Javascript Style Guide

Airbnb JavaScript Style Guide() {

A mostly reasonable approach to JavaScript

Table of Contents

  1. Types
  2. Objects
  3. Arrays
  4. Strings
  5. Functions
  6. Properties
  7. Variables
  8. Hoisting
  9. Conditional Expressions & Equality
  10. Blocks
  11. Comments
  12. Whitespace
  13. Leading Commas
  14. Semicolons
  15. Type Casting & Coercion
  16. Naming Conventions
  17. Accessors
  18. Constructors
  19. Modules
  20. jQuery
  21. ES5 Compatability
  22. Testing
  23. Performance
  24. Resources
  25. In the Wild
  26. The JavaScript Style Guide Guide
  27. Contributors
  28. License

Types

  • Primitives: When you access a primitive type you work directly on its value
    • String
    • Number
    • Boolean
    • null
    • undefined
    >var foo = 1,
        bar = foo;
    
    bar = 9;
    
    console.log(foo, bar); // => 1, 9
  • Complex: When you access a complex type you work on a reference to its value
    • Object
    • Array
    • Function
    >var foo = [1, 2],
        bar = foo;
    
    bar[0] = 9;
    
    console.log(foo[0], bar[0]); // => 9, 9

    [?]

Objects

  • Use the literal syntax for object creation.
    >// bad
    var item = new Object();
    
    // good
    var item = {};
  • Don’t use reserved wordsas keys.
    >// bad
    var superman = {
      class: 'superhero',
      default: { clark: kent },
      private: true
    };
    
    // good
    var superman = {
      klass: 'superhero',
      defaults: { clark: kent },
      hidden: true
    };

    [?]

Arrays

  • Use the literal syntax for array creation
    >// bad
    var items = new Array();
    
    // good
    var items = [];
  • For performance reasonsuse direct assignment over Array#push
    >var len = items.length,
        itemsCopy = [],
        i;
    
    // bad
    for (i = 0; i < len; i++) {
      itemsCopy.push(items[i])
    }
    
    // good
    for (i = 0; i < len; i++) {
      itemsCopy[i] = items[i];
    }

    [?]

Strings

  • Use single quotes ''for strings
    >// bad
    var name = "Bob Parr";
    
    // good
    var name = 'Bob Parr';
    
    // bad
    var fullName = "Bob" + this.lastName;
    
    // good
    var fullName = 'Bob' + this.lastName;
  • Strings longer than 80 characters should be written across multiple lines using string concatenation.
    >// bad
    var 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
    var 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
    var 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.';
  • When programatically building up a string, use Array#join instead of string concatenation. Mostly for IE: jsPerf.
    >var items,
        messages,
        length, i;
    
    messages = [{
        state: 'success',
        message: 'This one worked.'
    },{
        state: 'success',
        message: 'This one worked as well.'
    },{
        state: 'error',
        message: 'This one did not work.'
    }];
    
    length = messages.length;
    
    // bad
    function inbox(messages) {
      items = '<ul>';
    
      for (i = 0; i < length; i++) {
        items += '<li>' + messages[i].message + '</li>';
      }
    
      return items + '</ul>';
    }
    
    // good
    function inbox(messages) {
      items = [];
    
      for (i = 0; i < length; i++) {
        items[i] = messages[i].message;
      }
    
      return '<ul><li>' + items.join('</li><li>') + '</li></ul>';
    }

    [?]

Functions

  • Function expressions:
    >// anonymous function expression
    var anonymous = function() {
      return true;
    };
    
    // named function expression
    var named = function named() {
      return true;
    };
    
    // immediately-invoked function expression (IIFE)
    (function() {
      console.log('Welcome to the Internet. Please follow me.');
    })();
  • 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.
    >// bad
    if (currentUser) {
      function test() {
        console.log('Nope.');
      }
    }
    
    // good
    if (currentUser) {
      var test = function test() {
        console.log('Yup.');
      };
    }
  • Never name a parameter arguments, this will take precendence over the argumentsobject that is given to every function scope.
    >// bad
    function nope(name, options, arguments) {
      // ...stuff...
    }
    
    // good
    function yup(name, options, args) {
      // ...stuff...
    }

    [?]

