# JavaScript Coding Standards

This is The Orchard style guide. It's an adaptation of different community style guides, including those by [Airbnb](https://github.com/airbnb/) and Google.

> All code in any code-base should look like a single person typed it, no matter how many people contributed. – [Principles of Writing Consistent, Idiomatic JavaScript](https://github.com/rwaldron/idiomatic.js/)

All instructions are _mandatory_, unless otherwise denoted.

Rationale for each instruction is in blockquotes. Some instructions are under the _pure convention_ rationale: it means that when several, practically equivalent \(besides bikeshedding\) options are available, we pick one and stick to it to enforce consistency in the codebase.

## Table of Contents

1. [Types](javascript.md#types)
2. [References](javascript.md#references)
3. [Objects](javascript.md#objects)
4. [Arrays](javascript.md#arrays)
5. [Destructuring](javascript.md#destructuring)
6. [Strings](javascript.md#strings)
7. [Functions](javascript.md#functions)
8. [Arrow Functions](javascript.md#arrow-functions)
9. [Constructors](javascript.md#constructors)
10. [Modules](javascript.md#modules)
11. [Iterators and Generators](javascript.md#iterators-and-generators)
12. [Properties](javascript.md#properties)
13. [Variables](javascript.md#variables)
14. [Hoisting](javascript.md#hoisting)
15. [Comparison Operators & Equality](javascript.md#comparison-operators--equality)
16. [Blocks](javascript.md#blocks)
17. [Comments](javascript.md#comments)
18. [Whitespace](javascript.md#whitespace)
19. [Commas](javascript.md#commas)
20. [Semicolons](javascript.md#semicolons)
21. [Type Casting & Coercion](javascript.md#type-casting--coercion)
22. [Naming Conventions](javascript.md#naming-conventions)
23. [Accessors](javascript.md#accessors)
24. [Events](javascript.md#events)
25. [React](javascript.md#react)
26. [jQuery](javascript.md#jquery)
27. [Type Checks](javascript.md#type-checks)
28. [Switch Statements](javascript.md#switch-statements)
29. [ECMAScript 5 Compatibility](javascript.md#ecmascript-5-compatibility)
30. [ECMAScript 6 Styles](javascript.md#ecmascript-6-styles)
31. [Testing](javascript.md#testing)
32. [Performance](javascript.md#performance)
33. [Resources](javascript.md#resources)

## Types

* [1.1](javascript.md#1.1)  **Primitives**: When you access a primitive type you work directly on its value.
  * `string`
  * `number`
  * `boolean`
  * `null`
  * `undefined`

```javascript
const foo = 1;
let bar = foo;

bar = 9;

console.log(foo, bar); // => 1, 9
```

* [1.2](javascript.md#1.2)  **Complex**: When you access a complex type you work on a reference to its value.
  * `object`
  * `array`
  * `function`

```javascript
const foo = [1, 2];
const bar = foo;

bar[0] = 9;

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

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

## References

* [2.1](javascript.md#2.1)  Use `const` for all of your references; avoid using `var`.

  > Why? This ensures that you can't reassign your references \(mutation\), which can lead to bugs and difficult to comprehend code.

```javascript
// bad
var a = 1;
var b = 2;

// good
const a = 1;
const b = 2;
```

There's a curious case in the ES6 specification of JavaScript where variables declared with const are mutable. If a variable is declared as a primitive like the following, `const foo = 123`, it is immutable but if a variable is declared as an object \(`const bar = {}`\) it is mutable. This happens because the new variable always points to the same object but the properties on that object can change. [https://jack.ofspades.com/es6-const-not-immutable/](https://jack.ofspades.com/es6-const-not-immutable/)

**eslint** [`no-var`](http://eslint.org/docs/rules/no-var)

* [2.2](javascript.md#2.2)  If you must mutate references, use `let` instead of `var`.

  > Why? `let` is block-scoped rather than function-scoped like `var`.

```javascript
// bad
var count = 1;
if (true) {
  count += 1;
}

// good, use the let.
let count = 1;
if (true) {
  count += 1;
}
```

```text
**eslint** [`prefer-const`](http://eslint.org/docs/rules/prefer-const)
```

* [2.3](javascript.md#2.3)  Note that both `let` and `const` are block-scoped.

```javascript
// const and let only exist in the blocks they are defined in.
{
  let a = 1;
  const b = 1;
}
console.log(a); // ReferenceError
console.log(b); // ReferenceError
```

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

## Objects

* [3.1](javascript.md#3.1)  Use the literal syntax for object creation.

```javascript
// bad
const item = new Object();

// good
const item = {};
```

* [3.2](javascript.md#3.2)  If your code will be executed in browsers in script context, don't use [reserved words](http://es5.github.io/#x7.6.1) as keys. It won't work in IE8. [More info](https://github.com/airbnb/javascript/issues/61). It’s OK to use them in ES6 modules and server-side code.

