 2, 3 ];
  3console.log(foo[ 0 ]);
  4
  5// good
  6const foo = [1, 2, 3];
  7console.log(foo[0]);

• 19.12 Add spaces inside curly braces. eslint: object-curly-spacing

  1// bad
  2const foo = {clark: 'kent'};
  3
  4// good
  5const foo = { clark: 'kent' };

• 19.13 Avoid having lines of code that are longer than 100 characters (including whitespace). Note: per above, long strings are exempt from this rule, and should not be broken up. eslint: max-len

  Why? This ensures readability and maintainability.

  1// bad
  2const foo = jsonData && jsonData.foo && jsonData.foo.bar && jsonData.foo.bar.baz && jsonData.foo.bar.baz.quux && jsonData.foo.bar.baz.quux.xyzzy;
  3
  4// bad
  5$.ajax({ method: 'POST', url: 'https://airbnb.com/', data: { name: 'John' } }).done(() => console.log('Congratulations!')).fail(() => console.log('You have failed this city.'));
  6
  7// good
  8const foo = jsonData
  9  && jsonData.foo
  10  && jsonData.foo.bar
  11  && jsonData.foo.bar.baz
  12  && jsonData.foo.bar.baz.quux
  13  && jsonData.foo.bar.baz.quux.xyzzy;
  14
  15// better
  16const foo = jsonData
  17  ?.foo
  18  ?.bar
  19  ?.baz
  20  ?.quux
  21  ?.xyzzy;
  22
  23// good
  24$.ajax({
  25  method: 'POST',
  26  url: 'https://airbnb.com/',
  27  data: { name: 'John' },
  28})
  29  .done(() => console.log('Congratulations!'))
  30  .fail(() => console.log('You have failed this city.'));

• 19.14 Require consistent spacing inside an open block token and the next token on the same line. This rule also enforces consistent spacing inside a close block token and previous token on the same line. eslint: block-spacing

  1// bad
  2function foo() {return true;}
  3if (foo) { bar = 0;}
  4
  5// good
  6function foo() { return true; }
  7if (foo) { bar = 0; }

• 19.15 Avoid spaces before commas and require a space after commas. eslint: comma-spacing

  1// bad
  2const foo = 1,bar = 2;
  3const arr = [1 , 2];
  4
  5// good
  6const foo = 1, bar = 2;
  7const arr = [1, 2];

• 19.16 Enforce spacing inside of computed property brackets. eslint: computed-property-spacing

  1// bad
  2obj[foo ]
  3obj[ 'foo']
  4const x = {[ b ]: a}
  5obj[foo[ bar ]]
  6
  7// good
  8obj[foo]
  9obj['foo']
  10const x = { [b]: a }
  11obj[foo[bar]]

• 19.17 Avoid spaces between functions and their invocations. eslint: func-call-spacing

  1// bad
  2func ();
  3
  4func
  5();
  6
  7// good
  8func();

• 19.18 Enforce spacing between keys and values in object literal properties. eslint: key-spacing

  1// bad
  2const obj = { foo : 42 };
  3const obj2 = { foo:42 };
  4
  5// good
  6const obj = { foo: 42 };

• 19.19 Avoid trailing spaces at the end of lines. eslint: no-trailing-spaces

• 19.20 Avoid multiple empty lines, only allow one newline at the end of files, and avoid a newline at the beginning of files. eslint: no-multiple-empty-lines

  1// bad - multiple empty lines
  2const x = 1;
  3
  4
  5const y = 2;
  6
  7// bad - 2+ newlines at end of file
  8const x = 1;
  9const y = 2;
  10
  11
  12// bad - 1+ newline(s) at beginning of file
  13
  14const x = 1;
  15const y = 2;
  16
  17// good
  18const x = 1;
  19const y = 2;
  20

⬆ back to top

Commas

• 20.1 Leading commas: Nope. eslint: comma-style

  1// bad
  2const story = [
  3    once
  4  , upon
  5  , aTime
  6];
  7
  8// good
  9const story = [
  10  once,
  11  upon,
  12  aTime,
  13];
  14
  15// bad
  16const hero = {
  17    firstName: 'Ada'
  18  , lastName: 'Lovelace'
  19  , birthYear: 1815
  20  , superPower: 'computers'
  21};
  22
  23// good
  24const hero = {
  25  firstName: 'Ada',
  26  lastName: 'Lovelace',
  27  birthYear: 1815,
  28  superPower: 'computers',
  29};

• 20.2 Additional trailing comma: Yup. eslint: comma-dangle

  Why? This leads to cleaner git diffs. Also, transpilers like Babel will remove the additional trailing comma in the transpiled code which means you don’t have to worry about the trailing comma problem in legacy browsers.