Properties

  • Use dot notation when accessing properties.
    >var luke = {
      jedi: true,
      age: 28
    };
    
    // bad
    var isJedi = luke['jedi'];
    
    // good
    var isJedi = luke.jedi;
  • Use subscript notation []when accessing properties with a variable.
    >var luke = {
      jedi: true,
      age: 28
    };
    
    function getProp(prop) {
      return luke[prop];
    }
    
    var isJedi = getProp('jedi');

    [?]

Variables

  • Always use varto 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
    var superPower = new SuperPower();
  • Use one vardeclaration for multiple variables and declare each variable on a newline.
    >// bad
    var items = getItems();
    var goSportsTeam = true;
    var dragonball = 'z';
    
    // good
    var items = getItems(),
        goSportsTeam = true,
        dragonball = 'z';
  • Declare unassigned variables last. This is helpful when later on you might need to assign a variable depending on one of the previous assigned variables.
    >// bad
    var i, len, dragonball,
        items = getItems(),
        goSportsTeam = true;
    
    // bad
    var i, items = getItems(),
        dragonball,
        goSportsTeam = true,
        len;
    
    // good
    var items = getItems(),
        goSportsTeam = true,
        dragonball,
        i, length;
  • Assign variables at the top of their scope. This helps avoid issues with variable declaration and assignment hoisting related issues.
    >// bad
    function() {
      test();
      console.log('doing stuff..');
    
      //..other stuff..
    
      var name = getName();
    
      if (name === 'test') {
        return false;
      }
    
      return name;
    }
    
    // good
    function() {
      var name = getName();
    
      test();
      console.log('doing stuff..');
    
      //..other stuff..
    
      if (name === 'test') {
        return false;
      }
    
      return name;
    }
    
    // bad
    function() {
      var name = getName();
    
      if (!arguments.length) {
        return false;
      }
    
      return true;
    }
    
    // good
    function() {
      if (!arguments.length) {
        return false;
      }
    
      var name = getName();
    
      return true;
    }

    [?]

Hoisting

  • Variable declarations get hoisted to the top of their scope, their assignment does not.
    >// 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 interpretor is hoisting the variable
    // declaration to the top of the scope.
    // Which means our example could be rewritten as:
    function example() {
      var declaredButNotAssigned;
      console.log(declaredButNotAssigned); // => undefined
      declaredButNotAssigned = true;
    }
  • Anonymous function expression 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');
      };
    }
  • 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');
        };
      }
    }
  • 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[?]

Conditional Expressions & Equality

  • Use === and !== over == and !=.
  • Conditional expressions are evaluated using coercion with the ToBooleanmethod 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 evalute 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
    }
  • Use shortcuts.
    >// bad
    if (name !== '') {
      // ...stuff...
    }
    
    // good
    if (name) {
      // ...stuff...
    }
    
    // bad
    if (collection.length > 0) {
      // ...stuff...
    }
    
    // good
    if (collection.length) {
      // ...stuff...
    }
  • For more information see Truth Equality and JavaScriptby Angus Croll[?]

Blocks

  • 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;
    }

    [?]

Comments

  • Use /** ... */for multiline 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;
    }
  • Use //for single line comments. Place single line comments on a newline above the subject of the comment. Put an emptyline before the comment.
    >// bad
    var active = true;  // is current tab
    
    // good
    // is current tab
    var active = true;
    
    // bad
    function getType() {
      console.log('fetching type...');
      // set the default type to 'no type'
      var type = this._type || 'no type';
    
      return type;
    }
    
    // good
    function getType() {
      console.log('fetching type...');
    
      // set the default type to 'no type'
      var type = this._type || 'no type';
    
      return type;
    }

    [?]

Whitespace

  • Use soft tabs set to 2 spaces
    >// bad
    function() {
    ····var name;
    }
    
    // bad
    function() {
    ·var name;
    }
    
    // good
    function() {
    ··var name;
    }
  • 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'
    });
  • Place an empty newline at the end of the file.
    >// bad
    (function(global) {
      // ...stuff...
    })(this);
    >// good
    (function(global) {
      // ...stuff...
    })(this);

    [?]