```javascript
// bad
const superman = {
    default: { clark: 'kent' },
    private: true,
};

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

**eslint** [`quote-props`](http://eslint.org/docs/rules/quote-props.html)

* [3.3](javascript.md#3.3)  Use readable synonyms in place of reserved words.

```javascript
// bad
const superman = {
    class: 'alien',
};

// bad
const superman = {
    klass: 'alien',
};

// good
const superman = {
    type: 'alien',
};
```

* [3.4](javascript.md#3.4)  Use computed property names when creating objects with dynamic property names.

> Why? They allow you to define all the properties of an object in one place.

```javascript
function getKey(k) {
    return `a key named ${k}`;
}

// bad
const obj = {
    id: 5,
    name: 'San Francisco',
};

obj[getKey('enabled')] = true;

// good
const obj = {
    id: 5,
    name: 'San Francisco',
    [getKey('enabled')]: true,
};
```

* [3.5](javascript.md#3.5)  Use object method shorthand.

```javascript
// bad
const atom = {
    value: 1,

    addValue: function (value) {
        return atom.value + value;
    },
};

// good
const atom = {
    value: 1,

    addValue(value) {
        return atom.value + value;
    },
};
```

* [3.6](javascript.md#3.6)  Use property value shorthand.

> Why? It is shorter to write and descriptive.

```javascript
const lukeSkywalker = 'Luke Skywalker';

// bad
const obj = {
    lukeSkywalker: lukeSkywalker,
};

// good
const obj = {
    lukeSkywalker,
};
```

* [3.7](javascript.md#3.7)  Group your shorthand properties at the beginning of your object declaration.

> Why? It's easier to tell which properties are using the shorthand.

```javascript
const anakinSkywalker = 'Anakin Skywalker';
const lukeSkywalker = 'Luke Skywalker';

// bad
const obj = {
    episodeOne: 1,
    twoJediWalkIntoACantina: 2,
    lukeSkywalker,
    episodeThree: 3,
    mayTheFourth: 4,
    anakinSkywalker,
};

// good
const obj = {
    lukeSkywalker,
    anakinSkywalker,
    episodeOne: 1,
    twoJediWalkIntoACantina: 2,
    episodeThree: 3,
    mayTheFourth: 4,
};
```

* [3.8](javascript.md#3.8)  Object declarations can be made on a single line if they are short \(remember the line length guidelines\). When an object declaration is too long to fit on one line, there **must** be one property per line.

```javascript
// Preferred
var map = {
    ready: 9,
    when: 4,
    'you are': 15
};

// Acceptable for small objects
var map = { ready: 9, when: 4, 'you are': 15 };

// Bad
var map = { ready: 9,
    when: 4, 'you are': 15 };
```

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

## Arrays

* [4.1](javascript.md#4.1)  Use the literal syntax for array creation.

```javascript
// bad
const items = new Array();

// good
const items = [];
```

* [4.2](javascript.md#4.2)  Use Array\#push instead of direct assignment to add items to an array.

```javascript
const someStack = [];

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

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

* [4.3](javascript.md#4.3)  Use array spreads `...` to copy arrays.

```javascript
// bad
const len = items.length;
const itemsCopy = [];
let i;

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

// good
const itemsCopy = [...items];
```

* [4.4](javascript.md#4.4)  To convert an array-like object to an array, use Array\#from.

```javascript
const foo = document.querySelectorAll('.foo');
const nodes = Array.from(foo);
```

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

## Destructuring

* [5.1](javascript.md#5.1)  Use object destructuring when accessing and using multiple properties of an object.

> Why? Destructuring saves you from creating temporary references for those properties.

```javascript
// bad
function getFullName(user) {
    const firstName = user.firstName;
    const lastName = user.lastName;
    return `${firstName} ${lastName}`;
}

// good
function getFullName(obj) {
    const { firstName, lastName } = obj;
    return `${firstName} ${lastName}`;
}

// best
function getFullName({ firstName, lastName }) {
    return `${firstName} ${lastName}`;
}
```

* [5.2](javascript.md#5.2)  Use array destructuring.

```javascript
const arr = [1, 2, 3, 4];

// bad
const first = arr[0];
const second = arr[1];

// good
const [first, second] = arr;
```

* [5.3](javascript.md#5.3)  Use object destructuring for multiple return values, not array destructuring.

> Why? You can add new properties over time or change the order of things without breaking call sites.

```javascript
// bad
function processInput(input) {
    // then a miracle occurs
    return [left, right, top, bottom];
}

// the caller needs to think about the order of return data
const [left, __, top] = processInput(input);

// good
function processInput(input) {
    // then a miracle occurs
    return { left, right, top, bottom };
}

// the caller selects only the data they need
const { left, right } = processInput(input);
```

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

## Strings

* [6.1](javascript.md#6.1)  Use single quotes `''` for strings.

