A mostly reasonable approach to JavaScript, based on AirBnb's style guide

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

• Primitives: When you access a primitive type you work directly on its value

  • string
  • number
  • boolean
  • null
  • undefined

  let 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

  let foo = [1, 2],
      bar = foo;
  
  bar[0] = 9;
  
  console.log(foo[0], bar[0]); // => 9, 9

  ↑ Back to top

• Use the literal syntax for object creation.

  // bad
  let item = new Object();
  
  // good
  let item = {};

• (ES6) Use enhanced object literal instantiation to reduce boilerplate when assigning values to properties, methods, and dynamically computed property names:

  // bad
  let private = true,
      weakness = 'kryptonite';
  
  let superman = {
      private: private,
      fly: function () {
          console.log('Faster than a speeding bullet');
      }
  }
  
  superman[weakness] = true;
  
  // good
  let private = true,
      weakness = 'kryptonite';
  
  let superman = {
      private,
      fly () {
          console.log('Faster than a speeding bullet');
      },
      [weakness]: true
  }
  

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

  // bad
  let superman = {
      default: { clark: 'kent' },
      private: true
  };
  
  // good
  let superman = {
      defaults: { clark: 'kent' },
      hidden: true
  };

• Use readable synonyms in place of reserved words.

  // bad
  let superman = {
      class: 'alien'
  };
  
  // bad
  let superman = {
      klass: 'alien'
  };
  
  // good
  let superman = {
      type: 'alien'
  };

  ↑ Back to top

• Use the literal syntax for array creation

  // bad
  let items = new Array();
  
  // good
  let items = [];

• If you don't know array length use Array#push.

  let someStack = [];
  
  
  // bad
  someStack[someStack.length] = 'abracadabra';
  
  // good
  someStack.push('abracadabra');

• (ES6) To copy an array, use the spread operator:

  let items = [1, 2, 3],
      itemsCopy;
  
  // bad
  for (let i = 0; i < items.length; i++) {
      itemsCopy[i] = items[i];
  }
  
  // good
  itemsCopy = [...items];

• (ES5) In ES5 environments, use Array#slice to copy an array.

  var items = [1, 2, 3],
      itemsCopy;
  
  // bad
  for (var i = 0; i < items.length; i++) {
      itemsCopy[i] = items[i];
  }
  
  // good
  itemsCopy = items.slice();

• (ES6) To convert an array-like object to an array, use the spread operator.

  let nodeList = document.getElementsByTagName('a'),
      nodesArray = [...nodesList];

• (ES5) To convert an array-like object to an array in ES5 environments, use Array#slice.

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

• (ES5) To convert an array-like object to an array in ES5 environments, use Array#slice.

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

  ↑ Back to top

• Use single quotes '' for strings

  // bad
  const NAME = "Bob Parr";
  
  // good
  const NAME = 'Bob Parr';

• (ES6) Use template strings for interpolating variables in strings

  // bad
  let fullName = 'Bob ' + this.lastName;
  
  // good
  let fullname = `Bob ${this.lastName}`;

• (ES6) Strings longer than 80 characters should be written across multiple lines using template strings:

  // bad
  let 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
  let 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
  let 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.`;

• (ES5) Strings longer than 80 characters should be written across multiple lines using string concatenation in ES5 environments.

• 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.

  let items,
      messages,
      length;
  
  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 (let i = 0; i < length; i++) {
          items += '<li>' + messages[i].message + '</li>';
      }
  
      return items + '</ul>';
  }
  
  // good
  function inbox (messages) {
      items = [];
  
      for (let i = 0; i < length; i++) {
          items[i] = messages[i].message;
      }
  
      return '<ul><li>' + items.join('</li><li>') + '</li></ul>';
  }

  ↑ Back to top

• There are lots of ways to declare and use functions. Here's a list:

  // anonymous function expression
  let anonymous = function () {
      return true;
  };
  
  // named function expression
  let named = function named () {
      return true;
  };
  
  // arrow function expression
  let dogFilter = animal => typeof animal === 'Dog';
  
