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
  
  

  const foo = 1; let bar = foo; bar = 9; console.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
  
  

  const foo = [1, 2]; const bar = foo; bar[0] = 9; console.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.

  // bad var a = 1; var b = 2; // good const a = 1; const 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.

  // bad var count = 1; if (true) { count += 1; } // good, use the let. let count = 1; if (true) { count += 1; }

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

  // const and let only exist in the blocks they are defined in. { let a = 1; const b = 1; var c = 1; } console.log(a); // ReferenceError console.log(b); // ReferenceError console.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

  // bad const item = new Object(); // good const 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.

  function getKey(k) { return `a key named ${k}`; } // bad const obj = { id: 5, name: 'San Francisco', }; obj[getKey('enabled')] = true; // good const obj = { id: 5, name: 'San Francisco', [getKey('enabled')]: true, };

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

  // bad const atom = { value: 1, addValue: function (value) { return atom.value + value; }, }; // good const atom = { value: 1, addValue(value) { return atom.value + value; }, };

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

  Why? It is shorter and descriptive.

  const lukeSkywalker = 'Luke Skywalker'; // bad const obj = { lukeSkywalker: lukeSkywalker, }; // good const obj = { lukeSkywalker, };

• 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.

  const anakinSkywalker = 'Anakin Skywalker'; const lukeSkywalker = 'Luke Skywalker'; // bad const obj = { episodeOne: 1, twoJediWalkIntoACantina: 2, lukeSkywalker, episodeThree: 3, mayTheFourth: 4, anakinSkywalker, }; // good const obj = { lukeSkywalker, anakinSkywalker, episodeOne: 1, twoJediWalkIntoACantina: 2, episodeThree: 3, mayTheFourth: 4, };

• 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.

  // bad const bad = { 'foo': 3, 'bar': 4, 'data-blah': 5, }; // good const good = { foo: 3, bar: 4, 'data-blah': 5, };

• 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.

  // bad console.log(object.hasOwnProperty(key)); // good console.log(Object.prototype.hasOwnProperty.call(object, key)); // better const has = Object.prototype.hasOwnProperty; // cache the lookup once, in module scope. console.log(has.call(object, key)); // best console.log(Object.hasOwn(object, key)); // only supported in browsers that support ES2022 /* or */ import has from 'has'; // https://www.npmjs.com/package/has console.log(has(object, key)); /* or */ console.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

  // very bad const original = { a: 1, b: 2 }; const copy = Object.assign(original, { c: 3 }); // this mutates `original` ಠ_ಠ delete copy.a; // so does this // bad const original = { a: 1, b: 2 }; const copy = Object.assign({}, original, { c: 3 }); // copy => { a: 1, b: 2, c: 3 } // good const original = { a: 1, b: 2 }; const copy = { ...original, c: 3 }; // copy => { a: 1, b: 2, c: 3 } const { 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

  // bad const items = new Array(); // good const items = [];

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

  const someStack = []; // bad someStack[someStack.length] = 'abracadabra'; // good someStack.push('abracadabra');

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

  // bad const len = items.length; const itemsCopy = []; let i; for (i = 0; i < len; i += 1) { itemsCopy[i] = items[i]; } // good const itemsCopy = [...items];

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

  const foo = document.querySelectorAll('.foo'); // good const nodes = Array.from(foo); // best const nodes = [...foo];

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

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

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

  // bad const baz = [...foo].map(bar); // good const 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

  // good [1, 2, 3].map((x) => { const y = x + 1; return x * y; }); // good [1, 2, 3].map((x) => x + 1); // bad - no returned value means `acc` becomes undefined after the first iteration [[0, 1], [2, 3], [4, 5]].reduce((acc, item, index) => { const flatten = acc.concat(item); }); // good [[0, 1], [2, 3], [4, 5]].reduce((acc, item, index) => { const flatten = acc.concat(item); return flatten; }); // bad inbox.filter((msg) => { const { subject, author } = msg; if (subject === 'Mockingbird') { return author === 'Harper Lee'; } else { return false; } }); // good inbox.filter((msg) => { const { subject, author } = msg; if (subject === 'Mockingbird') { return author === 'Harper Lee'; } return false; });