```javascript
// bad
const name = "Capt. Janeway";

// good
const name = 'Capt. Janeway';
```

* [6.2](javascript.md#6.2)  Strings longer than 120 characters should be written across multiple lines using string concatenation.
* [6.3](javascript.md#6.3)  Note: If overused, long strings with concatenation could impact performance. [jsPerf](http://jsperf.com/ya-string-concat) & [Discussion](https://github.com/airbnb/javascript/issues/40).

```javascript
// bad
const 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
const 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
const 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.';
```

* [6.4](javascript.md#6.4)  When programmatically building up strings, use template strings instead of concatenation.

> Why? Template strings give you a readable, concise syntax with proper newlines and string interpolation features.

```javascript
// bad
function sayHi(name) {
    return 'How are you, ' + name + '?';
}

// bad
function sayHi(name) {
    return ['How are you, ', name, '?'].join();
}

// good
function sayHi(name) {
    return `How are you, ${name}?`;
}
```

* [6.5](javascript.md#6.5)  Never use eval\(\) on a string, it opens too many vulnerabilities.

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

## Functions

* [7.1](javascript.md#7.1)  Use fat arrow function expressions when possible. Otherwise, use function declarations.

```javascript
// bad
const foo = function () {
};

// good

const foo = () => {}

// ok
function foo() {
}
```

* [7.2](javascript.md#7.2)  Function expressions:

```javascript
// immediately-invoked function expression (IIFE)
(() => {
    console.log('Welcome to the Internet. Please follow me.');
})();
```

* [7.3](javascript.md#7.3)  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.
* [7.4](javascript.md#7.4)  **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](http://www.ecma-international.org/publications/files/ECMA-ST/Ecma-262.pdf#page=97).

```javascript
// bad
if (currentUser) {
    function test() {
        console.log('Nope.');
    }
}

// good
let test;

if (currentUser) {
    test = () => {
        console.log('Yup.');
    };
}
```

* [7.5](javascript.md#7.5)  Never name a parameter `arguments`. This will take precedence over the `arguments` object that is given to every function scope.

```javascript
// bad
function nope(name, options, arguments) {
    // ...stuff...
}

// good
function yup(name, options, args) {
    // ...stuff...
}
```

* [7.6](javascript.md#7.6)  Never use `arguments`, opt to use rest syntax `...` instead.

> Why? `...` is explicit about which arguments you want pulled. Plus rest arguments are a real Array and not Array-like like `arguments`.

```javascript
// bad
function concatenateAll() {
    const args = Array.prototype.slice.call(arguments);
    return args.join('');
}

// good
function concatenateAll(...args) {
    return args.join('');
}
```

* [7.7](javascript.md#7.7)  Use default parameter syntax rather than mutating function arguments.

```javascript
// really bad
function handleThings(opts) {
    // No! We shouldn't mutate function arguments.
    // Double bad: if opts is falsy it'll be set to an object which may
    // be what you want but it can introduce subtle bugs.
    opts = opts || {};
    // ...
}

// still bad
function handleThings(opts) {
    if (opts === void 0) {
        opts = {};
    }
    // ...
}

// good
function handleThings(opts = {}) {
    // ...
}
```

* [7.8](javascript.md#7.8)  Avoid side effects with default parameters

> Why? They are confusing to reason about.

```javascript
var b = 1;
// bad
function count(a = b++) {
    console.log(a);
}
count();  // 1
count();  // 2
count(3); // 3
count();  // 3
```

* [7.9](javascript.md#7.9)  Never use the Function constructor to create a new function.

> Why? Creating a function in this way evaluates a string similarly to eval\(\), which opens vulnerabilities.

```javascript
// bad
var add = new Function('a', 'b', 'return a + b');

// still bad
var subtract = Function('a', 'b', 'return a - b');
```

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

## Arrow Functions

* [8.1](javascript.md#8.1)  If the function body consists of a single expression, feel free to omit the braces and use the implicit return. Otherwise use a `return` statement.

> Why? Syntactic sugar. It reads well when multiple functions are chained together.
>
> Why not? If you plan on returning an object.

```javascript
// good
[1, 2, 3].map(number =>
    `A string containing the ${number}.
`);

// bad
[1, 2, 3].map(number => {
    const nextNumber = number + 1;
    `A string containing the ${nextNumber}.`;
);

// good
[1, 2, 3].map(number => {
    const nextNumber = number + 1;
    return `A string containing the ${nextNumber}.`;
});
```

* [8.2](javascript.md#8.2)  If your function only takes a single argument, feel free to omit the parentheses.

> Why? Less visual clutter.

```javascript
// good
[1, 2, 3].map(x => x * x);

// good
[1, 2, 3].reduce((y, x) => x + y);
```

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

## Constructors

* [9.1](javascript.md#9.1)  Always use `class`. Avoid manipulating `prototype` directly.

> Why? `class` syntax is more concise and easier to reason about.