  1// bad - git diff without trailing comma
  2const hero = {
  3     firstName: 'Florence',
  4-    lastName: 'Nightingale'
  5+    lastName: 'Nightingale',
  6+    inventorOf: ['coxcomb chart', 'modern nursing']
  7};
  8
  9// good - git diff with trailing comma
  10const hero = {
  11     firstName: 'Florence',
  12     lastName: 'Nightingale',
  13+    inventorOf: ['coxcomb chart', 'modern nursing'],
  14};

  1// bad
  2const hero = {
  3  firstName: 'Dana',
  4  lastName: 'Scully'
  5};
  6
  7const heroes = [
  8  'Batman',
  9  'Superman'
  10];
  11
  12// good
  13const hero = {
  14  firstName: 'Dana',
  15  lastName: 'Scully',
  16};
  17
  18const heroes = [
  19  'Batman',
  20  'Superman',
  21];
  22
  23// bad
  24function createHero(
  25  firstName,
  26  lastName,
  27  inventorOf
  28) {
  29  // does nothing
  30}
  31
  32// good
  33function createHero(
  34  firstName,
  35  lastName,
  36  inventorOf,
  37) {
  38  // does nothing
  39}
  40
  41// good (note that a comma must not appear after a "rest" element)
  42function createHero(
  43  firstName,
  44  lastName,
  45  inventorOf,
  46  ...heroArgs
  47) {
  48  // does nothing
  49}
  50
  51// bad
  52createHero(
  53  firstName,
  54  lastName,
  55  inventorOf
  56);
  57
  58// good
  59createHero(
  60  firstName,
  61  lastName,
  62  inventorOf,
  63);
  64
  65// good (note that a comma must not appear after a "rest" element)
  66createHero(
  67  firstName,
  68  lastName,
  69  inventorOf,
  70  ...heroArgs
  71);

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Semicolons

• 21.1 Yup. eslint: semi

  Why? When JavaScript encounters a line break without a semicolon, it uses a set of rules called Automatic Semicolon Insertion to determine whether it should regard that line break as the end of a statement, and (as the name implies) place a semicolon into your code before the line break if it thinks so. ASI contains a few eccentric behaviors, though, and your code will break if JavaScript misinterprets your line break. These rules will become more complicated as new features become a part of JavaScript. Explicitly terminating your statements and configuring your linter to catch missing semicolons will help prevent you from encountering issues.

  1// bad - raises exception
  2const luke = {}
  3const leia = {}
  4[luke, leia].forEach((jedi) => jedi.father = 'vader')
  5
  6// bad - raises exception
  7const reaction = "No! That’s impossible!"
  8(async function meanwhileOnTheFalcon() {
  9  // handle `leia`, `lando`, `chewie`, `r2`, `c3p0`
  10  // ...
  11}())
  12
  13// bad - returns `undefined` instead of the value on the next line - always happens when `return` is on a line by itself because of ASI!
  14function foo() {
  15  return
  16    'search your feelings, you know it to be foo'
  17}
  18
  19// good
  20const luke = {};
  21const leia = {};
  22[luke, leia].forEach((jedi) => {
  23  jedi.father = 'vader';
  24});
  25
  26// good
  27const reaction = 'No! That’s impossible!';
  28(async function meanwhileOnTheFalcon() {
  29  // handle `leia`, `lando`, `chewie`, `r2`, `c3p0`
  30  // ...
  31}());
  32
  33// good
  34function foo() {
  35  return 'search your feelings, you know it to be foo';
  36}

  Read more.

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Type Casting & Coercion

• 22.1 Perform type coercion at the beginning of the statement.

