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JavaScript Style Guide

Airbnb JavaScript Style Guide() {

A mostly reasonable approach to JavaScript

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Table of Contents

  1. Types
  2. References
  3. Objects
  4. Arrays
  5. Destructuring
  6. Strings
  7. Functions
  8. Arrow Functions
  9. Classes & Constructors
  10. Modules
  11. Iterators and Generators
  12. Properties
  13. Variables
  14. Hoisting
  15. Comparison Operators & Equality
  16. Blocks
  17. Comments
  18. Whitespace
  19. Commas
  20. Semicolons
  21. Type Casting & Coercion
  22. Naming Conventions
  23. Accessors
  24. Events
  25. jQuery
  26. ECMAScript 5 Compatibility
  27. ECMAScript 6 Styles
  28. Testing
  29. Performance
  30. Resources
  31. In the Wild
  32. Translation
  33. The JavaScript Style Guide Guide
  34. Chat With Us About JavaScript
  35. Contributors
  36. License

Types

- 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 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

References

- 2.1 Use const for all of your references; avoid using var. eslint: prefer-const, no-const-assign

> Why? This ensures that you can't reassign your references, 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;
```

- 2.2 If you must reassign references, use let instead of var. eslint: no-var jscs: disallowVar

> 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;
}
```

- 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
```

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Objects

- 3.1 Use the literal syntax for object creation. eslint: no-new-object

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

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

- 3.2 If your code will be executed in browsers in script context, don’t use reserved words as keys. It won’t work in IE8. More info. It’s OK to use them in ES6 modules and server-side code. jscs: disallowIdentifierNames

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

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

- 3.3 Use readable synonyms in place of reserved words. jscs: disallowIdentifierNames

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

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

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

- 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 Use object method shorthand. eslint: object-shorthand jscs: requireEnhancedObjectLiterals

```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 Use property value shorthand. eslint: object-shorthand jscs: requireEnhancedObjectLiterals

> Why? It is shorter to write and descriptive.

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

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

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

- 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 Only quote properties that are invalid identifiers. eslint: quote-props jscs: disallowQuotedKeysInObjects

Why? In general we consider it subjectively easier to read. It improves syntax highlighting, and is also more easily optimized by many JS engines.

  // bad
  const bad = {
    'foo': 3,
    'bar': 4,
    'data-blah': 5,
  };

  // good
  const good = {
    foo: 3,
    bar: 4,
    'data-blah': 5,
  };

- 3.9 Do not call Object.prototype methods directly, such as hasOwnProperty, propertyIsEnumerable, and isPrototypeOf.

Why? These methods may be shadowed by properties on the object in question - consider { hasOwnProperty: false } - or, the object may be a null object (Object.create(null)).

  // bad
  console.log(object.hasOwnProperty(key));

  // good
  console.log(Object.prototype.hasOwnProperty.call(object, key));

  // best
  const has = Object.prototype.hasOwnProperty; // cache the lookup once, in module scope.
  /* or */
  const has = require('has');
  …
  console.log(has.call(object, key));

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Arrays

- 4.1 Use the literal syntax for array creation. eslint: no-array-constructor

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

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

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

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

- 4.5 Use return statements in array method callbacks. It’s ok to omit the return if the function body consists of a single statement following 8.2. eslint: array-callback-return

```javascript
// good
[1, 2, 3].map((x) => {
  const y = x + 1;
  return x * y;
});

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

// bad
const flat = {};
[[0, 1], [2, 3], [4, 5]].reduce((memo, item, index) => {
  const flatten = memo.concat(item);
  flat[index] = flatten;
});

// good
const flat = {};
[[0, 1], [2, 3], [4, 5]].reduce((memo, item, index) => {
  const flatten = memo.concat(item);
  flat[index] = flatten;
  return flatten;
});

// bad
inbox.filter((msg) => {
  const { subject, author } = msg;
  if (subject === 'Mockingbird') {
    return author === 'Harper Lee';
  } else {
    return false;
  }
});

// good
inbox.filter((msg) => {
  const { subject, author } = msg;
  if (subject === 'Mockingbird') {
    return author === 'Harper Lee';
  }

  return false;
});
```

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Destructuring

- 5.1 Use object destructuring when accessing and using multiple properties of an object. jscs: requireObjectDestructuring