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

  // bad const arr = [ [0, 1], [2, 3], [4, 5], ]; const objectInArray = [{ id: 1, }, { id: 2, }]; const numberInArray = [ 1, 2, ]; // good const arr = [[0, 1], [2, 3], [4, 5]]; const objectInArray = [ { id: 1, }, { id: 2, }, ]; const numberInArray = [ 1, 2, ];

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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.

  // bad function getFullName(user) { const firstName = user.firstName; const lastName = user.lastName; return `${firstName} ${lastName}`; } // good function getFullName(user) { const { firstName, lastName } = user; return `${firstName} ${lastName}`; } // best function getFullName({ firstName, lastName }) { return `${firstName} ${lastName}`; }

• 5.2 Use array destructuring. eslint: prefer-destructuring

  const arr = [1, 2, 3, 4]; // bad const first = arr[0]; const second = arr[1]; // good const [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.

  // bad function processInput(input) { // then a miracle occurs return [left, right, top, bottom]; } // the caller needs to think about the order of return data const [left, __, top] = processInput(input); // good function processInput(input) { // then a miracle occurs return { left, right, top, bottom }; } // the caller selects only the data they need const { left, top } = processInput(input);

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Strings

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

  // bad const name = "Capt. Janeway"; // bad - template literals should contain interpolation or newlines const name = `Capt. Janeway`; // good const 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.

  // bad const 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.'; // bad const 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.'; // good const 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.

  // bad function sayHi(name) { return 'How are you, ' + name + '?'; } // bad function sayHi(name) { return ['How are you, ', name, '?'].join(); } // bad function sayHi(name) { return `How are you, ${ name }?`; } // good function sayHi(name) { return `How are you, ${name}?`; }

• 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.

  // bad const foo = '\'this\' \i\s \"quoted\"'; // good const foo = '\'this\' is "quoted"'; const 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)

  // bad function foo() { // ... } // bad const foo = function () { // ... }; // good // lexical name distinguished from the variable-referenced invocation(s) const short = function longUniqueMoreDescriptiveLexicalFoo() { // ... };

• 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.

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

• 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.

  // bad if (currentUser) { function test() { console.log('Nope.'); } } // good let test; if (currentUser) { test = () => { console.log('Yup.'); }; }

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

  // bad function foo(name, options, arguments) { // ... } // good function foo(name, options, args) { // ... }

• 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.

  // bad function concatenateAll() { const args = Array.prototype.slice.call(arguments); return args.join(''); } // good function concatenateAll(...args) { return args.join(''); }

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

  // really bad function handleThings(opts) { // No! We shouldn’t mutate function arguments. // Double bad: if opts is falsy it'll be set to an object which may // be what you want but it can introduce subtle bugs. opts = opts || {}; // ... } // still bad function handleThings(opts) { if (opts === void 0) { opts = {}; } // ... } // good function handleThings(opts = {}) { // ... }

• 7.8 Avoid side effects with default parameters.

  Why? They are confusing to reason about.

  let b = 1; // bad function count(a = b++) { console.log(a); } count(); // 1 count(); // 2 count(3); // 3 count(); // 3

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

  // bad function handleThings(opts = {}, name) { // ... } // good function handleThings(name, opts = {}) { // ... }

• 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.

  // bad const add = new Function('a', 'b', 'return a + b'); // still bad const 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.

  // bad const f = function(){}; const g = function (){}; const h = function() {}; // good const x = function () {}; const 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.

  // bad function f1(obj) { obj.key = 1; } // good function f2(obj) { const key = Object.prototype.hasOwnProperty.call(obj, 'key') ? obj.key : 1; }

• 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.

