JavaScript Style Guide
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README.md

Gitter

Airbnb JavaScript Style Guide() {

A mostly reasonable approach to JavaScript

For the ES5-only guide click here.

Table of Contents

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

Types

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

    • string
    • number
    • boolean
    • null
    • undefined
    const foo = 1;
    let bar = foo;
    
    bar = 9;
    
    console.log(foo, bar); // => 1, 9
  • 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.

    Why? This ensures that you can't reassign your references (mutation), 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 mutate references, use let instead of 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.

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

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Objects

  • 3.1 Use the literal syntax for object creation.

    // bad
    const item = new Object();
    
    // good
    const item = {};
  • 3.2 If your code will be executed in browsers in script context, don't use reserved words as keys. It won't work in IE8. More info. It’s OK to use them in ES6 modules and server-side code.

    // bad
    const superman = {
      default: { clark: 'kent' },
      private: true,
    };
    
    // good
    const superman = {
      defaults: { clark: 'kent' },
      hidden: true,
    };
  • 3.3 Use readable synonyms in place of reserved words.

    // bad
    const superman = {
      class: 'alien',
    };
    
    // bad
    const superman = {
      klass: 'alien',
    };
    
    // good
    const superman = {
      type: 'alien',
    };

  • 3.4 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,
      };

  • 3.5 Use object method shorthand.

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

  • 3.6 Use property value shorthand.

    Why? It is shorter to write and descriptive.

      const lukeSkywalker = 'Luke Skywalker';
    
      // bad
      const obj = {
        lukeSkywalker: lukeSkywalker,
      };
    
      // good
      const obj = {
        lukeSkywalker,
      };
  • 3.7 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,
        twoJedisWalkIntoACantina: 2,
        lukeSkywalker,
        episodeThree: 3,
        mayTheFourth: 4,
        anakinSkywalker,
      };
    
      // good
      const obj = {
        lukeSkywalker,
        anakinSkywalker,
        episodeOne: 1,
        twoJedisWalkIntoACantina: 2,
        episodeThree: 3,
        mayTheFourth: 4,
      };

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Arrays

  • 4.1 Use the literal syntax for array creation.

    // 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++) {
      itemsCopy[i] = items[i];
    }
    
    // good
    const itemsCopy = [...items];
  • 4.4 To convert an array-like object to an array, use Array#from.

    const foo = document.querySelectorAll('.foo');
    const nodes = Array.from(foo);

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Destructuring

  • 5.1 Use object destructuring when accessing and using multiple properties of an object.

    Why? Destructuring saves you from creating temporary references for those properties.

      // bad
      function getFullName(user) {
        const firstName = user.firstName;
        const lastName = user.lastName;
    
        return `${firstName} ${lastName}`;
      }
    
      // good
      function getFullName(obj) {
        const { firstName, lastName } = obj;
        return `${firstName} ${lastName}`;
      }
    
      // best
      function getFullName({ firstName, lastName }) {
        return `${firstName} ${lastName}`;
      }
  • 5.2 Use array 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, right } = processInput(input);

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Strings

  • 6.1 Use single quotes '' for strings.

    // bad
    const name = "Capt. Janeway";
    
    // good
    const name = 'Capt. Janeway';
  • 6.2 Strings longer than 80 characters should be written across multiple lines using string concatenation.

  • 6.3 Note: If overused, long strings with concatenation could impact performance. jsPerf & Discussion.

    // 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.';

  • 6.4 When programmatically building up strings, use template strings instead of concatenation.

    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();
      }
    
      // good
      function sayHi(name) {
        return `How are you, ${name}?`;
      }

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Functions

  • 7.1 Use function declarations instead of function expressions.

    Why? Function declarations are named, so they're easier to identify in call stacks. Also, the whole body of a function declaration is hoisted, whereas only the reference of a function expression is hoisted. This rule makes it possible to always use Arrow Functions in place of function expressions.

