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Airbnb JavaScript Style Guide() {

A mostly reasonable approach to JavaScript

Types
Objects
Arrays
Strings
Functions
Properties
Variables
Hoisting
Comparison Operators & Equality
Blocks
Comments
Whitespace
Commas
Semicolons
Type Casting & Coercion
Naming Conventions
Accessors
Constructors
Events
Modules
jQuery
ECMAScript 5 Compatibility
Testing
Performance
Resources
In the Wild
Translation
The JavaScript Style Guide Guide
Chat With Us About Javascript
Contributors
License

Types

Primitives: When you access a primitive type you work directly on its value.
- string
- number
- boolean
- null
- undefined
```
var foo = 1;
var 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];
var bar = foo;

bar[0] = 9;

console.log(foo[0], bar[0]); // => 9, 9

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Objects

Use the literal syntax for object creation.

// bad
var item = new Object();

// good
var item = {};

Don't use reserved words as keys. It won't work in IE8. More info.

// bad
var superman = {
  default: { clark: 'kent' },
  private: true
};

// good
var superman = {
  defaults: { clark: 'kent' },
  hidden: true
};

Use readable synonyms in place of reserved words.

// bad
var superman = {
  class: 'alien'
};

// bad
var superman = {
  klass: 'alien'
};

// good
var superman = {
  type: 'alien'
};

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Arrays

Use the literal syntax for array creation.

// bad
var items = new Array();

// good
var items = [];

Use Array#push instead of direct assignment to add items to an array.

var someStack = [];


// bad
someStack[someStack.length] = 'abracadabra';

// good
someStack.push('abracadabra');

When you need to copy an array use Array#slice. jsPerf

var len = items.length;
var itemsCopy = [];
var i;

// bad
for (i = 0; i < len; i++) {
  itemsCopy[i] = items[i];
}

// good
itemsCopy = items.slice();

To convert an array-like object to an array, use Array#slice.

function trigger() {
  var args = Array.prototype.slice.call(arguments);
  ...
}

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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.

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

// 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 programmatically building up a string, use Array#join instead of string concatenation. Mostly for IE: jsPerf.

var items;
var messages;
var length;
var 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++) {
    // use direct assignment in this case because we're micro-optimizing.
    items[i] = '<li>' + messages[i].message + '</li>';
  }

  return '<ul>' + items.join('') + '</ul>';
}

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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.

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
var test;
if (currentUser) {
  test = function test() {
    console.log('Yup.');
  };
}

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...
}

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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');

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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 per variable. 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
var items = getItems(),
    goSportsTeam = true,
    dragonball = 'z';

// bad
// (compare to above, and try to spot the mistake)
var items = getItems(),
    goSportsTeam = true;
    dragonball = 'z';

// good
var items = getItems();
var goSportsTeam = true;
var 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;
var items = getItems();
var dragonball;
var goSportsTeam = true;
var len;

// good
var items = getItems();
var goSportsTeam = true;
var dragonball;
var length;
var i;

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 - unnessary function call
function() {
  var name = getName();

  if (!arguments.length) {
    return false;
  }

  this.setFirstName(name);

  return true;
}

// good
function() {
  var name;

  if (!arguments.length) {
    return false;
  }

  name = getName();
  this.setFirstName(name);

  return true;
}

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Hoisting

Variable declarations get hoisted to the top of their scope, but 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 interpreter 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 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');
  };
}

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.

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Comparison Operators & Equality

Use === and !== over == and !=.
Comparison operators 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 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
}
```

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.

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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;
}

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

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;
}

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
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;
}

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.