  // bad function f1(a) { a = 1; // ... } function f2(a) { if (!a) { a = 1; } // ... } // good function f3(a) { const b = a || 1; // ... } function f4(a = 1) { // ... }

• 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.

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

  // bad function foo(bar, baz, quux) { // ... } // good function foo( bar, baz, quux, ) { // ... } // bad console.log(foo, bar, baz); // good console.log( foo, bar, baz, );

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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.

  // bad [1, 2, 3].map(function (x) { const y = x + 1; return x * y; }); // good [1, 2, 3].map((x) => { const y = x + 1; return x * y; });

• 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.

  // bad [1, 2, 3].map((number) => { const nextNumber = number + 1; `A string containing the ${nextNumber}.`; }); // good [1, 2, 3].map((number) => `A string containing the ${number + 1}.`); // good [1, 2, 3].map((number) => { const nextNumber = number + 1; return `A string containing the ${nextNumber}.`; }); // good [1, 2, 3].map((number, index) => ({ [index]: number, })); // No implicit return with side effects function foo(callback) { const val = callback(); if (val === true) { // Do something if callback returns true } } let bool = false; // bad foo(() => bool = true); // good foo(() => { bool = true; });

• 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.

  // bad ['get', 'post', 'put'].map((httpMethod) => Object.prototype.hasOwnProperty.call( httpMagicObjectWithAVeryLongName, httpMethod, ) ); // good ['get', 'post', 'put'].map((httpMethod) => ( Object.prototype.hasOwnProperty.call( httpMagicObjectWithAVeryLongName, httpMethod, ) ));

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

  Why? Minimizes diff churn when adding or removing arguments.

  // bad [1, 2, 3].map(x => x * x); // good [1, 2, 3].map((x) => x * x); // bad [1, 2, 3].map(number => ( `A long string with the ${number}. It’s so long that we don’t want it to take up space on the .map line!` )); // good [1, 2, 3].map((number) => ( `A long string with the ${number}. It’s so long that we don’t want it to take up space on the .map line!` )); // bad [1, 2, 3].map(x => { const y = x + 1; return x * y; }); // good [1, 2, 3].map((x) => { const y = x + 1; return x * y; });

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

  // bad const itemHeight = (item) => item.height <= 256 ? item.largeSize : item.smallSize; // bad const itemHeight = (item) => item.height >= 256 ? item.largeSize : item.smallSize; // good const itemHeight = (item) => (item.height <= 256 ? item.largeSize : item.smallSize); // good const itemHeight = (item) => { const { height, largeSize, smallSize } = item; return height <= 256 ? largeSize : smallSize; };

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

  // bad (foo) => bar; (foo) => (bar); // good (foo) => bar; (foo) => (bar); (foo) => ( bar )

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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.

  // bad function Queue(contents = []) { this.queue = [...contents]; } Queue.prototype.pop = function () { const value = this.queue[0]; this.queue.splice(0, 1); return value; }; // good class Queue { constructor(contents = []) { this.queue = [...contents]; } pop() { const value = this.queue[0]; this.queue.splice(0, 1); return value; } }

• 9.2 Use extends for inheritance.

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

  // bad const inherits = require('inherits'); function PeekableQueue(contents) { Queue.apply(this, contents); } inherits(PeekableQueue, Queue); PeekableQueue.prototype.peek = function () { return this.queue[0]; }; // good class PeekableQueue extends Queue { peek() { return this.queue[0]; } }

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

  // bad Jedi.prototype.jump = function () { this.jumping = true; return true; }; Jedi.prototype.setHeight = function (height) { this.height = height; }; const luke = new Jedi(); luke.jump(); // => true luke.setHeight(20); // => undefined // good class Jedi { jump() { this.jumping = true; return this; } setHeight(height) { this.height = height; return this; } } const luke = new Jedi(); luke.jump() .setHeight(20);

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

  class Jedi { constructor(options = {}) { this.name = options.name || 'no name'; } getName() { return this.name; } toString() { return `Jedi - ${this.getName()}`; } }

• 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

  // bad class Jedi { constructor() {} getName() { return this.name; } } // bad class Rey extends Jedi { constructor(...args) { super(...args); } } // good class Rey extends Jedi { constructor(...args) { super(...args); this.name = 'Rey'; } }

• 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.

