A mostly reasonable approach to JavaScript
- Types
- Objects
- Arrays
- Strings
- Functions
- Properties
- Variables
- Hoisting
- Conditional Expressions & Equality
- Blocks
- Comments
- Whitespace
- Leading Commas
- Semicolons
- Type Coercion
- Naming Conventions
- Accessors
- Constructors
- Modules
- jQuery
- ES5 Compatability
- Testing
- Performance
- Resources
- The JavaScript Style Guide Guide
- Contributors
- License
-
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
-
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 };
-
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]; }
-
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>'; }
-
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 thearguments
object that is given to every function scope.// bad function nope(name, options, arguments) { // ...stuff... } // good function yup(name, options, args) { // ...stuff... }
-
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');
-
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 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; }
-
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
-
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 }
- Objects evaluate to
-
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
-
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; }
-
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; }
-
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);
-
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' };
-
Yup.
// bad (function() { var name = 'Skywalker' return name })() // good (function() { var name = 'Skywalker'; return name; })(); // good ;(function() { var name = 'Skywalker'; return name; })();
-
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;
-
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); };
-
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]; };
-
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(); };
-
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);
-
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();
- Refer to Kangax's ES5 compatibility table
-
Yup.
function() { return true; }
- String vs Array Concat
- Try/Catch Cost In a Loop
- Bang Function
- jQuery Find vs Context, Selector
- Loading...
Read This
Other Styleguides
- Google JavaScript Style Guide
- jQuery Core Style Guidelines
- Principles of Writing Consistent, Idiomatic JavaScript
Books
- 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
Blogs
- DailyJS
- JavaScript Weekly
- JavaScript, JavaScript...
- Bocoup Weblog
- Adequately Good
- NCZOnline
- Perfection Kills
- Ben Alman
- Dmitry Baranovskiy
- Dustin Diaz
- net.tutsplus
(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.