```javascript
// bad
function Queue(contents = []) {
    this._queue = [...contents];
}

Queue.prototype.pop = function() {
    const value = this._queue[0];
    this._queue.splice(0, 1);
    return value;
}

// good
class Queue {
    constructor(contents = []) {
        this._queue = [...contents];
    }

    pop() {
        const value = this._queue[0];
        this._queue.splice(0, 1);
        return value;
    }
}
```

* [9.2](javascript.md#9.2)  Use `extends` for inheritance.

> Why? It is a built-in way to inherit prototype functionality without breaking `instanceof`.

```javascript
// bad
const inherits = require('inherits');

function PeekableQueue(contents) {
    Queue.apply(this, contents);
}

inherits(PeekableQueue, Queue);

PeekableQueue.prototype.peek = function() {
    return this._queue[0];
}

// good
class PeekableQueue extends Queue {
    peek() {
        return this._queue[0];
    }
}
```

* [9.3](javascript.md#9.3)  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;
};

const 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;
    }
}

const luke = new Jedi();

luke.jump()
    .setHeight(20);
```

* [9.4](javascript.md#9.4)  It's okay to write a custom toString\(\) method, just make sure it works successfully and causes no side effects.

```javascript
class Jedi {
    constructor(options = {}) {
        this.name = options.name || 'no name';
    }

    getName() {
        return this.name;
    }

    toString() {
        return `Jedi - ${this.getName()}`;
    }
}
```

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

## Modules

* [10.1](javascript.md#10.1)  Always use modules \(`import`/`export`\) over a non-standard module system. You can always transpile to your preferred module system.

> Why? Modules are the future, let's start using the future now.

```javascript
// bad
const AirbnbStyleGuide = require('./AirbnbStyleGuide');
module.exports = AirbnbStyleGuide.es6;

// ok
import AirbnbStyleGuide from './AirbnbStyleGuide';
export default AirbnbStyleGuide.es6;

// best
import { es6 } from './AirbnbStyleGuide';
export default es6;
```

* [10.2](javascript.md#10.2)  Use destructuring assignment when applicable.

```javascript
// bad
const moduleA = require('moduleA');
const func1 = moduleA.func1;
const func2 = moduleA.func2;

// good
import { func1, func2 } from 'moduleA';
```

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

## Iterators and Generators

* [11.1](javascript.md#11.1)  Don't use iterators. Prefer JavaScript's higher-order functions like `map()` and `reduce()` instead of loops like `for-of`.

> Why? This enforces our immutable rule. Dealing with pure functions that return values is easier to reason about than side-effects.

```javascript
const numbers = [1, 2, 3, 4, 5];

// bad
let sum = 0;
for (let num of numbers) {
  sum += num;
}

sum === 15;

// good
let sum = 0;
numbers.forEach((num) => sum += num);
sum === 15;

// best (use the functional force)
const sum = numbers.reduce((total, num) => total + num, 0);
sum === 15;
```

* [11.2](javascript.md#11.2)  Prefer [Lo-Dash](https://lodash.com/) over native higher-order functions.

> Why? Lo-Dash provides an extensive support for efficient and readable high-order functions in JavaScript for both collections and object hashes. Mixing both native and Lo-Dash higher-rder functions would be inconsistent. It also permits jQuery-style chaining with regular JavaScript objects.

```javascript
var obj = {
    first: 'thing 1',
    second: 'thing 2',
    third: 'lox'
};

var arr = _.chain(obj)
    .keys()
    .map(key => `${key} comes ${obj[key]}`)
    // Exit the chain
    .value();

// arr === [ 'first comes thing 1', 'second comes thing 2', 'third comes lox' ]
```

* [11.3](javascript.md#11.3)  Never use jQuery to iterate over raw data or vanilla JavaScript objects.

The only time jQuery **should** be used for iteration is when iterating over a collection of jQuery objects:

```javascript
$tabs.each(function(index, element) {
    var $element = $(element);

    // Do stuff to $element
});
```

* [11.4](javascript.md#11.4)  Don't use generators for now.

> Why? They don't transpile well to ES5.

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

## Properties

* [12.1](javascript.md#12.1)  Use dot notation when accessing properties.

```javascript
const luke = {
    jedi: true,
    age: 28,
};

// bad
const isJedi = luke['jedi'];

// good
const isJedi = luke.jedi;
```

* [12.2](javascript.md#12.2)  Use subscript notation `[]` when accessing properties with a variable.

```javascript
const luke = {
    jedi: true,
    age: 28,
};

function getProp(prop) {
    return luke[prop];
}

const isJedi = getProp('jedi');
```

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

## Variables

* [13.1](javascript.md#13.1)  Always use `const` 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.

```javascript
// bad
superPower = new SuperPower();

// good
const superPower = new SuperPower();
```

* [13.2](javascript.md#13.2)  Use one `const` declaration per variable.

> Why? 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.

```javascript
// bad
const items = getItems(),
    goSportsTeam = true,
    dragonball = 'z';

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

// good
const items = getItems();
const goSportsTeam = true;
const dragonball = 'z';
```

* [13.3](javascript.md#13.3)  Group all your `const`s and then group all your `let`s.