> 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(user) {
  const { firstName, lastName } = user;
  return `${firstName} ${lastName}`;
}

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

- 5.2 Use array destructuring. jscs: requireArrayDestructuring

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

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

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

- 5.3 Use object destructuring for multiple return values, not array destructuring. jscs: disallowArrayDestructuringReturn

> 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, top } = processInput(input);
```

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Strings

- 6.1 Use single quotes '' for strings. eslint: quotes jscs: validateQuoteMarks

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

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

- 6.2 Strings that cause the line to go over 100 characters should be written across multiple lines using string concatenation.

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

```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 When programmatically building up strings, use template strings instead of concatenation. eslint: prefer-template template-curly-spacing jscs: requireTemplateStrings

> 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();
}

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

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

- 6.5 Never use eval() on a string, it opens too many vulnerabilities.

- 6.6 Do not unnecessarily escape characters in strings. eslint: no-useless-escape

> Why? Backslashes harm readability, thus they should only be present when necessary.

```javascript
// bad
const foo = '\'this\' \i\s \"quoted\"';

// good
const foo = '\'this\' is "quoted"';
const foo = `'this' is "quoted"`;
```

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Functions

- 7.1 Use function declarations instead of function expressions. jscs: requireFunctionDeclarations

> Why? Function declarations are named, so they're easier to identify in call stacks. Also, the whole body of a function declaration is hoisted, whereas only the reference of a function expression is hoisted. This rule makes it possible to always use [Arrow Functions](#arrow-functions) in place of function expressions.

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

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

- 7.2 Wrap immediately invoked function expressions in parentheses. eslint: wrap-iife jscs: requireParenthesesAroundIIFE

> Why? An immediately invoked function expression is a single unit - wrapping both it, and its invocation parens, in parens, cleanly expresses this. Note that in a world with modules everywhere, you almost never need an IIFE.

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

- 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. eslint: no-loop-func

- 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.

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

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

- 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 Never use arguments, opt to use rest syntax ... instead. eslint: prefer-rest-params

> Why? `...` is explicit about which arguments you want pulled. Plus, rest arguments are a real Array, and not merely 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 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 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 Always put default parameters last.

```javascript
// bad
function handleThings(opts = {}, name) {
  // ...
}

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

- 7.10 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');
```

- 7.11 Spacing in a function signature.

> Why? Consistency is good, and you shouldn’t have to add or remove a space when adding or removing a name.

```javascript
// bad
const f = function(){};
const g = function (){};
const h = function() {};

// good
const x = function () {};
const y = function a() {};
```

- 7.12 Never mutate parameters. eslint: no-param-reassign

> Why? Manipulating objects passed in as parameters can cause unwanted variable side effects in the original caller.

```javascript
// bad
function f1(obj) {
  obj.key = 1;
};

// good
function f2(obj) {
  const key = Object.prototype.hasOwnProperty.call(obj, 'key') ? obj.key : 1;
};
```

- 7.13 Never reassign parameters. eslint: no-param-reassign

> Why? Reassigning parameters can lead to unexpected behavior, especially when accessing the `arguments` object. It can also cause optimization issues, especially in V8.

```javascript
// bad
function f1(a) {
  a = 1;
}

function f2(a) {
  if (!a) { a = 1; }
}

// good
function f3(a) {
  const b = a || 1;
}

function f4(a = 1) {
}
```

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Arrow Functions

- 8.1 When you must use function expressions (as when passing an anonymous function), use arrow function notation. eslint: prefer-arrow-callback, arrow-spacing jscs: requireArrowFunctions

> Why? It creates a version of the function that executes in the context of `this`, which is usually what you want, and is a more concise syntax.

> Why not? If you have a fairly complicated function, you might move that logic out into its own function declaration.

```javascript
// bad
[1, 2, 3].map(function (x) {
  const y = x + 1;
  return x * y;
});

// good
[1, 2, 3].map((x) => {
  const y = x + 1;
  return x * y;
});
```

- 8.2 If the function body consists of a single expression, omit the braces and use the implicit return. Otherwise, keep the braces and use a return statement. eslint: arrow-parens, arrow-body-style jscs: disallowParenthesesAroundArrowParam, requireShorthandArrowFunctions

> Why? Syntactic sugar. It reads well when multiple functions are chained together.