Use // FIXME: to annotate problems.

function Calculator() {

  // FIXME: shouldn't use a global here
  total = 0;

  return this;
}

Use // TODO: to annotate solutions to problems.

function Calculator() {

  // TODO: total should be configurable by an options param
  this.total = 0;

  return this;
}


**[⬆ back to top](#table-of-contents)**


## Whitespace

- Use soft tabs set to 2 spaces.

  ```javascript
  // bad
  function() {
  ∙∙∙∙var name;
  }

  // bad
  function() {
  ∙var name;
  }

  // good
  function() {
  ∙∙var name;
  }
  ```

- Place 1 space before the leading brace.

  ```javascript
  // 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 1 space before the opening parenthesis in control statements (`if`, `while` etc.). Place no space before the argument list in function calls and declarations.

  ```javascript
  // bad
  if(isJedi) {
    fight ();
  }

  // good
  if (isJedi) {
    fight();
  }

  // bad
  function fight () {
    console.log ('Swooosh!');
  }

  // good
  function fight() {
    console.log('Swooosh!');
  }
  ```

- Set off operators with spaces.

  ```javascript
  // bad
  var x=y+5;

  // good
  var x = y + 5;
  ```

- End files with a single newline character.

  ```javascript
  // bad
  (function(global) {
    // ...stuff...
  })(this);
  ```

  ```javascript
  // bad
  (function(global) {
    // ...stuff...
  })(this);↵
  ↵
  ```

  ```javascript
  // good
  (function(global) {
    // ...stuff...
  })(this);↵
  ```

- Use indentation when making long method chains. Use a leading dot, which
  emphasizes that the line is a method call, not a new statement.

  ```javascript
  // 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
  var leds = stage.selectAll('.led').data(data).enter().append('svg:svg').classed('led', true)
      .attr('width', (radius + margin) * 2).append('svg:g')
      .attr('transform', 'translate(' + (radius + margin) + ',' + (radius + margin) + ')')
      .call(tron.led);

  // good
  var 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);
  ```

- Leave a blank line after blocks and before the next statement

  ```javascript
  // bad
  if (foo) {
    return bar;
  }
  return baz;

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

  return baz;

  // bad
  var obj = {
    foo: function() {
    },
    bar: function() {
    }
  };
  return obj;

  // good
  var obj = {
    foo: function() {
    },

    bar: function() {
    }
  };

  return obj;
  ```


**[⬆ back to top](#table-of-contents)**

## Commas

- Leading commas: **Nope.**

  ```javascript
  // bad
  var story = [
      once
    , upon
    , aTime
  ];

  // good
  var story = [
    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'
  };
  ```

- Additional trailing comma: **Nope.** This can cause problems with IE6/7 and IE9 if it's in quirksmode. Also, in some implementations of ES3 would add length to an array if it had an additional trailing comma. This was clarified in ES5 ([source](http://es5.github.io/#D)):

> Edition 5 clarifies the fact that a trailing comma at the end of an ArrayInitialiser does not add to the length of the array. This is not a semantic change from Edition 3 but some implementations may have previously misinterpreted this.

  ```javascript
  // bad
  var hero = {
    firstName: 'Kevin',
    lastName: 'Flynn',
  };

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

  // good
  var hero = {
    firstName: 'Kevin',
    lastName: 'Flynn'
  };

  var heroes = [
    'Batman',
    'Superman'
  ];
  ```

**[⬆ back to top](#table-of-contents)**


## Semicolons

- **Yup.**

  ```javascript
  // bad
  (function() {
    var name = 'Skywalker'
    return name
  })()

  // good
  (function() {
    var name = 'Skywalker';
    return name;
  })();

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

  [Read more](http://stackoverflow.com/a/7365214/1712802).

**[⬆ back to top](#table-of-contents)**


## Type Casting & Coercion

- Perform type coercion at the beginning of the statement.
- Strings:

  ```javascript
  //  => 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.

  ```javascript
  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);
  ```

- If for whatever reason you are doing something wild and `parseInt` is your bottleneck and need to use Bitshift for [performance reasons](http://jsperf.com/coercion-vs-casting/3), leave a comment explaining why and what you're doing.