• 22.2 Strings: eslint: no-new-wrappers

  1// => this.reviewScore = 9;
  2
  3// bad
  4const totalScore = new String(this.reviewScore); // typeof totalScore is "object" not "string"
  5
  6// bad
  7const totalScore = this.reviewScore + ''; // invokes this.reviewScore.valueOf()
  8
  9// bad
  10const totalScore = this.reviewScore.toString(); // isn’t guaranteed to return a string
  11
  12// good
  13const totalScore = String(this.reviewScore);

• 22.3 Numbers: Use Number for type casting and parseInt always with a radix for parsing strings. eslint: radix no-new-wrappers

  Why? The parseInt function produces an integer value dictated by interpretation of the contents of the string argument according to the specified radix. Leading whitespace in string is ignored. If radix is undefined or 0, it is assumed to be 10 except when the number begins with the character pairs 0x or 0X, in which case a radix of 16 is assumed. This differs from ECMAScript 3, which merely discouraged (but allowed) octal interpretation. Many implementations have not adopted this behavior as of 2013. And, because older browsers must be supported, always specify a radix.

  1const inputValue = '4';
  2
  3// bad
  4const val = new Number(inputValue);
  5
  6// bad
  7const val = +inputValue;
  8
  9// bad
  10const val = inputValue >> 0;
  11
  12// bad
  13const val = parseInt(inputValue);
  14
  15// good
  16const val = Number(inputValue);
  17
  18// good
  19const val = parseInt(inputValue, 10);

• 22.4 If for whatever reason you are doing something wild and parseInt is your bottleneck and need to use Bitshift for performance reasons, leave a comment explaining why and what you’re doing.

  1// good
  2/**
  3 * parseInt was the reason my code was slow.
  4 * Bitshifting the String to coerce it to a
  5 * Number made it a lot faster.
  6 */
  7const val = inputValue >> 0;

• 22.5 Note: Be careful when using bitshift operations. Numbers are represented as 64-bit values, but bitshift operations always return a 32-bit integer (source). Bitshift can lead to unexpected behavior for integer values larger than 32 bits. Discussion. Largest signed 32-bit Int is 2,147,483,647:

  12147483647 >> 0; // => 2147483647
  22147483648 >> 0; // => -2147483648
  32147483649 >> 0; // => -2147483647

• 22.6 Booleans: eslint: no-new-wrappers

  1const age = 0;
  2
  3// bad
  4const hasAge = new Boolean(age);
  5
  6// good
  7const hasAge = Boolean(age);
  8
  9// best
  10const hasAge = !!age;

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Naming Conventions

• 23.1 Avoid single letter names. Be descriptive with your naming. eslint: id-length

  1// bad
  2function q() {
  3  // ...
  4}
  5
  6// good
  7function query() {
  8  // ...
  9}

• 23.2 Use camelCase when naming objects, functions, and instances. eslint: camelcase

  1// bad
  2const OBJEcttsssss = {};
  3const this_is_my_object = {};
  4function c() {}
  5
  6// good
  7const thisIsMyObject = {};
  8function thisIsMyFunction() {}

• 23.3 Use PascalCase only when naming constructors or classes. eslint: new-cap

  1// bad
  2function user(options) {
  3  this.name = options.name;
  4}
  5
  6const bad = new user({
  7  name: 'nope',
  8});
  9
  10// good
  11class User {
  12  constructor(options) {
  13    this.name = options.name;
  14  }
  15}
  16
  17const good = new User({
  18  name: 'yup',
  19});

• 23.4 Do not use trailing or leading underscores. eslint: no-underscore-dangle

  Why? JavaScript does not have the concept of privacy in terms of properties or methods. Although a leading underscore is a common convention to mean “private”, in fact, these properties are fully public, and as such, are part of your public API contract. This convention might lead developers to wrongly think that a change won’t count as breaking, or that tests aren’t needed. tl;dr: if you want something to be “private”, it must not be observably present.

  1// bad
  2this.__firstName__ = 'Panda';
  3this.firstName_ = 'Panda';
  4this._firstName = 'Panda';
  5
  6// good
  7this.firstName = 'Panda';
  8
  9// good, in environments where WeakMaps are available
  10// see https://compat-table.github.io/compat-table/es6/#test-WeakMap
  11const firstNames = new WeakMap();
  12firstNames.set(this, 'Panda');

• 23.5 Don’t save references to this. Use arrow functions or Function#bind.

  1// bad
  2function foo() {
  3  const self = this;
  4  return function () {
  5    console.log(self);
  6  };
  7}
  8
  9// bad
  10function foo() {
  11  const that = this;
  12  return function () {
  13    console.log(that);
  14  };
  15}
  16
  17// good
  18function foo() {
  19  return () => {
  20    console.log(this);
  21  };
  22}

• 23.6 A base filename should exactly match the name of its default export.