> Why? This is helpful when later on you might need to assign a variable depending on one of the previous assigned variables.

```javascript
// bad
let i, len, dragonball,
    items = getItems(),
    goSportsTeam = true;

// bad
let i;
const items = getItems();
let dragonball;
const goSportsTeam = true;
let len;

// good
const goSportsTeam = true;
const items = getItems();
let dragonball;
let i;
let length;
```

* [13.4](javascript.md#13.4)  Assign variables where you need them, but place them in a reasonable place.

> Why? `let` and `const` are block scoped and not function scoped.

```javascript
// good
function () {
    test();
    console.log('doing stuff..');

    //..other stuff..

    const name = getName();

    if (name === 'test') {
        return false;
    }

    return name;
}

// bad - unnecessary function call
function (hasName) {
    const name = getName();

    if (!hasName) {
        return false;
    }

    this.setFirstName(name);

    return true;
}

// good
function (hasName) {
    if (!hasName) {
        return false;
    }

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

    return true;
}
```

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

## Hoisting

* [14.1](javascript.md#14.1)  `var` declarations get hoisted to the top of their scope, their assignment does not. `const` and `let` declarations are blessed with a new concept called [Temporal Dead Zones \(TDZ\)](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/let#Temporal_dead_zone_and_errors_with_let). It's important to know why [typeof is no longer safe](http://es-discourse.com/t/why-typeof-is-no-longer-safe/15).

```javascript
// 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() {
    let declaredButNotAssigned;
    console.log(declaredButNotAssigned); // => undefined
    declaredButNotAssigned = true;
}

// using const and let
function example() {
    console.log(declaredButNotAssigned); // => throws a ReferenceError
    console.log(typeof declaredButNotAssigned); // => throws a ReferenceError
    const declaredButNotAssigned = true;
}
```

* [14.2](javascript.md#14.2)  Anonymous function expressions hoist their variable name, but not the function assignment.

```javascript
function example() {
    console.log(anonymous); // => undefined

    anonymous(); // => TypeError anonymous is not a function

    var anonymous = function() {
        console.log('anonymous function expression');
    };
}
```

* [14.3](javascript.md#14.3)  Named function expressions hoist the variable name, not the function name or the function body.

```javascript
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');
    }
}
```

* [14.4](javascript.md#14.4)  Function declarations hoist their name and the function body.

```javascript
function example() {
    superPower(); // => Flying

    function superPower() {
        console.log('Flying');
    }
}
```

* For more information refer to [JavaScript Scoping & Hoisting](http://www.adequatelygood.com/2010/2/JavaScript-Scoping-and-Hoisting) by [Ben Cherry](http://www.adequatelygood.com/).

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

## Comparison Operators & Equality

* [15.1](javascript.md#15.1)  Use `===` and `!==` over `==` and `!=`.
* [15.2](javascript.md#15.2)  Conditional statements such as the `if` statement evaluate their expression using coercion with the `ToBoolean` abstract 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**

```javascript
if ([0]) {
    // true
    // An array is an object, objects evaluate to true
}
```

* [15.3](javascript.md#15.3)  Use shortcuts.

```javascript
// bad
if (name !== '') {
    // ...stuff...
}

// good
if (name) {
    // ...stuff...
}

// bad
if (collection.length > 0) {
    // ...stuff...
}

// good
if (collection.length) {
    // ...stuff...
}
```

* [15.4](javascript.md#15.4)  For more information see [Truth Equality and JavaScript](http://javascriptweblog.wordpress.com/2011/02/07/truth-equality-and-javascript/#more-2108) by Angus Croll.

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

## Blocks

* [16.1](javascript.md#16.1)  Use braces with multi-line blocks only.

```javascript
// good
if (test)
    return false;

// good
if (test) return false;

// bad
if (test) {
    return false;
}

// good
function() {
    doSomething()
    return false;
}
```

* [16.2](javascript.md#16.2)  If you're using multi-line blocks with `if` and `else`, put `else` on the same line as your

  `if` block's closing brace.

```javascript
// bad
if (test) {
    thing1();
    thing2();
}
else {
    thing3();
}

// good
if (test) {
    thing1();
    thing2();
} else {
    thing3();
}
```

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

## Comments

* [17.1](javascript.md#17.1)  Use `/** ... */` for multi-line comments. Include a description, specify types and values for all parameters and return values. We use [JSDoc](http://usejsdoc.org/) convention for our comment blocks. Be sure to include documentation for the following scenarios:
  * Files \([@file](http://usejsdoc.org/tags-file.html)\)
  * Functions: [@function](http://usejsdoc.org/tags-function.html); [@param](http://usejsdoc.org/tags-param.html); [@returns](http://usejsdoc.org/tags-returns.html); [@callback](http://usejsdoc.org/tags-callback.html)

```javascript
// 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;
}
```

* [17.2](javascript.md#17.2)  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.

```javascript
// 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'
    const type = this._type || 'no type';

    return type;
}

// good
function getType() {
    console.log('fetching type...');

    // set the default type to 'no type'
    const type = this._type || 'no type';

    return type;
}
```

* [17.3](javascript.md#17.3)  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`.
* [17.4](javascript.md#17.4)  Use `// FIXME:` to annotate problems.

```javascript
class Calculator extends Abacus {
    constructor() {
        super();

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

* [17.5](javascript.md#17.5)  Use `// TODO:` to annotate solutions to problems.