  ```javascript
  // 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;
  ```

- **Note:** Be careful when using bitshift operations. Numbers are represented as [64-bit values](http://es5.github.io/#x4.3.19), but Bitshift operations always return a 32-bit integer ([source](http://es5.github.io/#x11.7)). Bitshift can lead to unexpected behavior for integer values larger than 32 bits. [Discussion](https://github.com/airbnb/javascript/issues/109). Largest signed 32-bit Int is 2,147,483,647:

  ```javascript
  2147483647 >> 0 //=> 2147483647
  2147483648 >> 0 //=> -2147483648
  2147483649 >> 0 //=> -2147483647
  ```

- Booleans:

  ```javascript
  var age = 0;

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

  // good
  var hasAge = Boolean(age);

  // good
  var hasAge = !!age;
  ```

**[⬆ back to top](#table-of-contents)**


## Naming Conventions

- Avoid single letter names. Be descriptive with your naming.

  ```javascript
  // bad
  function q() {
    // ...stuff...
  }

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

- Use camelCase when naming objects, functions, and instances.

  ```javascript
  // bad
  var OBJEcttsssss = {};
  var this_is_my_object = {};
  var o = {};
  function c() {}

  // good
  var thisIsMyObject = {};
  function thisIsMyFunction() {}
  ```

- Use PascalCase when naming constructors or classes.

  ```javascript
  // 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.

  ```javascript
  // bad
  this.__firstName__ = 'Panda';
  this.firstName_ = 'Panda';

  // good
  this._firstName = 'Panda';
  ```

- When saving a reference to `this` use `_this`.

  ```javascript
  // bad
  function() {
    var self = this;
    return function() {
      console.log(self);
    };
  }

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

  // good
  function() {
    var _this = this;
    return function() {
      console.log(_this);
    };
  }
  ```

- Name your functions. This is helpful for stack traces.

  ```javascript
  // bad
  var log = function(msg) {
    console.log(msg);
  };

  // good
  var log = function log(msg) {
    console.log(msg);
  };
  ```

- **Note:** IE8 and below exhibit some quirks with named function expressions.  See [http://kangax.github.io/nfe/](http://kangax.github.io/nfe/) for more info.

- If your file exports a single class, your filename should be exactly the name of the class.
  ```javascript
  // file contents
  class CheckBox {
    // ...
  }
  module.exports = CheckBox;

  // in some other file
  // bad
  var CheckBox = require('./checkBox');

  // bad
  var CheckBox = require('./check_box');

  // good
  var CheckBox = require('./CheckBox');
  ```

**[⬆ back to top](#table-of-contents)**


## Accessors

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

  ```javascript
  // bad
  dragon.age();

  // good
  dragon.getAge();

  // bad
  dragon.age(25);

  // good
  dragon.setAge(25);
  ```

- If the property is a boolean, use isVal() or hasVal().

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

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

- It's okay to create get() and set() functions, but be consistent.

  ```javascript
  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];
  };
  ```

**[⬆ back to top](#table-of-contents)**


## 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!

  ```javascript
  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 `this` to help with method chaining.

  ```javascript
  // 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.

  ```javascript
  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();
  };
  ```

**[⬆ back to top](#table-of-contents)**


## Events

- 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:

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

  ...

  $(this).on('listingUpdated', function(e, listingId) {
    // do something with listingId
  });
  ```

  prefer:

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

  ...

  $(this).on('listingUpdated', function(e, data) {
    // do something with data.listingId
  });
  ```

**[⬆ back to top](#table-of-contents)**


## 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. [Explanation](https://github.com/airbnb/javascript/issues/44#issuecomment-13063933)
- 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 and returns this one.
- Always declare `'use strict';` at the top of the module.

  ```javascript
  // fancyInput/fancyInput.js

  !function(global) {
    'use strict';

    var previousFancyInput = global.FancyInput;

    function FancyInput(options) {
      this.options = options || {};
    }

    FancyInput.noConflict = function noConflict() {
      global.FancyInput = previousFancyInput;
      return FancyInput;
    };

    global.FancyInput = FancyInput;
  }(this);
  ```

**[⬆ back to top](#table-of-contents)**


## jQuery

- Prefix jQuery object variables with a `$`.