      // bad
      const foo = function () {
      };
    
      // good
      function foo() {
      }
  • 7.2 Function expressions:

    // immediately-invoked function expression (IIFE)
    (() => {
      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.

  • 7.4 Note: ECMA-262 defines a block as a list of statements. A function declaration is not a statement. Read ECMA-262's note on this issue.

    // 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 nope(name, options, arguments) {
      // ...stuff...
    }
    
    // good
    function yup(name, options, args) {
      // ...stuff...
    }

  • 7.6 Never use arguments, opt to use rest syntax ... instead.

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

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

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Arrow Functions

  • 8.1 When you must use function expressions (as when passing an anonymous function), use arrow function notation.

    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 function declaration.

      // bad
      [1, 2, 3].map(function (x) {
        return x * x;
      });
    
      // good
      [1, 2, 3].map((x) => {
        return x * x;
      });
  • 8.2 If the function body fits on one line and there is only a single argument, feel free to omit the braces and parentheses, and use the implicit return. Otherwise, add the parentheses, braces, and use a return statement.

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

    Why not? If you plan on returning an object.

      // good
      [1, 2, 3].map(x => x * x);
    
      // good
      [1, 2, 3].reduce((total, n) => {
        return total + n;
      }, 0);

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

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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;

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Iterators and Generators

  • 11.1 Don't use iterators. Prefer JavaScript's higher-order functions like map() and reduce() instead of loops like for-of.

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

      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;
  • 11.2 Don't use generators for now.

    Why? They don't transpile well to ES5.

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Properties

  • 12.1 Use dot notation when accessing properties.

    const luke = {
      jedi: true,
      age: 28,
    };
    
    // bad
    const isJedi = luke['jedi'];
    
    // good
    const isJedi = luke.jedi;
  • 12.2 Use subscript notation [] when accessing properties with a variable.

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

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Variables

  • 13.1 Always use const 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.

    // bad
    superPower = new SuperPower();
    
    // good
    const superPower = new SuperPower();
  • 13.2 Use one const declaration per variable.

    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.

    // 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 previous 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.

      // good
      function() {
        test();
        console.log('doing stuff..');
    
        //..other stuff..
    
        const name = getName();
    
        if (name === 'test') {
          return false;
        }
    
        return name;
      }
    
      // bad - unnessary function call
      function(hasName) {
        const name = getName();
    
        if (!hasName) {
          return false;
        }
    
        this.setFirstName(name);
    
        return true;
      }
    
      // good
      function(hasName) {
        if (!hasName) {
          return false;
        }
    
        const name = getName();
        this.setFirstName(name);
    
        return true;
      }

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Hoisting

  • 14.1 var declarations get hoisted to the top of their 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');
      }
    }
  • For more information refer to JavaScript Scoping & Hoisting by Ben Cherry.

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Comparison Operators & Equality

  • 15.1 Use === and !== over == and !=.
  • 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 is an object, objects evaluate to true
    }
  • 15.3 Use shortcuts.

    // bad
    if (name !== '') {
      // ...stuff...
    }
    
    // good
    if (name) {
      // ...stuff...
    }
    
    // bad
    if (collection.length > 0) {
      // ...stuff...
    }
    
    // good
    if (collection.length) {
      // ...stuff...
    }
  • 15.4 For more information see Truth Equality and JavaScript by Angus Croll.

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Blocks

  • 16.1 Use braces with all multi-line blocks.

    // bad
    if (test)
      return false;
    
    // good
    if (test) return false;
    
    // good
    if (test) {
      return false;
    }
    
    // bad
    function() { return false; }
    
    // good
    function() {
      return false;
    }
  • 16.2 If you're using multi-line blocks with if and else, put else on the same line as your if block's closing brace.

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

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Comments

  • 17.1 Use /** ... */ for multi-line comments. Include a description, specify types and values for all parameters and return values.