  // bad class Foo { bar() { return 1; } bar() { return 2; } } // good class Foo { bar() { return 1; } } // good class Foo { bar() { return 2; } }

• 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

  // bad class Foo { bar() { console.log('bar'); } } // good - this is used class Foo { bar() { console.log(this.bar); } } // good - constructor is exempt class Foo { constructor() { // ... } } // good - static methods aren't expected to use this class Foo { static bar() { console.log('bar'); } }

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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.

  // bad const AirbnbStyleGuide = require('./AirbnbStyleGuide'); module.exports = AirbnbStyleGuide.es6; // ok import AirbnbStyleGuide from './AirbnbStyleGuide'; export default AirbnbStyleGuide.es6; // best import { es6 } from './AirbnbStyleGuide'; export default es6;

• 10.2 Do not use wildcard imports.

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

  // bad import * as AirbnbStyleGuide from './AirbnbStyleGuide'; // good import 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.

  // bad // filename es6.js export { es6 as default } from './AirbnbStyleGuide'; // good // filename es6.js import { es6 } from './AirbnbStyleGuide'; export 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.

  // bad import foo from 'foo'; // … some other imports … // import { named1, named2 } from 'foo'; // good import foo, { named1, named2 } from 'foo'; // good import foo, { named1, named2, } 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.

  // bad let foo = 3; export { foo }; // good const foo = 3; export { 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.

  // bad export function foo() {} // good export 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.

  // bad import foo from 'foo'; foo.init(); import bar from 'bar'; // good import foo from 'foo'; import bar from 'bar'; foo.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.

  // bad import {longNameA, longNameB, longNameC, longNameD, longNameE} from 'path'; // good import { longNameA, longNameB, longNameC, longNameD, longNameE, } 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.

  // bad import fooSass from 'css!sass!foo.scss'; import barCss from 'style!css!bar.css'; // good import fooSass from 'foo.scss'; import 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.

  // bad import foo from './foo.js'; import bar from './bar.jsx'; import baz from './baz/index.jsx'; // good import foo from './foo'; import bar from './bar'; import 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.

  const numbers = [1, 2, 3, 4, 5]; // bad let sum = 0; for (let num of numbers) { sum += num; } sum === 15; // good let sum = 0; numbers.forEach((num) => { sum += num; }); sum === 15; // best (use the functional force) const sum = numbers.reduce((total, num) => total + num, 0); sum === 15; // bad const increasedByOne = []; for (let i = 0; i < numbers.length; i++) { increasedByOne.push(numbers[i] + 1); } // good const increasedByOne = []; numbers.forEach((num) => { increasedByOne.push(num + 1); }); // best (keeping it functional) const 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.

  // bad function * foo() { // ... } // bad const bar = function * () { // ... }; // bad const baz = function *() { // ... }; // bad const quux = function*() { // ... }; // bad function*foo() { // ... } // bad function *foo() { // ... } // very bad function * foo() { // ... } // very bad const wat = function * () { // ... }; // good function* foo() { // ... } // good const foo = function* () { // ... };

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Properties

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

  const luke = { jedi: true, age: 28, }; // bad const isJedi = luke['jedi']; // good const isJedi = luke.jedi;

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

  const luke = { jedi: true, age: 28, }; function getProp(prop) { return luke[prop]; } const isJedi = getProp('jedi');

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

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

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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

  // bad superPower = new SuperPower(); // good const 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.