  ```javascript
  // bad
  var sidebar = $('.sidebar');

  // good
  var $sidebar = $('.sidebar');
  ```

- Cache jQuery lookups.

  ```javascript
  // 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](http://jsperf.com/jquery-find-vs-context-sel/16)
- Use `find` with scoped jQuery object queries.

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

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

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

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

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

**[⬆ back to top](#table-of-contents)**


## ECMAScript 5 Compatibility

- Refer to [Kangax](https://twitter.com/kangax/)'s ES5 [compatibility table](http://kangax.github.com/es5-compat-table/).

**[⬆ back to top](#table-of-contents)**


## Testing

- **Yup.**

  ```javascript
  function() {
    return true;
  }
  ```

**[⬆ back to top](#table-of-contents)**


## Performance

- [On Layout & Web Performance](http://kellegous.com/j/2013/01/26/layout-performance/)
- [String vs Array Concat](http://jsperf.com/string-vs-array-concat/2)
- [Try/Catch Cost In a Loop](http://jsperf.com/try-catch-in-loop-cost)
- [Bang Function](http://jsperf.com/bang-function)
- [jQuery Find vs Context, Selector](http://jsperf.com/jquery-find-vs-context-sel/13)
- [innerHTML vs textContent for script text](http://jsperf.com/innerhtml-vs-textcontent-for-script-text)
- [Long String Concatenation](http://jsperf.com/ya-string-concat)
- Loading...

**[⬆ back to top](#table-of-contents)**


## Resources


**Read This**

- [Annotated ECMAScript 5.1](http://es5.github.com/)

**Tools**

- Code Style Linters
  + [JSHint](http://www.jshint.com/) - [Airbnb Style .jshintrc](https://github.com/airbnb/javascript/blob/master/linters/jshintrc)
  + [JSCS](https://github.com/jscs-dev/node-jscs) - [Airbnb Style Preset](https://github.com/jscs-dev/node-jscs/blob/master/presets/airbnb.json)

**Other Styleguides**

- [Google JavaScript Style Guide](http://google-styleguide.googlecode.com/svn/trunk/javascriptguide.xml)
- [jQuery Core Style Guidelines](http://docs.jquery.com/JQuery_Core_Style_Guidelines)
- [Principles of Writing Consistent, Idiomatic JavaScript](https://github.com/rwldrn/idiomatic.js/)
- [JavaScript Standard Style](https://github.com/feross/standard)

**Other Styles**

- [Naming this in nested functions](https://gist.github.com/4135065) - Christian Johansen
- [Conditional Callbacks](https://github.com/airbnb/javascript/issues/52) - Ross Allen
- [Popular JavaScript Coding Conventions on Github](http://sideeffect.kr/popularconvention/#javascript) - JeongHoon Byun
- [Multiple var statements in JavaScript, not superfluous](http://benalman.com/news/2012/05/multiple-var-statements-javascript/) - Ben Alman

**Further Reading**

- [Understanding JavaScript Closures](http://javascriptweblog.wordpress.com/2010/10/25/understanding-javascript-closures/) - Angus Croll
- [Basic JavaScript for the impatient programmer](http://www.2ality.com/2013/06/basic-javascript.html) - Dr. Axel Rauschmayer
- [You Might Not Need jQuery](http://youmightnotneedjquery.com/) - Zack Bloom & Adam Schwartz
- [ES6 Features](https://github.com/lukehoban/es6features) - Luke Hoban
- [Frontend Guidelines](https://github.com/bendc/frontend-guidelines) - Benjamin De Cock