  // immediately-invoked function expression (IIFE)
  (function () {
      console.log('Welcome to the Internet. Please follow me.');
  })();
  
  // function declaration
  function greetings (yourName) {
      console.log(`Glad you could join us, ${yourName}`);
  }

• Um, that's a lot. Which to use? Here are some rules of thumb:

• (ES6) For simple callbacks and functions passed as parameters, use arrow function shorthand. Arrow functions are lexically scoped, which makes them great for things like event handlers:

  // bad
  let self = this;
  $('#hit-me').on('click', function () { self.hitCount++; });
  
  // good
  $('#hit-me').on('click', e => this.hitCount++);

• For multiline functions, use function declarations. Function declarations are hoisted, so you can use the functions anywhere in the scope where they are declared.

  // bad
  let mapMyLocation = function () {
      //do a lot of stuff
  }
  
  // good
  function mapMyLocation () {
      // do a lot of stuff
  }

• Don't abuse function parameters by deeply nesting anonymous functions. Instead, declare the function in the top-level scope and pass it by name.

  // bad
  let contents;
  fs.readFile('first.txt', function (err, data) {
      if (err) {
          fs.readFile('url.txt', function (err, data) {
              if (!err) {
                  http.get(data).then(function (resp) {
                      contents = resp.body;
                  });
              }
          });
      } else {
          contents = data;
      }
  });
  
  // good
  let contents;
  fs.readFile('first.txt', handleFileRead);
  
  function handleFileRead (err, data) {
      if (err) {
          fs.readFile('url.txt', handleFallbackRead);
      } else {
          contents = data;
      }
  }
  
  function handleFallbackRead (err, data) {
      if (err) return;
      http.get(data).then(resp => contents = resp.body);
  }

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

• (ES6) Prefer the use of rest params instead of the arguments keyword.

  // bad
  function stuff () {
      console.log(arguments.length);
  }
  
  // good
  function betterStuff (...args) {
      console.log(args.length);
  }
  

• 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...
  }

  ↑ Back to top

• Use dot notation when accessing properties.

  let luke = {
      jedi: true,
      age: 28
  };
  
  // bad
  let isJedi = luke['jedi'];
  
  // good
  let isJedi = luke.jedi;

• Use subscript notation [] when accessing properties with a variable.

  let luke = {
      jedi: true,
      age: 28
  };
  
  function getProp (prop) {
      return luke[prop];
  }
  
  let isJedi = getProp('jedi');

  ↑ Back to top

• Always use var, let or const to declare variables. Not doing so will result in strict mode errors.

  // bad
  superPower = new SuperPower();
  
  // good
  let superPower = new SuperPower();

• (ES6) Use only let or const instead of var. If you have a specific need for a function-scoped variable instead of a block-scoped one, write a comment explaining why.

  // bad
  var howManyTimes = 5;
  for (var i = 0; i < howManyTimes; i++) {
      ...
  }
  
  // good
  const HOW_MANY_TIMES = 5;
  for (let i = 0; i < howManyTimes, i++) {
      ...
  }

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

  // bad
  let items = getItems();
  let goSportsTeam = true;
  let dragonball = 'z';
  
  // good
  let items = getItems(),
      goSportsTeam = true,
      dragonball = 'z';

• (ES6) Use one const declaration per line, to emphasize the immutability of the assignment.

  // bad
  const SHAPE = 'circle',
      WEIGHT = 25;
  
  // good
  const SHAPE = 'circle';
  const WEIGHT = 25;

• 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. One variable per line. Move multiline declarations -- like object literals -- to their own let or var expression.