  1// file 1 contents
  2class CheckBox {
  3  // ...
  4}
  5export default CheckBox;
  6
  7// file 2 contents
  8export default function fortyTwo() { return 42; }
  9
  10// file 3 contents
  11export default function insideDirectory() {}
  12
  13// in some other file
  14// bad
  15import CheckBox from './checkBox'; // PascalCase import/export, camelCase filename
  16import FortyTwo from './FortyTwo'; // PascalCase import/filename, camelCase export
  17import InsideDirectory from './InsideDirectory'; // PascalCase import/filename, camelCase export
  18
  19// bad
  20import CheckBox from './check_box'; // PascalCase import/export, snake_case filename
  21import forty_two from './forty_two'; // snake_case import/filename, camelCase export
  22import inside_directory from './inside_directory'; // snake_case import, camelCase export
  23import index from './inside_directory/index'; // requiring the index file explicitly
  24import insideDirectory from './insideDirectory/index'; // requiring the index file explicitly
  25
  26// good
  27import CheckBox from './CheckBox'; // PascalCase export/import/filename
  28import fortyTwo from './fortyTwo'; // camelCase export/import/filename
  29import insideDirectory from './insideDirectory'; // camelCase export/import/directory name/implicit "index"
  30// ^ supports both insideDirectory.js and insideDirectory/index.js

• 23.7 Use camelCase when you export-default a function. Your filename should be identical to your function’s name.

  1function makeStyleGuide() {
  2  // ...
  3}
  4
  5export default makeStyleGuide;

• 23.8 Use PascalCase when you export a constructor / class / singleton / function library / bare object.

  1const AirbnbStyleGuide = {
  2  es6: {
  3  },
  4};
  5
  6export default AirbnbStyleGuide;

• 23.9 Acronyms and initialisms should always be all uppercased, or all lowercased.

  Why? Names are for readability, not to appease a computer algorithm.

  1// bad
  2import SmsContainer from './containers/SmsContainer';
  3
  4// bad
  5const HttpRequests = [
  6  // ...
  7];
  8
  9// good
  10import SMSContainer from './containers/SMSContainer';
  11
  12// good
  13const HTTPRequests = [
  14  // ...
  15];
  16
  17// also good
  18const httpRequests = [
  19  // ...
  20];
  21
  22// best
  23import TextMessageContainer from './containers/TextMessageContainer';
  24
  25// best
  26const requests = [
  27  // ...
  28];

• 23.10 You may optionally uppercase a constant only if it (1) is exported, (2) is a const (it can not be reassigned), and (3) the programmer can trust it (and its nested properties) to never change.

  Why? This is an additional tool to assist in situations where the programmer would be unsure if a variable might ever change. UPPERCASE_VARIABLES are letting the programmer know that they can trust the variable (and its properties) not to change.

  • What about all const variables? - This is unnecessary, so uppercasing should not be used for constants within a file. It should be used for exported constants however.
  • What about exported objects? - Uppercase at the top level of export (e.g. EXPORTED_OBJECT.key) and maintain that all nested properties do not change.

  1// bad
  2const PRIVATE_VARIABLE = 'should not be unnecessarily uppercased within a file';
  3
  4// bad
  5export const THING_TO_BE_CHANGED = 'should obviously not be uppercased';
  6
  7// bad
  8export let REASSIGNABLE_VARIABLE = 'do not use let with uppercase variables';
  9
  10// ---
  11
  12// allowed but does not supply semantic value
  13export const apiKey = 'SOMEKEY';
  14
  15// better in most cases
  16export const API_KEY = 'SOMEKEY';
  17
  18// ---
  19
  20// bad - unnecessarily uppercases key while adding no semantic value
  21export const MAPPING = {
  22  KEY: 'value'
  23};
  24
  25// good
  26export const MAPPING = {
  27  key: 'value',
  28};

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Accessors

• 24.1 Accessor functions for properties are not required.

• 24.2 Do not use JavaScript getters/setters as they cause unexpected side effects and are harder to test, maintain, and reason about. Instead, if you do make accessor functions, use getVal() and setVal('hello').

  1// bad
  2class Dragon {
  3  get age() {
  4    // ...
  5  }
  6
  7  set age(value) {
  8    // ...
  9  }
  10}
  11
  12// good
  13class Dragon {
  14  getAge() {
  15    // ...
  16  }
  17
  18  setAge(value) {
  19    // ...
  20  }
  21}

• 24.3 If the property/method is a boolean, use isVal() or hasVal().

