Airbnb JavaScript Style Guide

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
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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 = [];
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);
  ...
}
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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,
    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>';
}
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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 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;
}

more:https://github.com/airbnb/javascript