  // bad const items = getItems(), goSportsTeam = true, dragonball = 'z'; // bad // (compare to above, and try to spot the mistake) const items = getItems(), goSportsTeam = true; dragonball = 'z'; // good const items = getItems(); const goSportsTeam = true; const 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.

  // bad let i, len, dragonball, items = getItems(), goSportsTeam = true; // bad let i; const items = getItems(); let dragonball; const goSportsTeam = true; let len; // good const goSportsTeam = true; const items = getItems(); let dragonball; let i; let 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.

  // bad - unnecessary function call function checkName(hasName) { const name = getName(); if (hasName === 'test') { return false; } if (name === 'test') { this.setName(''); return false; } return name; } // good function checkName(hasName) { if (hasName === 'test') { return false; } const name = getName(); if (name === 'test') { this.setName(''); return false; } return name; }

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

  Why? Chaining variable assignments creates implicit global variables.

  // bad (function example() { // JavaScript interprets this as // let a = ( b = ( c = 1 ) ); // The let keyword only applies to variable a; variables b and c become // global variables. let a = b = c = 1; }()); console.log(a); // throws ReferenceError console.log(b); // 1 console.log(c); // 1 // good (function example() { let a = 1; let b = a; let c = a; }()); console.log(a); // throws ReferenceError console.log(b); // throws ReferenceError console.log(c); // throws ReferenceError // 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.

  // bad const array = [1, 2, 3]; let num = 1; num++; --num; let sum = 0; let truthyCount = 0; for (let i = 0; i < array.length; i++) { let value = array[i]; sum += value; if (value) { truthyCount++; } } // good const array = [1, 2, 3]; let num = 1; num += 1; num -= 1; const sum = array.reduce((a, b) => a + b, 0); const 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.

  // bad const foo = superLongLongLongLongLongLongLongLongFunctionName(); // bad const foo = 'superLongLongLongLongLongLongLongLongString'; // good const foo = ( superLongLongLongLongLongLongLongLongFunctionName() ); // good const 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.

  // bad const some_unused_var = 42; // Write-only variables are not considered as used. let y = 10; y = 5; // A read for a modification of itself is not considered as used. let z = 0; z = z + 1; // Unused function arguments. function getX(x, y) { return x; } // good function getXPlusY(x, y) { return x + y; } const x = 1; const y = a + 2; alert(getXPlusY(x, y)); // 'type' is ignored even if unused because it has a rest property sibling. // This is a form of extracting an object that omits the specified keys. const { type, ...coords } = data; // '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.

  // we know this wouldn’t work (assuming there // is no notDefined global variable) function example() { console.log(notDefined); // => throws a ReferenceError } // creating a variable declaration after you // reference the variable will work due to // variable hoisting. Note: the assignment // value of `true` is not hoisted. function example() { console.log(declaredButNotAssigned); // => undefined var declaredButNotAssigned = true; } // the interpreter is hoisting the variable // declaration to the top of the scope, // which means our example could be rewritten as: function example() { let declaredButNotAssigned; console.log(declaredButNotAssigned); // => undefined declaredButNotAssigned = true; } // using const and let function example() { console.log(declaredButNotAssigned); // => throws a ReferenceError console.log(typeof declaredButNotAssigned); // => throws a ReferenceError const declaredButNotAssigned = true; }

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

  function example() { console.log(anonymous); // => undefined anonymous(); // => TypeError anonymous is not a function var anonymous = function () { console.log('anonymous function expression'); }; }

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

  function example() { console.log(named); // => undefined named(); // => TypeError named is not a function superPower(); // => ReferenceError superPower is not defined var named = function superPower() { console.log('Flying'); }; } // the same is true when the function name // is the same as the variable name. function example() { console.log(named); // => undefined named(); // => TypeError named is not a function var named = function named() { console.log('named'); }; }

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

  function example() { superPower(); // => Flying function superPower() { console.log('Flying'); } }

• 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.