```javascript
class Calculator extends Abacus {
    constructor() {
        super();

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

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

## Whitespace

Use spaces liberally throughout your code. "When in doubt, space it out."

These rules encourage liberal spacing for improved developer readability. The minification process creates a file that is optimized for browsers to read and process.

* [18.1](javascript.md#18.1)  Use soft tabs set to 4 spaces.

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

// bad
function() {
∙const name;
}

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

* [18.2](javascript.md#18.2)  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',
});
```

* [18.3](javascript.md#18.3)  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!');
}
```

* [18.4](javascript.md#18.4)  Set off infix operators with spaces.

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

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

* [18.5](javascript.md#18.5)  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);↵
```

* [18.5](javascript.md#18.5)  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
const 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
const 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);
```

* [18.6](javascript.md#18.6)  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
const obj = {
    foo() {
    },
    bar() {
    },
};
return obj;

// good
const obj = {
    foo() {
    },

    bar() {
    },
};

return obj;

// bad
const arr = [
    function foo() {
    },
    function bar() {
    },
];
return arr;

// good
const arr = [
    function foo() {
    },

    function bar() {
    },
];

return arr;
```

* [18.7](javascript.md#18.7)  No trailing whitespace at the end of line or on blank lines.
* [18.8](javascript.md#18.8)  Lines **should** usually be no longer than 80 characters, and **should not** exceed 120. This is a "soft" rule, but long lines generally indicate unreadable or disorganized code.
* [18.9](javascript.md#18.9)  `if`/`else`/`for`/`while`/`try` blocks **must** always use braces, and **must** always go on multiple lines.
* [18.10](javascript.md#18.10)  1 space **must** be used after the leading brace `{` and before the ending brace `}` in single-line object literals or function declaration.
* [18.11](javascript.md#18.11)  Space **must not** be used after the leading bracket `[` or before the ending bracket `]` in single-line array literals.
* [18.12](javascript.md#18.12)  Space **must not** be used after the leading parenthesis `(` or before the trailing parenthesis `)` in function declarations.
* [18.13](javascript.md#18.13)  Space **must not** be used between `function` and `*` when defining generator. Space **must** be used between `*` and the function name when defining named generators.
* [18.14](javascript.md#18.14)  Space **must not** be used between `*` and the generator name when using the shorthand generator method syntax.
* [18.16](javascript.md#18.16)  Unary special-character operators \(e.g., `++`, `--`\) **must not** have space next to their operand.
* [18.17](javascript.md#18.17)  Any `,` and `;` **must not** have preceding space.
* [18.18](javascript.md#18.18)  Any `;` used as a statement terminator **must** not have a preceding space.
* [18.19](javascript.md#18.19)  Any `:` after a property name in an object definition **must not** have preceding space.
* [18.20](javascript.md#18.20)  The `?` and `:` in a ternary conditional **must** have space on both sides.
* [18.21](javascript.md#18.21)  Any empty constructs \(e.g., `{}`, `[]`, `fn()`\) **must not** have filler spaces.
* [18.22](javascript.md#18.22)  Any `!` negation operator **should not** have a following space.
* [18.23](javascript.md#18.23)  All function bodies **must** be indented by four spaces, even if the entire file is wrapped in a closure.

```javascript
(function ($) {
    // Expressions indented

    function doSomething() {
        // Expressions indented
    }

})(jQuery);
```

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

## Commas

* [19.1](javascript.md#19.1)  Leading commas: **Nope.**

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

// good
const story = [
  once,
  upon,
  aTime,
];

// bad
const hero = {
    firstName: 'Ada'
  , lastName: 'Lovelace'
  , birthYear: 1815
  , superPower: 'computers'
};

// good
const hero = {
  firstName: 'Ada',
  lastName: 'Lovelace',
  birthYear: 1815,
  superPower: 'computers',
};
```

* [19.2](javascript.md#19.2)  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 in legacy browsers.

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

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

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

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

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

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

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

## Semicolons

* [20.1](javascript.md#20.1)  **Yup.**

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

// good
(() => {
  const name = 'Skywalker';
  return name;
})();

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

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

* [20.2](javascript.md#20.2)  Any `;` used as a statement terminator **must** be at the end of the line.