  // bad
  let i, len, dragonball,
      items = getItems(),
      goSportsTeam = true;
  
  // bad
  let i, items = getItems(),
      dragonball,
      goSportsTeam = true,
      len;
  
  // bad
  let items = getItems(),
      startingInventory = {
          potions: 10,
          weapons: 5,
          staffs: 1
      },
      dragonball;
  
  // good
  let items = getItems(),
      goSportsTeam = true,
      dragonball,
      length,
      i;
  
  // good
  let items = getItems(),
      dragonball;
  
  let startingInventory = {
      potions: 10,
      weapons: 5,
      staffs: 1
  };

• 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..
  
      let name = getName();
  
      if (name === 'test') {
          return false;
      }
  
      return name;
  }
  
  // good
  function () {
      let name = getName();
  
      test();
      console.log('doing stuff..');
  
      //..other stuff..
  
      if (name === 'test') {
          return false;
      }
  
      return name;
  }
  
  // bad
  function () {
      let name = getName();
  
      if (!arguments.length) {
          return false;
      }
  
      return true;
  }
  
  // good
  function () {
      if (!arguments.length) {
          return false;
      }
  
      let name = getName();
  
      return true;
  }

  ↑ Back to top

• 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
      let 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 () {
      let 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
  
      let 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
  
      let 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
  
      let 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

  ↑ Back to top

• 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

  ↑ Back to top

• 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;
  }

  ↑ Back to top

• 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
  const ACTIVE = true;  // is current tab
  
  // good
  // is current tab
  const ACTIVE = true;
  
  // bad
  function getType () {
      console.log('fetching type...');
      // set the default type to 'no type'
      let type = this._type || 'no type';
  
      return type;
  }
  
  // good
  function getType () {
      console.log('fetching type...');
  
      // set the default type to 'no type'
      let 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

• In general, keep lines short. Use newlines between blocks or chunks of related code.

  // bad
  this.weightUnits = ko.computed(() =>
      (this.systemOfMeasurement() === 'Metric') ? uship.localization['MainKg'] : uship.localization['MainLbs']
  );
  function anotherFunction () {
      let foo = 1;
  }
  
  // good
  this.weightUnits = ko.computed(() =>
      (this.systemOfMeasurement() === 'Metric') ?
          uship.localization['MainKg'] :
          uship.localization['MainLbs']
  );
  
  function anotherFunction () {
      let foo = 1;
  }

• Use soft tabs set to 4 spaces. Don't mix hard tabs and spaces.

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

• Place 1 space before the leading brace. For function declarations, place 1 space between the function keyword and the arguments list.

  // bad
  function test(){
      console.log('test');
  }
  
  // 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.

  // bad
  let x=y+5;
  
  // good
  let x = y + 5;

• Use a newline between key/value pairs in an object literal. Place 1 space between keys and values. You can choose to tab-align values as long as the result is more readable.

  // bad
  var dog = { breed: 'Maltese', color: 'white', age: '3 years' };
  
  // bad
  let dog = { 
      breed:'Maltese',
      color:'white',
      age:'3 years' 
  };
  
  // good
  let dog = { 
      breed: 'Maltese',
      color: 'white',
      age: '3 years' 
  };
  
  // bad
  let dog = { 
      breed:                                             'Maltese',
      color:                                             'white',
      age:                                               '3 years',
      aReallyLongPropertyNameMakesAlignmentHardToRead: true
  };
  
  // acceptable
  let dog = { 
      breed:    'Maltese',
      color:    'white',
      age:      '3 years',
      adorable: 'obviously'
  };

• Use indentation when making long method chains.

  // bad
  $('#items').find('.selected').highlight().end().find('.open').updateCount();
  
  // good
  $('#items')
      .find('.selected')
          .highlight()
          .end()
      .find('.open')
          .updateCount();
  
  // bad
  var leds = stage.selectAll('.led').data(data).enter().append('svg:svg').class('led', true)
      .attr('width',  (radius + margin) * 2).append('svg:g')
      .attr('transform', 'translate(' + (radius + margin) + ',' + (radius + margin) + ')')
      .call(tron.led);
  
  // good
  var leds = stage.selectAll('.led')
          .data(data)
      .enter().append('svg:svg')
          .class('led', true)
          .attr('width',  (radius + margin) * 2)
      .append('svg:g')
          .attr('transform', 'translate(' + (radius + margin) + ',' + (radius + margin) + ')')
          .call(tron.led);

  ↑ Back to top

• Leading commas: Nope.