  // bad // Variable a is being used before it is being defined. console.log(a); // this will be undefined, since while the declaration is hoisted, the initialization is not var a = 10; // Function fun is being called before being defined. fun(); function fun() {} // Class A is being used before being defined. new A(); // ReferenceError: Cannot access 'A' before initialization class A { } // `let` and `const` are hoisted, but they don't have a default initialization. // The variables 'a' and 'b' are in a Temporal Dead Zone where JavaScript // knows they exist (declaration is hoisted) but they are not accessible // (as they are not yet initialized). console.log(a); // ReferenceError: Cannot access 'a' before initialization console.log(b); // ReferenceError: Cannot access 'b' before initialization let a = 10; const b = 5; // good var a = 10; console.log(a); // 10 function fun() {} fun(); class A { } new A(); let a = 10; const b = 5; console.log(a); // 10 console.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

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

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

  // bad if (isValid === true) { // ... } // good if (isValid) { // ... } // bad if (name) { // ... } // good if (name !== '') { // ... } // bad if (collection.length) { // ... } // good if (collection.length > 0) { // ... }

• 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.

  // bad switch (foo) { case 1: let x = 1; break; case 2: const y = 2; break; case 3: function f() { // ... } break; default: class C {} } // good switch (foo) { case 1: { let x = 1; break; } case 2: { const y = 2; break; } case 3: { function f() { // ... } break; } case 4: bar(); break; default: { class C {} } }

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

  // bad const foo = maybe1 > maybe2 ? "bar" : value1 > value2 ? "baz" : null; // split into 2 separated ternary expressions const maybeNull = value1 > value2 ? 'baz' : null; // better const foo = maybe1 > maybe2 ? 'bar' : maybeNull; // best const foo = maybe1 > maybe2 ? 'bar' : maybeNull;

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

  // bad const foo = a ? a : b; const bar = c ? true : false; const baz = c ? false : true; const quux = a != null ? a : b; // good const foo = a || b; const bar = !!c; const baz = !c; const 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.

  // bad const foo = a && b < 0 || c > 0 || d + 1 === 0; // bad const bar = a ** b - 5 % d; // bad // one may be confused into thinking (a || b) && c if (a || b && c) { return d; } // bad const bar = a + b / c * d; // good const foo = (a && b < 0) || c > 0 || (d + 1 === 0); // good const bar = a ** b - (5 % d); // good if (a || (b && c)) { return d; } // good const 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.

  // bad const value = 0 ?? 'default'; // returns 0, not 'default' // bad const value = '' ?? 'default'; // returns '', not 'default' // good const value = null ?? 'default'; // returns 'default' // good const user = { name: 'John', age: null }; const age = user.age ?? 18; // returns 18

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Blocks

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

  // bad if (test) return false; // good if (test) return false; // good if (test) { return false; } // bad function foo() { return false; } // good function bar() { return false; }

• 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

  // bad if (test) { thing1(); thing2(); } else { thing3(); } // good if (test) { thing1(); thing2(); } else { thing3(); }

• 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

  // bad function foo() { if (x) { return x; } else { return y; } } // bad function cats() { if (x) { return x; } else if (y) { return y; } } // bad function dogs() { if (x) { return x; } else { if (y) { return y; } } } // good function foo() { if (x) { return x; } return y; } // good function cats() { if (x) { return x; } if (y) { return y; } } // good function dogs(x) { if (x) { if (z) { return y; } } else { return z; } }

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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.