**Books**

- [JavaScript: The Good Parts](http://www.amazon.com/JavaScript-Good-Parts-Douglas-Crockford/dp/0596517742) - Douglas Crockford
- [JavaScript Patterns](http://www.amazon.com/JavaScript-Patterns-Stoyan-Stefanov/dp/0596806752) - Stoyan Stefanov
- [Pro JavaScript Design Patterns](http://www.amazon.com/JavaScript-Design-Patterns-Recipes-Problem-Solution/dp/159059908X)  - Ross Harmes and Dustin Diaz
- [High Performance Web Sites: Essential Knowledge for Front-End Engineers](http://www.amazon.com/High-Performance-Web-Sites-Essential/dp/0596529309) - Steve Souders
- [Maintainable JavaScript](http://www.amazon.com/Maintainable-JavaScript-Nicholas-C-Zakas/dp/1449327680) - Nicholas C. Zakas
- [JavaScript Web Applications](http://www.amazon.com/JavaScript-Web-Applications-Alex-MacCaw/dp/144930351X) - Alex MacCaw
- [Pro JavaScript Techniques](http://www.amazon.com/Pro-JavaScript-Techniques-John-Resig/dp/1590597273) - John Resig
- [Smashing Node.js: JavaScript Everywhere](http://www.amazon.com/Smashing-Node-js-JavaScript-Everywhere-Magazine/dp/1119962595) - Guillermo Rauch
- [Secrets of the JavaScript Ninja](http://www.amazon.com/Secrets-JavaScript-Ninja-John-Resig/dp/193398869X) - John Resig and Bear Bibeault
- [Human JavaScript](http://humanjavascript.com/) - Henrik Joreteg
- [Superhero.js](http://superherojs.com/) - Kim Joar Bekkelund, Mads Mobæk, & Olav Bjorkoy
- [JSBooks](http://jsbooks.revolunet.com/) - Julien Bouquillon
- [Third Party JavaScript](http://manning.com/vinegar/) - Ben Vinegar and Anton Kovalyov
- [Effective JavaScript: 68 Specific Ways to Harness the Power of JavaScript](http://amzn.com/0321812182) - David Herman
- [Eloquent JavaScript](http://eloquentjavascript.net) - Marijn Haverbeke
- [You Don't Know JS](https://github.com/getify/You-Dont-Know-JS) - Kyle Simpson

**Blogs**

- [DailyJS](http://dailyjs.com/)
- [JavaScript Weekly](http://javascriptweekly.com/)
- [JavaScript, JavaScript...](http://javascriptweblog.wordpress.com/)
- [Bocoup Weblog](http://weblog.bocoup.com/)
- [Adequately Good](http://www.adequatelygood.com/)
- [NCZOnline](http://www.nczonline.net/)
- [Perfection Kills](http://perfectionkills.com/)
- [Ben Alman](http://benalman.com/)
- [Dmitry Baranovskiy](http://dmitry.baranovskiy.com/)
- [Dustin Diaz](http://dustindiaz.com/)
- [nettuts](http://net.tutsplus.com/?s=javascript)

**Podcasts**

- [JavaScript Jabber](http://devchat.tv/js-jabber/)