    // bad
    // make() returns a new element
    // based on the passed in tag name
    //
    // @param {String} tag
    // @return {Element} element
    function make(tag) {
    
      // ...stuff...
    
      return element;
    }
    
    // good
    /**
     * make() returns a new element
     * based on the passed in tag name
     *
     * @param {String} tag
     * @return {Element} element
     */
    function make(tag) {
    
      // ...stuff...
    
      return element;
    }
  • 17.2 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.

    // 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;
    }
  • 17.3 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.

  • 17.4 Use // FIXME: to annotate problems.

    class Calculator {
      constructor() {
        // FIXME: shouldn't use a global here
        total = 0;
      }
    }
  • 17.5 Use // TODO: to annotate solutions to problems.

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

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Whitespace

  • 18.1 Use soft tabs set to 2 spaces.

    // bad
    function() {
    ∙∙∙∙const name;
    }
    
    // bad
    function() {
    ∙const name;
    }
    
    // good
    function() {
    ∙∙const name;
    }
  • 18.2 Place 1 space before the leading brace.

    // 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',
    });
  • 18.3 Place 1 space before the opening parenthesis in control statements (if, while etc.). Place no space before the argument list in function calls and declarations.

    // bad
    if(isJedi) {
      fight ();
    }
    
    // good
    if (isJedi) {
      fight();
    }
    
    // bad
    function fight () {
      console.log ('Swooosh!');
    }
    
    // good
    function fight() {
      console.log('Swooosh!');
    }
  • 18.4 Set off operators with spaces.

    // bad
    const x=y+5;
    
    // good
    const x = y + 5;
  • 18.5 End files with a single newline character.

    // bad
    (function(global) {
      // ...stuff...
    })(this);
    // bad
    (function(global) {
      // ...stuff...
    })(this);↵
    ↵
    // good
    (function(global) {
      // ...stuff...
    })(this);↵
  • 18.5 Use indentation when making long method chains. Use a leading dot, which emphasizes that the line is a method call, not a new statement.

    // 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').class('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);
  • 18.6 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;

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Commas

  • 19.1 Leading commas: Nope.

    // 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',
    };
  • 19.2 Additional trailing comma: Yup.

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

      // bad - git diff without trailing comma
      const hero = {
           firstName: 'Florence',
      -    lastName: 'Nightingale'
      +    lastName: 'Nightingale',
      +    inventorOf: ['coxcomb graph', 'mordern nursing']
      }
    
      // good - git diff with trailing comma
      const hero = {
           firstName: 'Florence',
           lastName: 'Nightingale',
      +    inventorOf: ['coxcomb chart', 'mordern nursing'],
      }
    
      // bad
      const hero = {
        firstName: 'Dana',
        lastName: 'Scully'
      };
    
      const heroes = [
        'Batman',
        'Superman'
      ];
    
      // good
      const hero = {
        firstName: 'Dana',
        lastName: 'Scully',
      };
    
      const heroes = [
        'Batman',
        'Superman',
      ];

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Semicolons

  • 20.1 Yup.

    // bad
    (function() {
      const name = 'Skywalker'
      return name
    })()
    
    // good
    (() => {
      const name = 'Skywalker';
      return name;
    })();
    
    // good (guards against the function becoming an argument when two files with IIFEs are concatenated)
    ;(() => {
      const name = 'Skywalker';
      return name;
    })();

    Read more.

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Type Casting & Coercion

  • 21.1 Perform type coercion at the beginning of the statement.
  • 21.2 Strings:

    //  => this.reviewScore = 9;
    
    // bad
    const totalScore = this.reviewScore + '';
    
    // good
    const totalScore = String(this.reviewScore);
  • 21.3 Use parseInt for Numbers and always with a radix for type casting.

    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);
  • 21.4 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;
  • 21.5 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
  • 21.6 Booleans:

    const age = 0;
    
    // bad
    const hasAge = new Boolean(age);
    
    // good
    const hasAge = Boolean(age);
    
    // good
    const hasAge = !!age;

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Naming Conventions

  • 22.1 Avoid single letter names. Be descriptive with your naming.