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

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
let hero = {
    firstName: 'Kevin',
    lastName: 'Flynn',
};

// bad
let heroes = [
    'Batman',
    'Superman',
];

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

// good
let heroes = [
    'Batman',
    'Superman'
];
```

**[↑ Back to top](#TOC)**

• Yup. Javascript will automatically insert semicolons -- that it's they're optional in the strict sense. But leaving out semicolons can cause some pernicious bugs, particularly when multiple people are working on the same code, or when not everyone is an expert on the edge case rules for semicolon insertion. So for consistency's sake, always use them.

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

  ↑ Back to top

• Perform type coercion at the beginning of the statement.

• Strings:

  //  => this.reviewScore = 9;
  
  // bad
  let totalScore = this.reviewScore + '';
  
  // good
  let totalScore = '' + this.reviewScore;
  
  // bad
  let totalScore = '' + this.reviewScore + ' total score';
  
  // good
  let totalScore = this.reviewScore + ' total score';

• Use parseInt for Numbers and always with a radix for type casting.

  let inputValue = '4';
  
  // bad
  let val = new Number(inputValue);
  
  // bad
  let val = +inputValue;
  
  // bad
  let val = inputValue >> 0;
  
  // bad
  let val = parseInt(inputValue);
  
  // good
  let 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, leave a comment explaining why and what you're doing.

• Note: Be careful when using bitshift operations. Numbers are represented as 64-bit values, but Bitshift operations always return a 32-bit integer (source). Bitshift can lead to unexpected behavior for integer values larger than 32 bits. Discussion

  // good
  /**
   * parseInt was the reason my code was slow.
   * Bitshifting the String to coerce it to a
   * Number made it a lot faster.
   */
  let val = inputValue >> 0;

• Booleans:

  let age = 0;
  
  // bad
  let hasAge = new Boolean(age);
  
  // good
  let hasAge = Boolean(age);
  
  // good
  let hasAge = !!age;

  ↑ Back to top

• 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
  let OBJEcttsssss = {};
  let this_is_my_object = {};
  function c () {};
  let u = new user({
      name: 'Bob Parr'
  });
  
  // good
  let thisIsMyObject = {};
  function thisIsMyFunction () {};
  let 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
  
  class User {
  
      constructor (options) {
          this.name = options.name;
      }
  
  }
  
  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';

• When saving a reference to this use self. Prefer arrow function expressions or bind to replace the need for saving a reference to this. Stick to a single paradigm -- don't mix this and self in the same context.

  // bad
  function () {
      let that = this;
      return function () {
          console.log(that);
      };
  }
  
  function () {
      let self = this;
      this.trait = 'Inconsistency';
      return function () {
          console.log(self);
      };
  }
  
  // good
  function () {
      let self = this;
      return function () {
          console.log(self);
      };
  }
  
  // better
  function () {
      return function () {
          console.log(this);
      }.bind(this);
  }
  
  // best
  function () {
      return () => console.log(this);
  }

• Name your functions. This is helpful for stack traces. (You don't have to worry about this if you use function declarations as recommended.)

  // bad
  let log = function (msg) {
      console.log(msg);
  };
  
  // good
  let log = function log (msg) {
      console.log(msg);
  };

  ↑ Back to top

• 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, make the property name a "statement of fact" using a prefix like "is", "has", or "should"

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

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

  class Jedi {
      constructor (options) {
          options || (options = {});
          var lightsaber = options.lightsaber || 'blue';
          this.set('lightsaber', lightsaber);
      }
  
      set (key, val) {
          this[key] = val;
      }
  
      get (key) {
          return this[key];
      }
  }

  ↑ Back to top

• (ES6) Use the class syntax instead of constructor functions and prototypes

      // old
      function Jedi (options) {
          ForceUser.call(this);
          this.name = options.name;
      }
  
      Jedi.prototype = Object.create(ForceUser);
  
      Jedi.prototype.jump = function jump () {
