// 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 }

⬆ back to top

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,
    ];

⬆ back to top

Destructuring

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

// 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.

// 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);

⬆ back to top

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.

// 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

// 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

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

    // good
    const foo = '\'this\' is "quoted"';
    const foo = `my name is '${name}'`;

⬆ back to top

Functions

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

// 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

// 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

// 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.

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

// 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

// 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

// 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

// 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

// 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,
    );

⬆ back to top

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 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

// 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.

// 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

// 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
    )

⬆ back to top

Classes & Constructors

• 9.1 Always use class. Avoid manipulating prototype directly.

// 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.

// 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

// 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');
      }
    }

⬆ back to top

Modules

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

// 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.

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

    // good
    import AirbnbStyleGuide from './AirbnbStyleGuide';

• 10.3 And do not export directly from an import.

// 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.

// 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.

// 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.

// bad
    export function foo() {}

    // good
    export default function foo() {}

• 10.7 Put all imports above non-import statements.

// 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.

> 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.

// 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

// 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';

⬆ back to top

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

> 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.

• 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

// 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* () {
      // ...
    };

⬆ back to top

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;

⬆ 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

// bad
    superPower = new SuperPower();

    // good
    const superPower = new SuperPower();

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

// 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.

// 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.

// 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

// 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

// 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.

// 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

// 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.

⬆ back to top

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

// 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.

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

// 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.

> 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.

// 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

⬆ back to top

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;
      }
    }

⬆ back to top

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.

// 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();
    }

⬆ back to top

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;
      }
    }

⬆ back to top

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

// 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;

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

// bad
    function bar() {

      console.log(foo);

    }

    // bad
    if (baz) {

      console.log(quux);
    } else {
      console.log(foo);

    }

    // bad
    class Foo {

      constructor(bar) {
        this.bar = bar;
      }
    }

    // good
    function bar() {
      console.log(foo);
    }

    // good
    if (baz) {
      console.log(quux);
    } else {
      console.log(foo);
    }

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

// bad
    class Person {
      constructor(fullName, email, birthday) {
        this.fullName = fullName;


        this.email = email;


        this.setAge(birthday);
      }


      setAge(birthday) {
        const today = new Date();


        const age = this.getAge(today, birthday);


        this.age = age;
      }


      getAge(today, birthday) {
        // ..
      }
    }

    // good
    class Person {
      constructor(fullName, email, birthday) {
        this.fullName = fullName;
        this.email = email;
        this.setAge(birthday);
      }

      setAge(birthday) {
        const today = new Date();
        const age = getAge(today, birthday);
        this.age = age;
      }

      getAge(today, birthday) {
        // ..
      }
    }

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

// bad
    function bar( foo ) {
      return foo;
    }

    // good
    function bar(foo) {
      return foo;
    }

    // bad
    if ( foo ) {
      console.log(foo);
    }

    // good
    if (foo) {
      console.log(foo);
    }

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

// bad
    const foo = [ 1, 2, 3 ];
    console.log(foo[ 0 ]);

    // good
    const foo = [1, 2, 3];
    console.log(foo[0]);

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

// bad
    const foo = {clark: 'kent'};

    // good
    const 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

// bad
    const foo = jsonData && jsonData.foo && jsonData.foo.bar && jsonData.foo.bar.baz && jsonData.foo.bar.baz.quux && jsonData.foo.bar.baz.quux.xyzzy;

    // bad
    $.ajax({ method: 'POST', url: 'https://airbnb.com/', data: { name: 'John' } }).done(() => console.log('Congratulations!')).fail(() => console.log('You have failed this city.'));

    // good
    const foo = jsonData
      && jsonData.foo
      && jsonData.foo.bar
      && jsonData.foo.bar.baz
      && jsonData.foo.bar.baz.quux
      && jsonData.foo.bar.baz.quux.xyzzy;

    // better
    const foo = jsonData
      ?.foo
      ?.bar
      ?.baz
      ?.quux
      ?.xyzzy;

    // good
    $.ajax({
      method: 'POST',
      url: 'https://airbnb.com/',
      data: { name: 'John' },
    })
      .done(() => console.log('Congratulations!'))
      .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

// bad
    function foo() {return true;}
    if (foo) { bar = 0;}

    // good
    function foo() { return true; }
    if (foo) { bar = 0; }

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

// bad
    const foo = 1,bar = 2;
    const arr = [1 , 2];