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

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

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

// good
[1, 2, 3].map((number, index) => ({
  index: number
}));
```

- 8.3 In case the expression spans over multiple lines, wrap it in parentheses for better readability.

> Why? It shows clearly where the function starts and ends.

```js
// bad
[1, 2, 3].map(number => 'As time went by, the string containing the ' +
  `${number} became much longer. So we needed to break it over multiple ` +
  'lines.'
);

// good
[1, 2, 3].map(number => (
  `As time went by, the string containing the ${number} became much ` +
  'longer. So we needed to break it over multiple lines.'
));
```

- 8.4 If your function takes a single argument and doesn’t use braces, omit the parentheses. Otherwise, always include parentheses around arguments. eslint: arrow-parens jscs: disallowParenthesesAroundArrowParam

> Why? Less visual clutter.

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

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

// good
[1, 2, 3].map(number => (
  `A long string with the ${number}. It’s so long that we’ve broken it ` +
  'over multiple lines!'
));

// bad
[1, 2, 3].map(x => {
  const y = x + 1;
  return x * y;
});

// good
[1, 2, 3].map((x) => {
  const y = x + 1;
  return x * y;
});
```

- 8.5 Avoid confusing arrow function syntax (=>) with comparison operators (<=, >=). eslint: no-confusing-arrow

```js
// bad
const itemHeight = item => item.height > 256 ? item.largeSize : item.smallSize;

// bad
const itemHeight = (item) => item.height > 256 ? item.largeSize : item.smallSize;

// good
const itemHeight = (item) => { return item.height > 256 ? item.largeSize : item.smallSize; };
```

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Classes & Constructors

- 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 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 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 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()}`;
  }
}
```

- 9.5 Classes have a default constructor if one is not specified. An empty constructor function or one that just delegates to a parent class is unnecessary. eslint: no-useless-constructor

```javascript
// bad
class Jedi {
  constructor() {}

  getName() {
    return this.name;
  }
}

// bad
class Rey extends Jedi {
  constructor(...args) {
    super(...args);
  }
}

// good
class Rey extends Jedi {
  constructor(...args) {
    super(...args);
    this.name = 'Rey';
  }
}
```

- 9.6 Avoid duplicate class members. eslint: no-dupe-class-members

> Why? Duplicate class member declarations will silently prefer the last one - having duplicates is almost certainly a bug.

```javascript
// bad
class Foo {
  bar() { return 1; }
  bar() { return 2; }
}

// good
class Foo {
  bar() { return 1; }
}

// good
class Foo {
  bar() { return 2; }
}
```

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Modules

- 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 Do not use wildcard imports.

> Why? This makes sure you have a single default export.

```javascript
// bad
import * as AirbnbStyleGuide from './AirbnbStyleGuide';

// good
import AirbnbStyleGuide from './AirbnbStyleGuide';
```

- 10.3 And do not export directly from an import.

> Why? Although the one-liner is concise, having one clear way to import and one clear way to export makes things consistent.

```javascript
// bad
// filename es6.js
export { es6 as default } from './airbnbStyleGuide';

// good
// filename es6.js
import { es6 } from './AirbnbStyleGuide';
export default es6;
```

- 10.4 Only import from a path in one place. eslint: no-duplicate-imports > Why? Having multiple lines that import from the same path can make code harder to maintain.

```javascript
// bad
import foo from 'foo';
// … some other imports … //
import { named1, named2 } from 'foo';

// good
import foo, { named1, named2 } from 'foo';

// good
import foo, {
  named1,
  named2,
} from 'foo';
```

- 10.5 Do not export mutable bindings. eslint: import/no-mutable-exports > Why? Mutation should be avoided in general, but in particular when exporting mutable bindings. While this technique may be needed for some special cases, in general, only constant references should be exported.

```javascript
// bad
let foo = 3;
export { foo }

// good
const foo = 3;
export { foo }
```

- 10.6 In modules with a single export, prefer default export over named export. eslint: import/prefer-default-export

```javascript
// bad
export function foo() {}

// good
export default function foo() {}
```

- 10.7 Put all imports above non-import statements. eslint: import/imports-first > Why? Since imports are hoisted, keeping them all at the top prevents surprising behavior.