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

## Type Casting & Coercion

* [21.1](javascript.md#21.1)  Perform type coercion at the beginning of the statement.
* [21.2](javascript.md#21.2)  Strings:

```javascript
//  => this.reviewScore = 9;

// bad
const totalScore = this.reviewScore + '';

// good
const totalScore = String(this.reviewScore);
```

* [21.3](javascript.md#21.3)  Use `parseInt` for Numbers and always with a radix for type casting.

```javascript
const inputValue = '4';

// bad
const val = new Number(inputValue);

// bad
const val = +inputValue;

// bad
const val = inputValue >> 0;

// bad
const val = parseInt(inputValue);

// good
const val = Number(inputValue);

// good
const val = parseInt(inputValue, 10);
```

* [21.4](javascript.md#21.4)  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.
 */
const val = inputValue >> 0;
```

* [21.5](javascript.md#21.5)  **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
```

* [21.6](javascript.md#21.6)  Booleans:

```javascript
const age = 0;

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

// good
const hasAge = Boolean(age);

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

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

## Naming Conventions

* [22.1](javascript.md#22.1)  Avoid single letter names. Be descriptive with your naming.

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

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

* [22.2](javascript.md#22.2)  Use camelCase when naming objects, functions, and instances.

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

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

* [22.3](javascript.md#22.3)  Use PascalCase when naming constructors or classes.

```javascript
// bad
function user(options) {
  this.name = options.name;
}

const bad = new user({
  name: 'nope',
});

// good
class User {
  constructor(options) {
    this.name = options.name;
  }
}

const good = new User({
  name: 'yup',
});
```

* [22.4](javascript.md#22.4)  Use a leading underscore `_` when naming private properties.

> Why? Enforcing lexical 'privacy' of methods and properties is possible using closures, but it comes with a performance cost and more coding efforts, when in the vast majority of cases you just want to enforce a conventional privacy: users can violate privacy, at their own risk.

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

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

* [22.5](javascript.md#22.5)  Don't save references to `this`. Use arrow functions or Function\#bind.

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

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

// good
function foo() {
  return () => {
    console.log(this);
  };
}
```

* [22.6](javascript.md#22.6)  If your file exports a single class, your filename should be exactly the name of the class.

  \`\`\`javascript

  // file contents

  class CheckBox {

  // ...

  }

  export default CheckBox;

// in some other file // bad import CheckBox from './checkBox';

// bad import CheckBox from './check\_box';

// good import CheckBox from './CheckBox';

```text
- [22.7](#22.7) <a name='22.7'></a> Use camelCase when you export-default a function. Your filename should be identical to your function's name.

```javascript
function makeStyleGuide() {
}

export default makeStyleGuide;
```

* [22.8](javascript.md#22.8)  Use PascalCase when you export a singleton / function library / bare object.

```javascript
const AirbnbStyleGuide = {
    es6: {
    }
};

export default AirbnbStyleGuide;
```

* [22.9](javascript.md#22.9)  Use CONSTANT\_CASE when naming magic values \(e.g. numerical constants\)

```javascript
const SOME_MAGIC_NUMBER = 1337;
```

* [22.10](javascript.md#22.10)  Use `Element` suffix when naming variables or properties that reference a DOM element.

```javascript
const titleElement = document.getElementById('title-element');
```

* [22.11](javascript.md#22.11)  Whenever possible, methods that return a boolean result should start with the prefix "is" \(e.g. `isValidFormat()`, `isCanadianCitizen()`, `isDeclawed()`, etc\). However, certain words dictate another prefix, \(e.g. `usesPostOffice()`, `hasMoustache()`\). As long as the method name is posed as a yes/no question, you're on the right track.

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

## Accessors

* [23.1](javascript.md#23.1)  Accessor functions for properties are not required.

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

// good
dragon.getAge();

// bad
dragon.age(25);

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

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

## Events

* [24.1](javascript.md#24.1)  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:

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

...

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

prefer:

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

...

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

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

## React

* [25.1](javascript.md#25.1)  Component class names should always begin with an upper case character.

> Why? React expects elements starting with a lower case character to be built-in HTML elements and will throw an error if your component starts with one.

```javascript
// Bad
var myComponent = React.createClass({ /* ... */ });

// Good
var MyComponent = React.createClass({ /* ... */ });
```

* [25.2](javascript.md#25.2)  Always define the component's props in a `propTypes` object.

> Why? This makes it easy to quickly see all of the props that are used in the component, gives the reader an idea of what the component does, and adds validation to incoming props on a new instance. Comment these props generously and alphabetize them if there are many.

* [25.3](javascript.md#25.3)  Event handlers should be named `handle{EventName}`. Props that pass event handlers should be named `on{EventName}`.

```javascript
handleButtonClick: function () {}
```

```javascript
<Component onButtonClick={this.handleButtonClick.bind(this)} />
```

* [25.4](javascript.md#25.4)  Prefer stateless components

> [Stateless components](https://facebook.github.io/react/docs/reusable-components.html#stateless-functions) are lightweight, easier to test, and more functional. Use them whenever a component just needs to render props.

