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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 Coercion
  16. Naming Conventions
  17. Accessors
  18. Constructors
  19. Modules
  20. jQuery
  21. ES5 Compatability
  22. Testing
  23. Performance
  24. Resources
  25. The JavaScript Style Guide Guide
  26. Contributors
  27. 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 words as 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 reasons use 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] = '<li>' + messages[i].message + '</li>';
  }

  return '<ul>' + items.join('') + '</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 arguments object 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 var 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
var superPower = new SuperPower();

  • Use one var declaration for multiple variables and declare each variable on a newline.

    // bad
var items = getItems();
var goGiants = true;
var dragonball = 'z';

// good
var items = getItems(),
    goGiants = 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(),
    goGiants = true;

// bad
var i, items = getItems(),
    dragonball,
    goGiants = true,
    len;

// good
var items = getItems(),
    goGiants = 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 ToBoolean 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 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 JavaScript by 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 pased in tag name
//
// @param <String> tag
// @return <Element> element
function make(tag) {

  // ...stuff...

  return element;
}

// good
/**
 * make() returns a new element
 * based on the pased 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';i

  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);

    [⬆]

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 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';

  • Numbers:

    var inputValue = '4';

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

// good
var val = Number(inputValue);

// good
var val = +inputValue;

  • 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';

  • 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');
};

  • Constructor methods should try to return this. This helps with method chaining which is often useful.

    // 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'
  });
}

  • Scope jQuery object queries with find. jsPerf

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

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

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

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

    [⬆]

ECMAScript 5 Compatability

[⬆]

Testing

  • Yup.

    function() {
  return true;
}

    [⬆]

Performance

[⬆]

Resources

Read This

Other Styleguides

Books

Blogs

[⬆]

The JavaScript Style Guide Guide

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.

[⬆]

};

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