```javascript
// bad
import foo from 'foo';
foo.init();

import bar from 'bar';

// good
import foo from 'foo';
import bar from 'bar';

foo.init();
```

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Iterators and Generators

- 11.1 Don’t use iterators. Prefer JavaScript’s higher-order functions like map() and reduce() instead of loops like for-of. eslint: no-iterator

> 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 Don’t use generators for now.

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

- 11.3 If you must use generators, or if you disregard our advice, make sure their function signature is spaced properly. eslint: generator-star-spacing

> Why? `function` and `*` are part of the same conceptual keyword - `*` is not a modifier for `function`, `function*` is a unique construct, different from `function`.

```js
// bad
function * foo() {
}

const bar = function * () {
}

const baz = function *() {
}

const quux = function*() {
}

function*foo() {
}

function *foo() {
}

// very bad
function
*
foo() {
}

const wat = function
*
() {
}

// good
function* foo() {
}

const foo = function* () {
}
```

⬆ back to top

Properties

- 12.1 Use dot notation when accessing properties. eslint: dot-notation jscs: requireDotNotation

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

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

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

- 12.2 Use bracket 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

Variables

- 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 Use one const declaration per variable. eslint: one-var jscs: disallowMultipleVarDecl

> 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. You can also step through each declaration with the debugger, instead of jumping through all of them at once.

```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 Group all your consts and then group all your lets.

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

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

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

  return name;
}

// good
function checkName(hasName) {
  if (hasName === 'test') {
    return false;
  }

  const name = getName();

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

  return name;
}
```

⬆ back to top

Hoisting

- 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). It’s important to know why typeof is no longer safe.

```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 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 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 Function declarations hoist their name and the function body.

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

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

⬆ back to top

Comparison Operators & Equality

- 15.1 Use === and !== over == and !=. eslint: eqeqeq

- 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 (even an empty one) is an object, objects will evaluate to true
}
```

- 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 For more information see Truth Equality and JavaScript by Angus Croll.

- 15.5 Use braces to create blocks in case and default clauses that contain lexical declarations (e.g. let, const, function, and class).

Why? Lexical declarations are visible in the entire switch block but only get initialized when assigned, which only happens when its case is reached. This causes problems when multiple case clauses attempt to define the same thing.

eslint rules: no-case-declarations.

```javascript
// bad
switch (foo) {
  case 1:
    let x = 1;
    break;
  case 2:
    const y = 2;
    break;
  case 3:
    function f() {}
    break;
  default:
    class C {}
}

// good
switch (foo) {
  case 1: {
    let x = 1;
    break;
  }
  case 2: {
    const y = 2;
    break;
  }
  case 3: {
    function f() {}
    break;
  }
  case 4:
    bar();
    break;
  default: {
    class C {}
  }
}
```

- 15.6 Ternaries should not be nested and generally be single line expressions.

eslint rules: [`no-nested-ternary`](http://eslint.org/docs/rules/no-nested-ternary.html).

```javascript
// bad
const foo = maybe1 > maybe2
  ? "bar"
  : value1 > value2 ? "baz" : null;

// better
const maybeNull = value1 > value2 ? 'baz' : null;

const foo = maybe1 > maybe2
  ? 'bar'
  : maybeNull;

// best
const maybeNull = value1 > value2 ? 'baz' : null;

const foo = maybe1 > maybe2 ? 'bar' : maybeNull;
```

- 15.7 Avoid unneeded ternary statements.

eslint rules: [`no-unneeded-ternary`](http://eslint.org/docs/rules/no-unneeded-ternary.html).

```javascript
// bad
const foo = a ? a : b;
const bar = c ? true : false;
const baz = c ? false : true;

// good
const foo = a || b;
const bar = !!c;
const baz = !c;
```

⬆ back to top

Blocks

- 16.1 Use braces with all multi-line blocks.

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

// good
if (test) return false;

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

// bad
function foo() { return false; }

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

- 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. eslint: brace-style jscs: disallowNewlineBeforeBlockStatements

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

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

⬆ back to top

Comments

- 17.1 Use /** ... */ for multi-line comments. Include a description, specify types and values for all parameters and return values.

```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 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 unless it’s on the first line of a block.

```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;
}

// also good
function getType() {
  // set the default type to 'no type'
  const type = this._type || 'no type';

  return type;
}
```

- 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 Use // FIXME: to annotate problems.