    // good
    const foo = 1, bar = 2;
    const arr = [1, 2];

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

// bad
    obj[foo ]
    obj[ 'foo']
    const x = {[ b ]: a}
    obj[foo[ bar ]]

    // good
    obj[foo]
    obj['foo']
    const x = { [b]: a }
    obj[foo[bar]]

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

// bad
    func ();

    func
    ();

    // good
    func();

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

// bad
    const obj = { foo : 42 };
    const obj2 = { foo:42 };

    // good
    const 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

// bad - multiple empty lines
    const x = 1;


    const y = 2;

    // bad - 2+ newlines at end of file
    const x = 1;
    const y = 2;


    // bad - 1+ newline(s) at beginning of file

    const x = 1;
    const y = 2;

    // good
    const x = 1;
    const y = 2;

⬆ back to top

Commas

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

// bad
    const story = [
        once
      , upon
      , aTime
    ];

    // good
    const story = [
      once,
      upon,
      aTime,
    ];

    // bad
    const hero = {
        firstName: 'Ada'
      , lastName: 'Lovelace'
      , birthYear: 1815
      , superPower: 'computers'
    };

    // good
    const hero = {
      firstName: 'Ada',
      lastName: 'Lovelace',
      birthYear: 1815,
      superPower: 'computers',
    };

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

// bad - git diff without trailing comma
    const hero = {
         firstName: 'Florence',
    -    lastName: 'Nightingale'
    +    lastName: 'Nightingale',
    +    inventorOf: ['coxcomb chart', 'modern nursing']
    };

    // good - git diff with trailing comma
    const hero = {
         firstName: 'Florence',
         lastName: 'Nightingale',
    +    inventorOf: ['coxcomb chart', 'modern nursing'],
    };

// bad
    const hero = {
      firstName: 'Dana',
      lastName: 'Scully'
    };

    const heroes = [
      'Batman',
      'Superman'
    ];

    // good
    const hero = {
      firstName: 'Dana',
      lastName: 'Scully',
    };

    const heroes = [
      'Batman',
      'Superman',
    ];

    // bad
    function createHero(
      firstName,
      lastName,
      inventorOf
    ) {
      // does nothing
    }

    // good
    function createHero(
      firstName,
      lastName,
      inventorOf,
    ) {
      // does nothing
    }

    // good (note that a comma must not appear after a "rest" element)
    function createHero(
      firstName,
      lastName,
      inventorOf,
      ...heroArgs
    ) {
      // does nothing
    }

    // bad
    createHero(
      firstName,
      lastName,
      inventorOf
    );

    // good
    createHero(
      firstName,
      lastName,
      inventorOf,
    );

    // good (note that a comma must not appear after a "rest" element)
    createHero(
      firstName,
      lastName,
      inventorOf,
      ...heroArgs
    );

⬆ back to top

Semicolons

• 21.1 Yup. eslint: semi

// bad - raises exception
    const luke = {}
    const leia = {}
    [luke, leia].forEach((jedi) => jedi.father = 'vader')

    // bad - raises exception
    const reaction = "No! That’s impossible!"
    (async function meanwhileOnTheFalcon() {
      // handle `leia`, `lando`, `chewie`, `r2`, `c3p0`
      // ...
    }())

    // bad - returns `undefined` instead of the value on the next line - always happens when `return` is on a line by itself because of ASI!
    function foo() {
      return
        'search your feelings, you know it to be foo'
    }

    // good
    const luke = {};
    const leia = {};
    [luke, leia].forEach((jedi) => {
      jedi.father = 'vader';
    });

    // good
    const reaction = 'No! That’s impossible!';
    (async function meanwhileOnTheFalcon() {
      // handle `leia`, `lando`, `chewie`, `r2`, `c3p0`
      // ...
    }());

    // good
    function foo() {
      return 'search your feelings, you know it to be foo';
    }

Read more.

⬆ back to top

Type Casting & Coercion

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

• 22.2 Strings: eslint: no-new-wrappers

// => this.reviewScore = 9;

    // bad
    const totalScore = new String(this.reviewScore); // typeof totalScore is "object" not "string"

    // bad
    const totalScore = this.reviewScore + ''; // invokes this.reviewScore.valueOf()

    // bad
    const totalScore = this.reviewScore.toString(); // isn’t guaranteed to return a string

    // good
    const 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

const inputValue = '4';

    // bad
    const val = new Number(inputValue);

    // bad
    const val = +inputValue;

    // bad
    const val = inputValue >> 0;

    // bad
    const val = parseInt(inputValue);

    // good
    const val = Number(inputValue);

    // good
    const 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.