  // bad if ((foo === 123 || bar === 'abc') && doesItLookGoodWhenItBecomesThatLong() && isThisReallyHappening()) { thing1(); } // bad if (foo === 123 && bar === 'abc') { thing1(); } // bad if (foo === 123 && bar === 'abc') { thing1(); } // bad if ( foo === 123 && bar === 'abc' ) { thing1(); } // good if ( foo === 123 && bar === 'abc' ) { thing1(); } // good if ( (foo === 123 || bar === 'abc') && doesItLookGoodWhenItBecomesThatLong() && isThisReallyHappening() ) { thing1(); } // good if (foo === 123 && bar === 'abc') { thing1(); }

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

  // bad !isRunning && startRunning(); // good if (!isRunning) { startRunning(); }

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Comments

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

  // bad // make() returns a new element // based on the passed in tag name // // @param {String} tag // @return {Element} element function make(tag) { // ... return element; } // good /**  * make() returns a new element  * based on the passed-in tag name  */ function make(tag) { // ... return element; }

• 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.

  // bad const active = true; // is current tab // good // is current tab const active = true; // bad function getType() { console.log('fetching type...'); // set the default type to 'no type' const type = this.type || 'no type'; return type; } // good function getType() { console.log('fetching type...'); // set the default type to 'no type' const type = this.type || 'no type'; return type; } // also good function getType() { // set the default type to 'no type' const type = this.type || 'no type'; return type; }

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

  // bad //is current tab const active = true; // good // is current tab const active = true; // bad /**  *make() returns a new element  *based on the passed-in tag name  */ function make(tag) { // ... return element; } // good /**  * make() returns a new element  * based on the passed-in tag name  */ function make(tag) { // ... return element; }

• 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.

  class Calculator extends Abacus { constructor() { super(); // FIXME: shouldn’t use a global here total = 0; } }

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

  class Calculator extends Abacus { constructor() { super(); // TODO: total should be configurable by an options param this.total = 0; } }

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Whitespace

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

  // bad function foo() { ∙∙∙∙let name; } // bad function bar() { ∙let name; } // good function baz() { ∙∙let name; }

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

  // bad function test(){ console.log('test'); } // good function test() { console.log('test'); } // bad dog.set('attr',{ age: '1 year', breed: 'Bernese Mountain Dog', }); // good dog.set('attr', { age: '1 year', breed: 'Bernese Mountain Dog', });

• 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

  // bad if(isJedi) { fight (); } // good if (isJedi) { fight(); } // bad function fight () { console.log ('Swooosh!'); } // good function fight() { console.log('Swooosh!'); }

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

  // bad const x=y+5; // good const x = y + 5;

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

  // bad import { es6 } from './AirbnbStyleGuide'; // ... export default es6;

  // bad import { es6 } from './AirbnbStyleGuide'; // ... export default es6;↵ ↵

  // good import { es6 } from './AirbnbStyleGuide'; // ... export 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

  // bad $('#items').find('.selected').highlight().end().find('.open').updateCount(); // bad $('#items'). find('.selected'). highlight(). end(). find('.open'). updateCount(); // good $('#items') .find('.selected') .highlight() .end() .find('.open') .updateCount(); // bad const leds = stage.selectAll('.led').data(data).enter().append('svg:svg').classed('led', true) .attr('width', (radius + margin) * 2).append('svg:g') .attr('transform', `translate(${radius + margin}, ${radius + margin})`) .call(tron.led); // good const leds = stage.selectAll('.led') .data(data) .enter().append('svg:svg') .classed('led', true) .attr('width', (radius + margin) * 2) .append('svg:g') .attr('transform', `translate(${radius + margin}, ${radius + margin})`) .call(tron.led); // good const leds = stage.selectAll('.led').data(data); const svg = leds.enter().append('svg:svg'); svg.classed('led', true).attr('width', (radius + margin) * 2); const g = svg.append('svg:g'); g.attr('transform', `translate(${radius + margin}, ${radius + margin})`).call(tron.led);

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

  // bad if (foo) { return bar; } return baz; // good if (foo) { return bar; } return baz; // bad const obj = { foo() { }, bar() { }, }; return obj; // good const obj = { foo() { }, bar() { }, }; return obj; // bad const arr = [ function foo() { }, function bar() { }, ]; return arr; // good const arr = [ function foo() { }, function bar() { }, ]; return arr;