  • Use indentation when making long method chains.
    >// bad
    $('#items').find('.selected').highlight().end().find('.open').updateCount();
    
    // good
    $('#items')
    .find('.selected')
      .highlight()
      .end()
    .find('.open')
      .updateCount();
    
    // bad
    var 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
    var 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);

Leading Commas

  • Nope.
    >// bad
    var once
      , upon
      , aTime;
    
    // good
    var once,
        upon,
        aTime;
    
    // bad
    var hero = {
        firstName: 'Bob'
      , lastName: 'Parr'
      , heroName: 'Mr. Incredible'
      , superPower: 'strength'
    };
    
    // good
    var hero = {
      firstName: 'Bob',
      lastName: 'Parr',
      heroName: 'Mr. Incredible',
      superPower: 'strength'
    };

    [?]

Semicolons

  • Yup.
    >// bad
    (function() {
      var name = 'Skywalker'
      return name
    })()
    
    // good
    (function() {
      var name = 'Skywalker';
      return name;
    })();
    
    // good
    ;(function() {
      var name = 'Skywalker';
      return name;
    })();

    [?]

Type Casting & Coercion

  • Perform type coercion at the beginning of the statement.
  • Strings:
    >//  => this.reviewScore = 9;
    
    // bad
    var totalScore = this.reviewScore + '';
    
    // good
    var totalScore = '' + this.reviewScore;
    
    // bad
    var totalScore = '' + this.reviewScore + ' total score';
    
    // good
    var totalScore = this.reviewScore + ' total score';
  • Use parseInt for Numbers and always with a radix for type casting.
  • 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.
    >var inputValue = '4';
    
    // bad
    var val = new Number(inputValue);
    
    // bad
    var val = +inputValue;
    
    // bad
    var val = inputValue >> 0;
    
    // bad
    var val = parseInt(inputValue);
    
    // good
    var val = Number(inputValue);
    
    // good
    var val = parseInt(inputValue, 10);
    
    // good
    /**
     * parseInt was the reason my code was slow.
     * Bitshifting the String to coerce it to a
     * Number made it a lot faster.
     */
    var val = inputValue >> 0;
  • Booleans:
    >var age = 0;
    
    // bad
    var hasAge = new Boolean(age);
    
    // good
    var hasAge = Boolean(age);
    
    // good
    var hasAge = !!age;

    [?]

Naming Conventions

  • Avoid single letter names. Be descriptive with your naming.
    >// bad
    function q() {
      // ...stuff...
    }
    
    // good
    function query() {
      // ..stuff..
    }
  • Use camelCase when naming objects, functions, and instances
    >// bad
    var OBJEcttsssss = {};
    var this_is_my_object = {};
    var this-is-my-object = {};
    function c() {};
    var u = new user({
      name: 'Bob Parr'
    });
    
    // good
    var thisIsMyObject = {};
    function thisIsMyFunction() {};
    var user = new User({
      name: 'Bob Parr'
    });
  • Use PascalCase when naming constructors or classes
    >// bad
    function user(options) {
      this.name = options.name;
    }
    
    var bad = new user({
      name: 'nope'
    });
    
    // good
    function User(options) {
      this.name = options.name;
    }
    
    var good = new User({
      name: 'yup'
    });
  • Use a leading underscore _when naming private properties
    >// bad
    this.__firstName__ = 'Panda';
    this.firstName_ = 'Panda';
    
    // good
    this._firstName = 'Panda';
  • Name your functions. This is helpful for stack traces.
    >// bad
    var log = function(msg) {
      console.log(msg);
    };
    
    // good
    var log = function log(msg) {
      console.log(msg);
    };

    [?]