// good
    /**
     * parseInt was the reason my code was slow.
     * Bitshifting the String to coerce it to a
     * Number made it a lot faster.
     */
    const 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:

2147483647 >> 0; // => 2147483647
    2147483648 >> 0; // => -2147483648
    2147483649 >> 0; // => -2147483647

• 22.6 Booleans: eslint: no-new-wrappers

const age = 0;

    // bad
    const hasAge = new Boolean(age);

    // good
    const hasAge = Boolean(age);

    // best
    const hasAge = !!age;

⬆ back to top

Naming Conventions

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

// bad
    function q() {
      // ...
    }

    // good
    function query() {
      // ...
    }

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

// bad
    const OBJEcttsssss = {};
    const this_is_my_object = {};
    function c() {}

    // good
    const thisIsMyObject = {};
    function thisIsMyFunction() {}

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

// bad
    function user(options) {
      this.name = options.name;
    }

    const bad = new user({
      name: 'nope',
    });

    // good
    class User {
      constructor(options) {
        this.name = options.name;
      }
    }

    const good = new User({
      name: 'yup',
    });

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

// bad
    this.__firstName__ = 'Panda';
    this.firstName_ = 'Panda';
    this._firstName = 'Panda';

    // good
    this.firstName = 'Panda';

    // good, in environments where WeakMaps are available
    // see https://compat-table.github.io/compat-table/es6/#test-WeakMap
    const firstNames = new WeakMap();
    firstNames.set(this, 'Panda');

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

// bad
    function foo() {
      const self = this;
      return function () {
        console.log(self);
      };
    }

    // bad
    function foo() {
      const that = this;
      return function () {
        console.log(that);
      };
    }

    // good
    function foo() {
      return () => {
        console.log(this);
      };
    }

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

// file 1 contents
    class CheckBox {
      // ...
    }
    export default CheckBox;

    // file 2 contents
    export default function fortyTwo() { return 42; }

    // file 3 contents
    export default function insideDirectory() {}

    // in some other file
    // bad
    import CheckBox from './checkBox'; // PascalCase import/export, camelCase filename
    import FortyTwo from './FortyTwo'; // PascalCase import/filename, camelCase export
    import InsideDirectory from './InsideDirectory'; // PascalCase import/filename, camelCase export

    // bad
    import CheckBox from './check_box'; // PascalCase import/export, snake_case filename
    import forty_two from './forty_two'; // snake_case import/filename, camelCase export
    import inside_directory from './inside_directory'; // snake_case import, camelCase export
    import index from './inside_directory/index'; // requiring the index file explicitly
    import insideDirectory from './insideDirectory/index'; // requiring the index file explicitly

    // good
    import CheckBox from './CheckBox'; // PascalCase export/import/filename
    import fortyTwo from './fortyTwo'; // camelCase export/import/filename
    import insideDirectory from './insideDirectory'; // camelCase export/import/directory name/implicit "index"
    // ^ 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.

function makeStyleGuide() {
      // ...
    }

    export default makeStyleGuide;

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

const AirbnbStyleGuide = {
      es6: {
      },
    };

    export default AirbnbStyleGuide;

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

// bad
    import SmsContainer from './containers/SmsContainer';

    // bad
    const HttpRequests = [
      // ...
    ];

    // good
    import SMSContainer from './containers/SMSContainer';

    // good
    const HTTPRequests = [
      // ...
    ];

    // also good
    const httpRequests = [
      // ...
    ];

    // best
    import TextMessageContainer from './containers/TextMessageContainer';

    // best
    const requests = [
      // ...
    ];

• 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.

• 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.

// bad
    const PRIVATE_VARIABLE = 'should not be unnecessarily uppercased within a file';

    // bad
    export const THING_TO_BE_CHANGED = 'should obviously not be uppercased';

    // bad
    export let REASSIGNABLE_VARIABLE = 'do not use let with uppercase variables';

    // ---

    // allowed but does not supply semantic value
    export const apiKey = 'SOMEKEY';

    // better in most cases
    export const API_KEY = 'SOMEKEY';

    // ---

    // bad - unnecessarily uppercases key while adding no semantic value
    export const MAPPING = {
      KEY: 'value'
    };

    // good
    export const MAPPING = {
      key: 'value',
    };

⬆ back to top

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').