    // bad
    function q() {
      // ...stuff...
    }
    
    // good
    function query() {
      // ..stuff..
    }
  • 22.2 Use camelCase when naming objects, functions, and instances.

    // bad
    const OBJEcttsssss = {};
    const this_is_my_object = {};
    function c() {}
    
    // good
    const thisIsMyObject = {};
    function thisIsMyFunction() {}
  • 22.3 Use PascalCase when naming constructors or classes.

    // 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',
    });
  • 22.4 Use a leading underscore _ when naming private properties.

    // bad
    this.__firstName__ = 'Panda';
    this.firstName_ = 'Panda';
    
    // good
    this._firstName = 'Panda';
  • 22.5 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);
      };
    }
  • 22.6 If your file exports a single class, your filename should be exactly the name of the class.

    // file contents
    class CheckBox {
      // ...
    }
    export default CheckBox;
    
    // in some other file
    // bad
    import CheckBox from './checkBox';
    
    // bad
    import CheckBox from './check_box';
    
    // good
    import CheckBox from './CheckBox';
  • 22.7 Use camelCase when you export-default a function. Your filename should be identical to your function's name.

    function makeStyleGuide() {
    }
    
    export default makeStyleGuide;
  • 22.8 Use PascalCase when you export a singleton / function library / bare object.

    const AirbnbStyleGuide = {
      es6: {
      }
    };
    
    export default AirbnbStyleGuide;

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Accessors

  • 23.1 Accessor functions for properties are not required.
  • 23.2 If you do make accessor functions use getVal() and setVal('hello').

    // bad
    dragon.age();
    
    // good
    dragon.getAge();
    
    // bad
    dragon.age(25);
    
    // good
    dragon.setAge(25);
  • 23.3 If the property is a boolean, use isVal() or hasVal().

    // bad
    if (!dragon.age()) {
      return false;
    }
    
    // good
    if (!dragon.hasAge()) {
      return false;
    }
  • 23.4 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];
      }
    }

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Events

  • 24.1 When attaching data payloads to events (whether DOM events or something more proprietary like Backbone events), pass 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', function(e, listingId) {
      // do something with listingId
    });

    prefer:

    // good
    $(this).trigger('listingUpdated', { listingId : listing.id });
    
    ...
    
    $(this).on('listingUpdated', function(e, data) {
      // do something with data.listingId
    });

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jQuery

  • 25.1 Prefix jQuery object variables with a $.

    // bad
    const sidebar = $('.sidebar');
    
    // good
    const $sidebar = $('.sidebar');
  • 25.2 Cache jQuery lookups.

    // bad
    function setSidebar() {
      $('.sidebar').hide();
    
      // ...stuff...
    
      $('.sidebar').css({
        'background-color': 'pink'
      });
    }
    
    // good
    function setSidebar() {
      const $sidebar = $('.sidebar');
      $sidebar.hide();
    
      // ...stuff...
    
      $sidebar.css({
        'background-color': 'pink'
      });
    }
  • 25.3 For DOM queries use Cascading $('.sidebar ul') or parent > child $('.sidebar > ul'). jsPerf

  • 25.4 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();

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ECMAScript 5 Compatibility

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ECMAScript 6 Styles

  • 27.1 This is a collection of links to the various es6 features.
  1. Arrow Functions
  2. Classes
  3. Object Shorthand
  4. Object Concise
  5. Object Computed Properties
  6. Template Strings
  7. Destructuring
  8. Default Parameters
  9. Rest
  10. Array Spreads
  11. Let and Const
  12. Iterators and Generators
  13. Modules

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Testing

  • 28.1 Yup.

    function() {
      return true;
    }

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Performance

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Resources

Learning ES6

Read This

Tools

Other Styleguides

Other Styles

Further Reading

Books

Blogs

Podcasts

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In the Wild

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

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Translation

This style guide is also available in other languages:

The JavaScript Style Guide Guide

Chat With Us About JavaScript

Contributors

License

(The MIT License)

Copyright (c) 2014 Airbnb

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

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

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

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};