  1// bad
  2if (!dragon.age()) {
  3  return false;
  4}
  5
  6// good
  7if (!dragon.hasAge()) {
  8  return false;
  9}

• 24.4 It’s okay to create get() and set() functions, but be consistent.

  1class Jedi {
  2  constructor(options = {}) {
  3    const lightsaber = options.lightsaber || 'blue';
  4    this.set('lightsaber', lightsaber);
  5  }
  6
  7  set(key, val) {
  8    this[key] = val;
  9  }
  10
  11  get(key) {
  12    return this[key];
  13  }
  14}

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Events

• 25.1 When attaching data payloads to events (whether DOM events or something more proprietary like Backbone events), pass an object literal (also known as a "hash") instead of a raw value. This allows a subsequent contributor to add more data to the event payload without finding and updating every handler for the event. For example, instead of:

  1// bad
  2$(this).trigger('listingUpdated', listing.id);
  3
  4// ...
  5
  6$(this).on('listingUpdated', (e, listingID) => {
  7  // do something with listingID
  8});

  prefer:

  1// good
  2$(this).trigger('listingUpdated', { listingID: listing.id });
  3
  4// ...
  5
  6$(this).on('listingUpdated', (e, data) => {
  7  // do something with data.listingID
  8});

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jQuery

• 26.1 Prefix jQuery object variables with a $.

  1// bad
  2const sidebar = $('.sidebar');
  3
  4// good
  5const $sidebar = $('.sidebar');
  6
  7// good
  8const $sidebarBtn = $('.sidebar-btn');

• 26.2 Cache jQuery lookups.

  1// bad
  2function setSidebar() {
  3  $('.sidebar').hide();
  4
  5  // ...
  6
  7  $('.sidebar').css({
  8    'background-color': 'pink',
  9  });
  10}
  11
  12// good
  13function setSidebar() {
  14  const $sidebar = $('.sidebar');
  15  $sidebar.hide();
  16
  17  // ...
  18
  19  $sidebar.css({
  20    'background-color': 'pink',
  21  });
  22}

• 26.3 For DOM queries use Cascading $('.sidebar ul') or parent > child $('.sidebar > ul'). jsPerf

• 26.4 Use find with scoped jQuery object queries.

  1// bad
  2$('ul', '.sidebar').hide();
  3
  4// bad
  5$('.sidebar').find('ul').hide();
  6
  7// good
  8$('.sidebar ul').hide();
  9
  10// good
  11$('.sidebar > ul').hide();
  12
  13// good
  14$sidebar.find('ul').hide();

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ECMAScript 5 Compatibility

• 27.1 Refer to Kangax’s ES5 compatibility table.

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ECMAScript 6+ (ES 2015+) Styles

• 28.1 This is a collection of links to the various ES6+ features.

1. Arrow Functions
2. Classes
3. Object Shorthand
4. Object Concise
5. Object Computed Properties
6. Template Strings
7. Destructuring
8. Default Parameters
9. Rest
10. Array Spreads
11. Let and Const
12. Exponentiation Operator
13. Iterators and Generators
14. Modules

• 28.2 Do not use TC39 proposals that have not reached stage 3.

  Why? They are not finalized, and they are subject to change or to be withdrawn entirely. We want to use JavaScript, and proposals are not JavaScript yet.

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Standard Library

The Standard Library contains utilities that are functionally broken but remain for legacy reasons.

• 29.1 Use Number.isNaN instead of global isNaN. eslint: no-restricted-globals

  Why? The global isNaN coerces non-numbers to numbers, returning true for anything that coerces to NaN. If this behavior is desired, make it explicit.

  1// bad
  2isNaN('1.2'); // false
  3isNaN('1.2.3'); // true
  4
  5// good
  6Number.isNaN('1.2.3'); // false
  7Number.isNaN(Number('1.2.3')); // true

• 29.2 Use Number.isFinite instead of global isFinite. eslint: no-restricted-globals

  Why? The global isFinite coerces non-numbers to numbers, returning true for anything that coerces to a finite number. If this behavior is desired, make it explicit.

  1// bad
  2isFinite('2e3'); // true
  3
  4// good
  5Number.isFinite('2e3'); // false
  6Number.isFinite(parseInt('2e3', 10)); // true

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Testing

• 30.1 Yup.