// bad
    class Dragon {
      get age() {
        // ...
      }

      set age(value) {
        // ...
      }
    }

    // good
    class Dragon {
      getAge() {
        // ...
      }

      setAge(value) {
        // ...
      }
    }

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

// bad
    if (!dragon.age()) {
      return false;
    }

    // good
    if (!dragon.hasAge()) {
      return false;
    }

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

class Jedi {
      constructor(options = {}) {
        const lightsaber = options.lightsaber || 'blue';
        this.set('lightsaber', lightsaber);
      }

      set(key, val) {
        this[key] = val;
      }

      get(key) {
        return this[key];
      }
    }

⬆ back to top

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:

// bad
    $(this).trigger('listingUpdated', listing.id);

    // ...

    $(this).on('listingUpdated', (e, listingID) => {
      // do something with listingID
    });

prefer:

// good
    $(this).trigger('listingUpdated', { listingID: listing.id });

    // ...

    $(this).on('listingUpdated', (e, data) => {
      // do something with data.listingID
    });

⬆ back to top

jQuery

• 26.1 Prefix jQuery object variables with a $.

// bad
    const sidebar = $('.sidebar');

    // good
    const $sidebar = $('.sidebar');

    // good
    const $sidebarBtn = $('.sidebar-btn');

• 26.2 Cache jQuery lookups.

// bad
    function setSidebar() {
      $('.sidebar').hide();

      // ...

      $('.sidebar').css({
        'background-color': 'pink',
      });
    }

    // good
    function setSidebar() {
      const $sidebar = $('.sidebar');
      $sidebar.hide();

      // ...

      $sidebar.css({
        'background-color': 'pink',
      });
    }

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

• 26.4 Use find with scoped jQuery object queries.

// bad
    $('ul', '.sidebar').hide();

    // bad
    $('.sidebar').find('ul').hide();

    // good
    $('.sidebar ul').hide();

    // good
    $('.sidebar > ul').hide();

    // good
    $sidebar.find('ul').hide();

⬆ back to top

ECMAScript 5 Compatibility

• 27.1 Refer to Kangax’s ES5 compatibility table.

ECMAScript 6+ (ES 2015+) Styles

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

• Arrow Functions
• Classes
• Object Shorthand
• Object Concise
• Object Computed Properties
• Template Strings
• Destructuring
• Default Parameters
• Rest
• Array Spreads
• Let and Const
• Exponentiation Operator
• Iterators and Generators
• Modules

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

⬆ back to top

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.

> 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.

// bad
    isNaN('1.2'); // false
    isNaN('1.2.3'); // true

    // good
    Number.isNaN('1.2.3'); // false
    Number.isNaN(Number('1.2.3')); // true

• 29.2 Use Number.isFinite instead of global isFinite.

> 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.

// bad
    isFinite('2e3'); // true

    // good
    Number.isFinite('2e3'); // false
    Number.isFinite(parseInt('2e3', 10)); // true

⬆ back to top

Testing

• 30.1 Yup.

function foo() {
      return true;
    }

• 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.

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...

Resources

Learning ES6+

• Latest ECMA spec

• ExploringJS

• ES6 Compatibility Table

• Comprehensive Overview of ES6 Features

• JavaScript Roadmap

• Standard ECMA-262

• Code Style Linters

• ESlint - Airbnb Style .eslintrc

• JSHint - Airbnb Style .jshintrc

• Neutrino Preset - @neutrinojs/airbnb

• Google JavaScript Style Guide

• Google JavaScript Style Guide (Old)

• jQuery Core Style Guidelines

• Principles of Writing Consistent, Idiomatic JavaScript

• StandardJS

• 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

• 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

• 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

• JavaScript Weekly

• JavaScript, JavaScript...

• Bocoup Weblog

• Adequately Good

• NCZOnline

• Perfection Kills

• Ben Alman

• Dmitry Baranovskiy

• nettuts

• JavaScript Air

• JavaScript Jabber

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

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

• Reference

Chat With Us About JavaScript

• Find us on gitter.

Contributors

• View Contributors

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.

⬆ back to top

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.

};