JavaScript Style Guide
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README.md

README.md

Digitpaint JavaScript Style Guide() {

A mostly reasonable approach to JavaScript

Forked from the excellent Airbnb JavaScript Style Guide, slightly modified to fit Digitpaint's conventions.

Table of Contents

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

Files

  • Avoid version numbers in filenames; this makes updating hard and you probably do not want multiple versions of the same library in your project anyway.

  • Avoid adding minified versions in development; minification should be handled in the release process.

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. 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'
    };

    [⬆]

Arrays

  • Use the literal syntax for array creation

    // bad
    var items = new Array();
    
    // good
    var items = [];
  • If you don't know array length use Array#push.

    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,
        itemsCopy = [],
        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);
      ...
    }

    [⬆]

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 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.

  • 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...
    }

    [⬆]

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 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,
        length,
        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
    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 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

    [⬆]

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 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

    [⬆]

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

  • Always write comments in English. Translate where necessary.

  • 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 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;
    }

  **[[⬆]](#TOC)**


## <a name='whitespace'>Whitespace</a>

- 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'
  });
  ```

- Set off operators with spaces.

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

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

- Place an empty newline at the end of the file.

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

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

  ```

- Use indentation when making long method chains.

  ```javascript
  // 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);
  ```

  **[[⬆]](#TOC)**

## <a name='commas'>Commas</a>

- Leading commas: **Nope.**

  ```javascript
  // 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'
  };
  ```

- Additional trailing comma: **Yup.**

> Why? This leads to cleaner git diffs. Also, transpilers like Babel will remove the additional trailing comma in the transpiled code which means you don't have to worry about the [trailing comma problem](https://github.com/airbnb/javascript/blob/es5-deprecated/es5/README.md#commas) in legacy browsers.

  ```diff
  // bad - git diff without trailing comma
  const hero = {
       firstName: 'Florence',
  -    lastName: 'Nightingale'
  +    lastName: 'Nightingale',
  +    inventorOf: ['coxcomb chart', 'modern nursing']
  };

  // good - git diff with trailing comma
  const hero = {
       firstName: 'Florence',
       lastName: 'Nightingale',
  +    inventorOf: ['coxcomb chart', 'modern nursing'],
  };
  ```

  ```javascript
  // bad
  const hero = {
    firstName: 'Dana',
    lastName: 'Scully'
  };

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

  // good
  const hero = {
    firstName: 'Dana',
    lastName: 'Scully',
  };

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

  // bad
  function createHero(
    firstName,
    lastName,
    inventorOf
  ) {
    // does nothing
  }

  // good
  function createHero(
    firstName,
    lastName,
    inventorOf,
  ) {
    // does nothing
  }

  // good (note that a comma must not appear after a "rest" element)
  function createHero(
    firstName,
    lastName,
    inventorOf,
    ...heroArgs
  ) {
    // does nothing
  }

  // bad
  createHero(
    firstName,
    lastName,
    inventorOf
  );

  // good
  createHero(
    firstName,
    lastName,
    inventorOf,
  );

  // good (note that a comma must not appear after a "rest" element)
  createHero(
    firstName,
    lastName,
    inventorOf,
    ...heroArgs
  )
  ```

  **[[⬆]](#TOC)**


## <a name='semicolons'>Semicolons</a>

- **Yup.**

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

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

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

  **[[⬆]](#TOC)**


## <a name='type-coercion'>Type Casting & Coercion</a>

- 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.
- **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)

  ```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;
  ```

- Booleans:

  ```javascript
  var age = 0;

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

  // good
  var hasAge = Boolean(age);

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

  **[[⬆]](#TOC)**


## <a name='naming-conventions'>Naming Conventions</a>

- 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 = {};
  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

  ```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);
  };
  ```

  **[[⬆]](#TOC)**


## <a name='accessors'>Accessors</a>

- 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];
  };
  ```

  **[[⬆]](#TOC)**


## <a name='constructors'>Constructors</a>

- 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();
  };
  ```

  **[[⬆]](#TOC)**

## <a name='scope'>Scope</a>

- When handling scope (this) reassigning prefer the use of `.bind()` instead of saving a reference to `this` by setting `_this`. In some complex cases it's better to use `_this`.

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

  // ok
  function(){
    var _this = this;
    this.foo = "bar";
    return function(){
      console.log(_this.foo)
    }.bind(this);
  }


  // preferred
  function(){
    this.foo = "bar";
    return function(){
      console.log(this.foo)
    }.bind(this);
  }
  ```
- If using `Function.bind()` and you need to [support older browsers](http://kangax.github.io/es5-compat-table/#Function.prototype.bind), make sure you load an appropriate polyfill (or use `.apply(..)` or `.call(..)` instead.