**[⬆ back to top](#table-of-contents)**

## 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.

- **Aan Zee**: [AanZee/javascript](https://github.com/AanZee/javascript)
- **Adult Swim**: [adult-swim/javascript](https://github.com/adult-swim/javascript)
- **Airbnb**: [airbnb/javascript](https://github.com/airbnb/javascript)
- **American Insitutes for Research**: [AIRAST/javascript](https://github.com/AIRAST/javascript)
- **Apartmint**: [apartmint/javascript](https://github.com/apartmint/javascript)
- **Avalara**: [avalara/javascript](https://github.com/avalara/javascript)
- **Compass Learning**: [compasslearning/javascript-style-guide](https://github.com/compasslearning/javascript-style-guide)
- **DailyMotion**: [dailymotion/javascript](https://github.com/dailymotion/javascript)
- **Digitpaint** [digitpaint/javascript](https://github.com/digitpaint/javascript)
- **Evernote**: [evernote/javascript-style-guide](https://github.com/evernote/javascript-style-guide)
- **ExactTarget**: [ExactTarget/javascript](https://github.com/ExactTarget/javascript)
- **Flexberry**: [Flexberry/javascript-style-guide](https://github.com/Flexberry/javascript-style-guide)
- **Gawker Media**: [gawkermedia/javascript](https://github.com/gawkermedia/javascript)
- **GeneralElectric**: [GeneralElectric/javascript](https://github.com/GeneralElectric/javascript)
- **GoodData**: [gooddata/gdc-js-style](https://github.com/gooddata/gdc-js-style)
- **Grooveshark**: [grooveshark/javascript](https://github.com/grooveshark/javascript)
- **How About We**: [howaboutwe/javascript](https://github.com/howaboutwe/javascript)
- **InfoJobs**: [InfoJobs/JavaScript-Style-Guide](https://github.com/InfoJobs/JavaScript-Style-Guide)
- **Intent Media**: [intentmedia/javascript](https://github.com/intentmedia/javascript)
- **Jam3**: [Jam3/Javascript-Code-Conventions](https://github.com/Jam3/Javascript-Code-Conventions)
- **JSSolutions**: [JSSolutions/javascript](https://github.com/JSSolutions/javascript)
- **Kinetica Solutions**: [kinetica/javascript](https://github.com/kinetica/javascript)
- **Mighty Spring**: [mightyspring/javascript](https://github.com/mightyspring/javascript)
- **MinnPost**: [MinnPost/javascript](https://github.com/MinnPost/javascript)
- **ModCloth**: [modcloth/javascript](https://github.com/modcloth/javascript)
- **Money Advice Service**: [moneyadviceservice/javascript](https://github.com/moneyadviceservice/javascript)
- **Muber**: [muber/javascript](https://github.com/muber/javascript)
- **National Geographic**: [natgeo/javascript](https://github.com/natgeo/javascript)
- **National Park Service**: [nationalparkservice/javascript](https://github.com/nationalparkservice/javascript)
- **Nimbl3**: [nimbl3/javascript](https://github.com/nimbl3/javascript)
- **Nordic Venture Family**: [CodeDistillery/javascript](https://github.com/CodeDistillery/javascript)
- **Orion Health**: [orionhealth/javascript](https://github.com/orionhealth/javascript)
- **Peerby**: [Peerby/javascript](https://github.com/Peerby/javascript)
- **Razorfish**: [razorfish/javascript-style-guide](https://github.com/razorfish/javascript-style-guide)
- **reddit**: [reddit/styleguide/javascript](https://github.com/reddit/styleguide/tree/master/javascript)
- **REI**: [reidev/js-style-guide](https://github.com/reidev/js-style-guide)
- **Ripple**: [ripple/javascript-style-guide](https://github.com/ripple/javascript-style-guide)
- **SeekingAlpha**: [seekingalpha/javascript-style-guide](https://github.com/seekingalpha/javascript-style-guide)
- **Shutterfly**: [shutterfly/javascript](https://github.com/shutterfly/javascript)
- **StudentSphere**: [studentsphere/javascript](https://github.com/studentsphere/javascript)
- **Target**: [target/javascript](https://github.com/target/javascript)
- **TheLadders**: [TheLadders/javascript](https://github.com/TheLadders/javascript)
- **T4R Technology**: [T4R-Technology/javascript](https://github.com/T4R-Technology/javascript)
- **Userify**: [userify/javascript](https://github.com/userify/javascript)
- **VoxFeed**: [VoxFeed/javascript-style-guide](https://github.com/VoxFeed/javascript-style-guide)
- **Weggo**: [Weggo/javascript](https://github.com/Weggo/javascript)
- **Zillow**: [zillow/javascript](https://github.com/zillow/javascript)
- **ZocDoc**: [ZocDoc/javascript](https://github.com/ZocDoc/javascript)

## Translation

This style guide is also available in other languages:

- ![br](https://raw.githubusercontent.com/gosquared/flags/master/flags/flags/shiny/24/Brazil.png) **Brazilian Portuguese**: [armoucar/javascript-style-guide](https://github.com/armoucar/javascript-style-guide)
- ![bg](https://raw.githubusercontent.com/gosquared/flags/master/flags/flags/shiny/24/Bulgaria.png) **Bulgarian**: [borislavvv/javascript](https://github.com/borislavvv/javascript)
- ![ca](https://raw.githubusercontent.com/fpmweb/javascript-style-guide/master/img/catala.png) **Catalan**: [fpmweb/javascript-style-guide](https://github.com/fpmweb/javascript-style-guide)
- ![tw](https://raw.githubusercontent.com/gosquared/flags/master/flags/flags/shiny/24/Taiwan.png) **Chinese(Traditional)**: [jigsawye/javascript](https://github.com/jigsawye/javascript)
- ![cn](https://raw.githubusercontent.com/gosquared/flags/master/flags/flags/shiny/24/China.png) **Chinese(Simplified)**: [adamlu/javascript-style-guide](https://github.com/adamlu/javascript-style-guide)
- ![fr](https://raw.githubusercontent.com/gosquared/flags/master/flags/flags/shiny/24/France.png) **French**: [nmussy/javascript-style-guide](https://github.com/nmussy/javascript-style-guide)
- ![de](https://raw.githubusercontent.com/gosquared/flags/master/flags/flags/shiny/24/Germany.png) **German**: [timofurrer/javascript-style-guide](https://github.com/timofurrer/javascript-style-guide)
- ![it](https://raw.githubusercontent.com/gosquared/flags/master/flags/flags/shiny/24/Italy.png) **Italian**: [sinkswim/javascript-style-guide](https://github.com/sinkswim/javascript-style-guide)
- ![jp](https://raw.githubusercontent.com/gosquared/flags/master/flags/flags/shiny/24/Japan.png) **Japanese**: [mitsuruog/javacript-style-guide](https://github.com/mitsuruog/javacript-style-guide)
- ![kr](https://raw.githubusercontent.com/gosquared/flags/master/flags/flags/shiny/24/South-Korea.png) **Korean**: [tipjs/javascript-style-guide](https://github.com/tipjs/javascript-style-guide)
- ![pl](https://raw.githubusercontent.com/gosquared/flags/master/flags/flags/shiny/24/Poland.png) **Polish**: [mjurczyk/javascript](https://github.com/mjurczyk/javascript)
- ![ru](https://raw.githubusercontent.com/gosquared/flags/master/flags/flags/shiny/24/Russia.png) **Russian**: [uprock/javascript](https://github.com/uprock/javascript)
- ![es](https://raw.githubusercontent.com/gosquared/flags/master/flags/flags/shiny/24/Spain.png) **Spanish**: [paolocarrasco/javascript-style-guide](https://github.com/paolocarrasco/javascript-style-guide)
- ![th](https://raw.githubusercontent.com/gosquared/flags/master/flags/flags/shiny/24/Thailand.png) **Thai**: [lvarayut/javascript-style-guide](https://github.com/lvarayut/javascript-style-guide)

## The JavaScript Style Guide Guide

- [Reference](https://github.com/airbnb/javascript/wiki/The-JavaScript-Style-Guide-Guide)

## Chat With Us About JavaScript

- Find us on [gitter](https://gitter.im/airbnb/javascript).

## Contributors

- [View Contributors](https://github.com/airbnb/javascript/graphs/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.

**[⬆ back to top](#table-of-contents)**

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README.md

Airbnb JavaScript Style Guide() {

Table of Contents

Types

Objects

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Functions

Properties

Variables

Hoisting

Comparison Operators & Equality

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Airbnb JavaScript Style Guide() {

Table of Contents

Types

Objects

Arrays

Strings

Functions

Properties

Variables

Hoisting

Comparison Operators & Equality

Blocks

Comments