          this.jumping = true;
      }
  
      // preferred
      class Jedi : ForceUser {
          constructor (options) {
              this.name = options.name;
          }
  
          jump () {
              this.jumping = true;
          }
      }

• Methods can return this to help with method chaining.

  // bad
  class Jedi {
      jump () {
        this.jumping = true;
        return true;
      }
  
      setHeight (height) {
        this.height = height;
      }
  }
  
  let luke = new Jedi();
  luke.jump(); // => true
  luke.setHeight(20) // => undefined
  
  // good
  class Jedi {
      jump () {
        this.jumping = true;
        return this;
      }
  
      setHeight (height) {
        this.height = height;
        return this;
      }
  }
  
  let 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.

  class Jedi {
  
      constructor (options) {
          options || (options = {});
          this.name = options.name || 'no name';
      }
  
      getName () {
          return this.name;
      }
  
      toString () {
          return 'Jedi - ' + this.getName();
      }
  }

• (ES5) 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');
  };

  ↑ Back to top

• 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:

  // bad
  $(this).trigger('listingUpdated', listing.id);
  
  ...
  
  $(this).on('listingUpdated', (e, listingId) => /* do something with listingId */);

  prefer:

  // good
  $(this).trigger('listingUpdated', { listingId : listing.id });
  
  ...
  
  $(this).on('listingUpdated', (e, listingId) => /* do something with listingId */);

↑ Back to top

• Wrap the function expression in parentheses.

• 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

• The file name should match the name of the single export. Casing should follow the same rules as the export -- e.g. a module that exports a constructor should be PascalCase.

• 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.

  // fancyInput/fancyInput.js
  
  ;(function (global, $) {
      'use strict';
  
      let previousFancyInput = global.FancyInput;
  
      function FancyInput (options) {
          this.options = options || {};
      }
  
      FancyInput.noConflict = function noConflict () {
          global.FancyInput = previousFancyInput;
          return FancyInput;
      };
  
      global.FancyInput = FancyInput;
  })(window, window.jQuery);

  ↑ Back to top

• Don't prefix jQuery object variables with a $.

  // bad
  let $sidebar = $('.sidebar');
  
  // good
  let sidebar = $('.sidebar');

• Cache jQuery lookups.

  // bad
  function setSidebar () {
      $('.sidebar').hide();
  
      // ...stuff...
  
      $('.sidebar').css({
          'background-color': 'pink'
      });
  }
  
  // good
  function setSidebar () {
      let sidebar = $('.sidebar');
      sidebar.hide();
  
      // ...stuff...
  
      sidebar.css({
          'background-color': 'pink'
      });
  }

• For DOM queries use Cascading $('.sidebar ul') or parent > child $('.sidebar > ul'). jsPerf

• Use find with scoped jQuery object queries.

  // 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

• Refer to Kangax's ES5 compatibility table

↑ Back to top

• Yup.

  function () {
      return true;
  }

  ↑ Back to top

• On Layout & Web Performance
• String vs Array Concat
• Try/Catch Cost In a Loop
• Bang Function
• jQuery Find vs Context, Selector
• innerHTML vs textContent for script text
• Long String Concatenation
• Loading...

↑ Back to top

Read This

• Annotated ECMAScript 5.1

Other Styleguides

• Google JavaScript Style Guide
• jQuery Core Style Guidelines
• Principles of Writing Consistent, Idiomatic JavaScript

Other Styles

• Naming this in nested functions - Christian Johansen
• Conditional Callbacks
• Popular JavaScript Coding Conventions on Github

Further Reading

• Understanding JavaScript Closures - Angus Croll
• Basic JavaScript for the impatient programmer - Dr. Axel Rauschmayer

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
• Secrets of the JavaScript Ninja - John Resig and Bear Bibeault
• Human JavaScript - Henrik Joreteg
• Superhero.js - Kim Joar Bekkelund, Mads Mobæk, & Olav Bjorkoy
• JSBooks
• Third Party JavaScript - Ben Vinegar and Anton Kovalyov

Blogs

• DailyJS
• JavaScript Weekly
• JavaScript, JavaScript...
• Bocoup Weblog
• Adequately Good
• NCZOnline
• Perfection Kills
• Ben Alman
• Dmitry Baranovskiy
• Dustin Diaz
• nettuts

↑ Back to top

• Reference

• View Contributors

(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.

↑ Back to top