Accessors

  • Accessor functions for properties are not required
  • 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);
  • If the property is a boolean, use isVal() or hasVal()
    >// bad
    if (!dragon.age()) {
      return false;
    }
    
    // good
    if (!dragon.hasAge()) {
      return false;
    }
  • It’s okay to create get() and set() functions, but be consistent.
    >function Jedi(options) {
      options || (options = {});
      var lightsaber = options.lightsaber || 'blue';
      this.set('lightsaber', lightsaber);
    }
    
    Jedi.prototype.set = function(key, val) {
      this[key] = val;
    };
    
    Jedi.prototype.get = function(key) {
      return this[key];
    };

    [?]

Constructors

  • Assign methods to the prototype object, instead of overwriting the prototype with a new object. Overwriting the prototype makes inheritance impossible: by resetting the prototype you’ll overwrite the base!
    >function Jedi() {
      console.log('new jedi');
    }
    
    // bad
    Jedi.prototype = {
      fight: function fight() {
        console.log('fighting');
      },
    
      block: function block() {
        console.log('blocking');
      }
    };
    
    // good
    Jedi.prototype.fight = function fight() {
      console.log('fighting');
    };
    
    Jedi.prototype.block = function block() {
      console.log('blocking');
    };
  • Methods can return thisto help with method chaining.
    >// bad
    Jedi.prototype.jump = function() {
      this.jumping = true;
      return true;
    };
    
    Jedi.prototype.setHeight = function(height) {
      this.height = height;
    };
    
    var luke = new Jedi();
    luke.jump(); // => true
    luke.setHeight(20) // => undefined
    
    // good
    Jedi.prototype.jump = function() {
      this.jumping = true;
      return this;
    };
    
    Jedi.prototype.setHeight = function(height) {
      this.height = height;
      return this;
    };
    
    var luke = new Jedi();
    
    luke.jump()
      .setHeight(20);
  • It’s okay to write a custom toString() method, just make sure it works successfully and causes no side effects.
    >function Jedi(options) {
      options || (options = {});
      this.name = options.name || 'no name';
    }
    
    Jedi.prototype.getName = function getName() {
      return this.name;
    };
    
    Jedi.prototype.toString = function toString() {
      return 'Jedi - ' + this.getName();
    };

    [?]

Modules

  • The module should start with a !. This ensures that if a malformed module forgets to include a final semicolon there aren’t errors in production when the scripts get concatenated.
  • The file should be named with camelCase, live in a folder with the same name, and match the name of the single export.
  • Add a method called noConflict() that sets the exported module to the previous version.
  • Always declare 'use strict';at the top of the module.
    >// fancyInput/fancyInput.js
    
    !function(global) {
      'use strict';
    
      var previousFancyInput = global.FancyInput;
    
      function FancyInput(options) {
        options || (options = {});
      }
    
      FancyInput.noConflict = function noConflict() {
        global.FancyInput = previousFancyInput;
      };
    
      global.FancyInput = FancyInput;
    }(this);

    [?]

jQuery

  • Prefix jQuery object variables with a $.
    >// bad
    var sidebar = $('.sidebar');
    
    // good
    var $sidebar = $('.sidebar');
  • Cache jQuery lookups.
    >// bad
    function setSidebar() {
      $('.sidebar').hide();
    
      // ...stuff...
    
      $('.sidebar').css({
        'background-color': 'pink'
      });
    }
    
    // good
    function setSidebar() {
      var $sidebar = $('.sidebar');
      $sidebar.hide();
    
      // ...stuff...
    
      $sidebar.css({
        'background-color': 'pink'
      });
    }
  • For DOM queries use Cascading $('.sidebar ul') or parent > child $('.sidebar > .ul'). jsPerf
  • Use findwith scoped jQuery object queries.
    >// bad
    $('.sidebar', 'ul').hide();
    
    // bad
    $('.sidebar').find('ul').hide();
    
    // good
    $('.sidebar ul').hide();
    
    // good
    $('.sidebar > ul').hide();
    
    // good (slower)
    $sidebar.find('ul');
    
    // good (faster)
    $($sidebar[0]).find('ul');

    [?]

ECMAScript 5 Compatability

Testing

  • Yup.
    >function() {
      return true;
    }

    [?]

Performance

Resources

Read This

Other Styleguides

Books

Blogs

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.

The JavaScript Style Guide Guide

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