## <a name='events'>Events</a>

- 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
  });
  ```

**[[⬆]](#TOC)**


## <a name='modules'>Modules</a>

- 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);
  ```

  **[[⬆]](#TOC)**


## <a name='jquery'>jQuery</a>

- 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();
  ```

  **[[⬆]](#TOC)**


## <a name='es5'>ECMAScript 5 Compatibility</a>

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

**[[⬆]](#TOC)**


## <a name='testing'>Unit testing</a>

- Write unit tests wherever possible but don't test vendor or library code.

- Make sure all your tests pass.

- Write descriptive tests; they should be human readable.

- Write tests to requirements, not to your specific code

- Write positive as well as negative tests

- Use a testing framework like Karma to run tests in multiple environments/browsers. See our instructions on [how to set up a Karma environment](instructions/karma.md)


**Good reads:**

- [Set up a Karma/Mocha/Chai base system](instructions/karma.md)
- [Introducing to JS Unit testing](http://www.smashingmagazine.com/2012/06/27/introduction-to-javascript-unit-testing/)
- [Writing testable code](http://alistapart.com/article/writing-testable-javascript)
- [JS Unit testing and why](http://thomas.junghans.co.za/blog/2013/06/25/javascript-unit-testing-and-why/)


**[[⬆]](#TOC)**


## <a name='performance'>Performance</a>

- [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...

**[[⬆]](#TOC)**


## <a name='resources'>Resources</a>


**Read This**

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

**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/)

**Other Styles**

- [Naming this in nested functions](https://gist.github.com/4135065) - Christian Johansen
- [Conditional Callbacks](https://github.com/airbnb/javascript/issues/52)
- [Popular JavaScript Coding Conventions on Github](http://sideeffect.kr/popularconvention/#javascript)

**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

**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/)
- [Third Party JavaScript](http://manning.com/vinegar/) - Ben Vinegar and Anton Kovalyov

**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)

**[[⬆]](#TOC)**

## <a name='in-the-wild'>In the Wild</a>

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)
- **Airbnb**: [airbnb/javascript](https://github.com/airbnb/javascript)
- **American Insitutes for Research**: [AIRAST/javascript](https://github.com/AIRAST/javascript)
- **Compass Learning**: [compasslearning/javascript-style-guide](https://github.com/compasslearning/javascript-style-guide)
- **Digitpaint** [digitpaint/javascript](https://github.com/digitpaint/javascript)
- **ExactTarget**: [ExactTarget/javascript](https://github.com/ExactTarget/javascript)
- **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)
- **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)
- **National Geographic**: [natgeo/javascript](https://github.com/natgeo/javascript)
- **National Park Service**: [nationalparkservice/javascript](https://github.com/nationalparkservice/javascript)
- **Peerby**: [Peerby/javascript](https://github.com/Peerby/javascript)
- **Razorfish**: [razorfish/javascript-style-guide](https://github.com/razorfish/javascript-style-guide)
- **REI**: [reidev/js-style-guide](https://github.com/reidev/js-style-guide)
- **Shutterfly**: [shutterfly/javascript](https://github.com/shutterfly/javascript)
- **Userify**: [userify/javascript](https://github.com/userify/javascript)
- **Zillow**: [zillow/javascript](https://github.com/zillow/javascript)
- **ZocDoc**: [ZocDoc/javascript](https://github.com/ZocDoc/javascript)

## <a name='translation'>Translation</a>

This style guide is also available in other languages:

- :de: **German**: [timofurrer/javascript-style-guide](https://github.com/timofurrer/javascript-style-guide)
- :jp: **Japanese**: [mitsuruog/javacript-style-guide](https://github.com/mitsuruog/javacript-style-guide)
- :br: **Portuguese**: [armoucar/javascript-style-guide](https://github.com/armoucar/javascript-style-guide)
- :cn: **Chinese**: [adamlu/javascript-style-guide](https://github.com/adamlu/javascript-style-guide)
- :es: **Spanish**: [paolocarrasco/javascript-style-guide](https://github.com/paolocarrasco/javascript-style-guide)
- :kr: **Korean**: [tipjs/javascript-style-guide](https://github.com/tipjs/javascript-style-guide)
- :fr: **French**: [nmussy/javascript-style-guide](https://github.com/nmussy/javascript-style-guide)
- :ru: **Russian**: [uprock/javascript](https://github.com/uprock/javascript)
- :bg: **Bulgarian**: [borislavvv/javascript](https://github.com/borislavvv/javascript)

## <a name='guide-guide'>The JavaScript Style Guide Guide</a>

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

## <a name='authors'>Contributors</a>

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


## <a name='license'>License</a>

(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.

**[[⬆]](#TOC)**

# };