  1function foo() {
  2  return true;
  3}

• 30.2 No, but seriously:
  • Whichever testing framework you use, you should be writing tests!
  • Strive to write many small pure functions, and minimize where mutations occur.
  • Be cautious about stubs and mocks - they can make your tests more brittle.
  • We primarily use mocha and jest at Airbnb. tape is also used occasionally for small, separate modules.
  • 100% test coverage is a good goal to strive for, even if it’s not always practical to reach it.
  • Whenever you fix a bug, write a regression test. A bug fixed without a regression test is almost certainly going to break again in the future.

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Performance

• On Layout & Web Performance
• String vs Array Concat
• Try/Catch Cost In a Loop
• Bang Function
• jQuery Find vs Context, Selector
• innerHTML vs textContent for script text
• Long String Concatenation
• Are JavaScript functions like map(), reduce(), and filter() optimized for traversing arrays?
• Loading...

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Resources

Learning ES6+

• Latest ECMA spec
• ExploringJS
• ES6 Compatibility Table
• Comprehensive Overview of ES6 Features
• JavaScript Roadmap

Read This

• Standard ECMA-262

Tools

• Code Style Linters
  • ESlint - Airbnb Style .eslintrc
  • JSHint - Airbnb Style .jshintrc
• Neutrino Preset - @neutrinojs/airbnb

Other Style Guides

• Google JavaScript Style Guide
• Google JavaScript Style Guide (Old)
• jQuery Core Style Guidelines
• Principles of Writing Consistent, Idiomatic JavaScript
• StandardJS

Other Styles

• Naming this in nested functions - Christian Johansen
• Conditional Callbacks - Ross Allen
• Popular JavaScript Coding Conventions on GitHub - JeongHoon Byun
• Multiple var statements in JavaScript, not superfluous - Ben Alman

Further Reading

• Understanding JavaScript Closures - Angus Croll
• Basic JavaScript for the impatient programmer - Dr. Axel Rauschmayer
• You Might Not Need jQuery - Zack Bloom & Adam Schwartz
• ES6 Features - Luke Hoban
• Frontend Guidelines - Benjamin De Cock

Books

• JavaScript: The Good Parts - Douglas Crockford
• JavaScript Patterns - Stoyan Stefanov
• Pro JavaScript Design Patterns - Ross Harmes and Dustin Diaz
• High Performance Web Sites: Essential Knowledge for Front-End Engineers - Steve Souders
• Maintainable JavaScript - Nicholas C. Zakas
• JavaScript Web Applications - Alex MacCaw
• Pro JavaScript Techniques - John Resig
• Smashing Node.js: JavaScript Everywhere - Guillermo Rauch
• Secrets of the JavaScript Ninja - John Resig and Bear Bibeault
• Human JavaScript - Henrik Joreteg
• Superhero.js - Kim Joar Bekkelund, Mads Mobæk, & Olav Bjorkoy
• JSBooks - Julien Bouquillon
• Third Party JavaScript - Ben Vinegar and Anton Kovalyov
• Effective JavaScript: 68 Specific Ways to Harness the Power of JavaScript - David Herman
• Eloquent JavaScript - Marijn Haverbeke
• You Don’t Know JS: ES6 & Beyond - Kyle Simpson

Blogs

• JavaScript Weekly
• JavaScript, JavaScript...
• Bocoup Weblog
• Adequately Good
• NCZOnline
• Perfection Kills
• Ben Alman
• Dmitry Baranovskiy
• nettuts

Podcasts

• JavaScript Air
• JavaScript Jabber

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In the Wild

This is a list of organizations that are using this style guide. Send us a pull request and we'll add you to the list.