```javascript
// Good
const Component = () => <p>Hey</p>

// Good

class Component extends React.Component {
  componentDidMount() {
      // do stuff
  }

  render() {
      // render stuff
  }
}

// Bad

class Component extends React.Component {
    render() {
        // renderStuff()
    }
}
```

* [25.7](javascript.md#25.6)  Extending React.Component instead of React.createClass\(\)

### Redux

* Use selectors to get data from the state

> Only store unique and non-derivable data on the state tree. Anything that can be computed should use selectors via [reselect](https://github.com/reactjs/reselect)

* Logic should be done in action creators, not reducers

> Reducers are simple functions that compute state. All logic should be done in action creators. If you have to dispatch conditional or asynchronous actions, see [redux-thunk](https://github.com/gaearon/redux-thunk).

### ES6

* For default arguments of arrays or objects, use predefined variables.

> On a default parameter, e.g., `f = (a = []) => {}`, where `a` is set to an empty arra if no argument is supplied, it is more performant to predefine the default argument, i.e.,

```javascript
const defaultA = []

f = (a = default) => {}
```

This can improve performance of large data sets \(especially in React\), and allows memoization of arguments by reselect \(see Redux above\), since it is not creating a new reference every time.

* [25.4](javascript.md#25.4)  A constructor that calls `super(props)` is required.

```javascript
class FooComponent extends React.Component {
    constructor(props) {
        super(props);
    }
}

FooComponent.displayName = displayName;
FooComponent.propTypes = propTypes;
FooComponent.defaultProps = defaultProps;
```

### JSX

* [25.11](javascript.md#25.11)  Align props when you can't fit them all on a single line. The ending "&gt;" of the opening tag should go on the same line as the last prop.

```javascript
// Good
<div className="foo-container"
    data-bar="some value">
    { /* ... */ }
</div>

// Good
<div className="foo-container" data-bar="some value">
    { /* ... */ }
</div>

// Bad
<div
    className="foo-container"
    data-bar="some value"
>
    { /* ... */ }
</div>
```

* [25.12](javascript.md#25.12)  Open elements that span multiple lines on a new line.

```javascript
// Bad
var multilineJsx = <div>
    { /* ... */ }
</div>;

// Good
var multilineJsx = (
    <div>
        { /* ... */ }
    </div>
);

// Better
var multilineJsx = (
    <div
        className="really very long class"
        id="reallyReallyReallyLongId"
        data-foo="some important value">
        { /* ... */ }
    </div>
);
```

* [25.13](javascript.md#25.13)  Variables holding a JSX object can stay on one line if it is short enough.

```javascript
var singleLineJsx = <h1>My header</h1>;
```

* [25.14](javascript.md#25.14)  Components without children should be self-closing.

```javascript
// Bad
<FooComponent></FooComponent>

// Good
<FooComponent />
```

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

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

## Switch Statements

The usage of `switch` statements is generally discouraged, but can be useful when there are a large number of cases – especially when multiple cases can be handled by the same block, or fall-through logic \(the `default` case\) can be leveraged.

When using `switch` statements:

* Use a `break` for each case other than `default`. When allowing statements to "fall through," note that explicitly.
* Indent case statements one tab within the switch.

```javascript
switch (event.keyCode) {
  // ENTER and SPACE both trigger x()
  case $.ui.keyCode.ENTER:
  case $.ui.keyCode.SPACE:
      x();
      break;
  case $.ui.keyCode.ESCAPE:
      y();
      break;
  default:
      z();
}
```

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

## ECMAScript 6 Styles

* [28.1](javascript.md#28.1)  This is a collection of links to the various es6 features.
* [Arrow Functions](javascript.md#arrow-functions)
* [Classes](javascript.md#constructors)
* [Object Shorthand](javascript.md#es6-object-shorthand)
* [Object Concise](javascript.md#es6-object-concise)
* [Object Computed Properties](javascript.md#es6-computed-properties)
* [Template Strings](javascript.md#es6-template-literals)
* [Destructuring](javascript.md#destructuring)
* [Default Parameters](javascript.md#es6-default-parameters)
* [Rest](javascript.md#es6-rest)
* [Array Spreads](javascript.md#es6-array-spreads)
* [Let and Const](javascript.md#references)
* [Iterators and Generators](javascript.md#iterators-and-generators)
* [Modules](javascript.md#modules)

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

## Resources

**Learning ES6**

* [Draft ECMA 2015 \(ES6\) Spec](https://people.mozilla.org/~jorendorff/es6-draft.html)
* [ExploringJS](http://exploringjs.com/)
* [ES6 Compatibility Table](https://kangax.github.io/compat-table/es6/)
* [Comprehensive Overview of ES6 Features](http://es6-features.org/)

**Read This**

* [Standard ECMA-262](http://www.ecma-international.org/ecma-262/6.0/index.html)

**Tools**

* Code Style Linters
  * [ESlint](http://eslint.org/) - [Airbnb Style .eslintrc](https://github.com/airbnb/javascript/blob/master/linters/.eslintrc)
  * [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 Style Guides**

* [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) - 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: ES6 & Beyond](http://shop.oreilly.com/product/0636920033769.do) - 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/)