• 123erfasst: 123erfasst/javascript
• 4Catalyzer: 4Catalyzer/javascript
• Aan Zee: AanZee/javascript
• Airbnb: airbnb/javascript
• AloPeyk: AloPeyk
• AltSchool: AltSchool/javascript
• Apartmint: apartmint/javascript
• Ascribe: ascribe/javascript
• Avant: avantcredit/javascript
• Axept: axept/javascript
• Billabong: billabong/javascript
• Bisk: bisk
• Brainshark: brainshark/javascript
• CaseNine: CaseNine/javascript
• Cerner: Cerner
• Chartboost: ChartBoost/javascript-style-guide
• Coeur d'Alene Tribe: www.cdatribe-nsn.gov
• ComparaOnline: comparaonline/javascript
• Compass Learning: compasslearning/javascript-style-guide
• DailyMotion: dailymotion/javascript
• DoSomething: DoSomething/eslint-config
• Digitpaint digitpaint/javascript
• Drupal: www.drupal.org
• Ecosia: ecosia/javascript
• Evolution Gaming: evolution-gaming/javascript
• EvozonJs: evozonjs/javascript
• ExactTarget: ExactTarget/javascript
• Flexberry: Flexberry/javascript-style-guide
• Gawker Media: gawkermedia
• General Electric: GeneralElectric/javascript
• Generation Tux: GenerationTux/javascript
• GoodData: gooddata/gdc-js-style
• GreenChef: greenchef/javascript
• Grooveshark: grooveshark/javascript
• Grupo-Abraxas: Grupo-Abraxas/javascript
• Happeo: happeo/javascript
• How About We: howaboutwe/javascript
• HubSpot: HubSpot/javascript
• Hyper: hyperoslo/javascript-playbook
• ILIAS: ILIAS
• InterCity Group: intercitygroup/javascript-style-guide
• Jam3: Jam3/Javascript-Code-Conventions
• JSSolutions: JSSolutions/javascript
• Kaplan Komputing: kaplankomputing/javascript
• KickorStick: kickorstick
• Kinetica Solutions: kinetica/javascript
• LEINWAND: LEINWAND/javascript
• Lonely Planet: lonelyplanet/javascript
• M2GEN: M2GEN/javascript
• Mighty Spring: mightyspring/javascript
• MinnPost: MinnPost/javascript
• MitocGroup: MitocGroup/javascript
• Muber: muber
• National Geographic Society: natgeosociety
• NullDev: NullDevCo/JavaScript-Styleguide
• Nulogy: nulogy/javascript
• Orange Hill Development: orangehill/javascript
• Orion Health: orionhealth/javascript
• Peerby: Peerby/javascript
• Pier 1: Pier1/javascript
• Qotto: Qotto/javascript-style-guide
• React: reactjs.org/docs/how-to-contribute.html#style-guide
• Ripple: ripple/javascript-style-guide
• Sainsbury’s Supermarkets: jsainsburyplc
• Shutterfly: shutterfly/javascript
• Sourcetoad: sourcetoad/javascript
• Springload: springload
• StratoDem Analytics: stratodem/javascript
• SteelKiwi Development: steelkiwi/javascript
• StudentSphere: studentsphere/javascript
• SwoopApp: swoopapp/javascript
• SysGarage: sysgarage/javascript-style-guide
• Syzygy Warsaw: syzygypl/javascript
• Target: target/javascript
• Terra: terra
• TheLadders: TheLadders/javascript
• The Nerdery: thenerdery/javascript-standards
• Tomify: tomprats
• Traitify: traitify/eslint-config-traitify
• T4R Technology: T4R-Technology/javascript
• UrbanSim: urbansim
• VoxFeed: VoxFeed/javascript-style-guide
• WeBox Studio: weboxstudio/javascript
• Weggo: Weggo/javascript
• Zillow: zillow/javascript
• Zit Software: zit-software/javascript
• ZocDoc: ZocDoc/javascript

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Translation

This style guide is also available in other languages:

• Brazilian Portuguese: armoucar/javascript-style-guide
• Bulgarian: borislavvv/javascript
• Catalan: fpmweb/javascript-style-guide
• Chinese (Simplified): lin-123/javascript
• Chinese (Traditional): jigsawye/javascript
• French: nmussy/javascript-style-guide
• German: timofurrer/javascript-style-guide
• Italian: sinkswim/javascript-style-guide
• Japanese: mitsuruog/javascript-style-guide
• Korean: ParkSB/javascript-style-guide
• Russian: leonidlebedev/javascript-airbnb
• Spanish: paolocarrasco/javascript-style-guide
• Thai: lvarayut/javascript-style-guide
• Turkish: eraycetinay/javascript
• Ukrainian: ivanzusko/javascript
• Vietnam: dangkyokhoang/javascript-style-guide

The JavaScript Style Guide Guide

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License

(The MIT License)

Copyright (c) 2012 Airbnb

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the 'Software'), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED 'AS IS', WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

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Amendments

We encourage you to fork this guide and change the rules to fit your team’s style guide. Below, you may list some amendments to the style guide. This allows you to periodically update your style guide without having to deal with merge conflicts.

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