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

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

- 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

Whitespace

- 18.1 Use soft tabs set to 2 spaces. eslint: indent jscs: validateIndentation

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

// bad
function bar() {
∙const name;
}

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

- 18.2 Place 1 space before the leading brace. eslint: space-before-blocks jscs: requireSpaceBeforeBlockStatements

```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 Place 1 space before the opening parenthesis in control statements (if, while etc.). Place no space between the argument list and the function name in function calls and declarations. eslint: keyword-spacing jscs: requireSpaceAfterKeywords

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

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

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

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

- 18.4 Set off operators with spaces. eslint: space-infix-ops jscs: requireSpaceBeforeBinaryOperators, requireSpaceAfterBinaryOperators

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

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

- 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);↵
```

- 18.6 Use indentation when making long method chains (more than 2 method chains). Use a leading dot, which emphasizes that the line is a method call, not a new statement. eslint: newline-per-chained-call no-whitespace-before-property

```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').classed('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')
    .classed('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);
```

- 18.7 Leave a blank line after blocks and before the next statement. jscs: requirePaddingNewLinesAfterBlocks

```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;
```

- 18.8 Do not pad your blocks with blank lines. eslint: padded-blocks jscs: disallowPaddingNewlinesInBlocks

```javascript
// bad
function bar() {

  console.log(foo);

}

// also bad
if (baz) {

  console.log(qux);
} else {
  console.log(foo);

}

// good
function bar() {
  console.log(foo);
}

// good
if (baz) {
  console.log(qux);
} else {
  console.log(foo);
}
```

- 18.9 Do not add spaces inside parentheses. eslint: space-in-parens jscs: disallowSpacesInsideParentheses

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

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

// bad
if ( foo ) {
  console.log(foo);
}

// good
if (foo) {
  console.log(foo);
}
```

- 18.10 Do not add spaces inside brackets. eslint: array-bracket-spacing jscs: disallowSpacesInsideArrayBrackets

```javascript
// bad
const foo = [ 1, 2, 3 ];
console.log(foo[ 0 ]);

// good
const foo = [1, 2, 3];
console.log(foo[0]);
```

- 18.11 Add spaces inside curly braces. eslint: object-curly-spacing jscs: requireSpacesInsideObjectBrackets

```javascript
// bad
const foo = {clark: 'kent'};

// good
const foo = { clark: 'kent' };
```

- 18.12 Avoid having lines of code that are longer than 100 characters (including whitespace). eslint: max-len jscs: maximumLineLength

> Why? This ensures readability and maintainability.

```javascript
// bad
const foo = 'Whatever national crop flips the window. The cartoon reverts within the screw. Whatever wizard constrains a helpful ally. The counterpart ascends!';

// bad
$.ajax({ method: 'POST', url: 'https://airbnb.com/', data: { name: 'John' } }).done(() => console.log('Congratulations!')).fail(() => console.log('You have failed this city.'));

// good
const foo = 'Whatever national crop flips the window. The cartoon reverts within the screw. ' +
  'Whatever wizard constrains a helpful ally. The counterpart ascends!';

// good
$.ajax({
  method: 'POST',
  url: 'https://airbnb.com/',
  data: { name: 'John' },
})
  .done(() => console.log('Congratulations!'))
  .fail(() => console.log('You have failed this city.'));
```

⬆ back to top

Commas

- 19.1 Leading commas: Nope. eslint: comma-style jscs: requireCommaBeforeLineBreak

```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 Additional trailing comma: Yup. eslint: comma-dangle jscs: requireTrailingComma

> 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](es5/README.md#commas) 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

Semicolons

- 20.1 Yup. eslint: semi jscs: requireSemicolons

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

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

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

[Read more](http://stackoverflow.com/questions/7365172/semicolon-before-self-invoking-function/7365214%237365214).

⬆ back to top

Type Casting & Coercion

- 21.1 Perform type coercion at the beginning of the statement.

- 21.2 Strings:

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

// bad
const totalScore = this.reviewScore + ''; // invokes this.reviewScore.valueOf()

// bad
const totalScore = this.reviewScore.toString(); // isn't guaranteed to return a string

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

- 21.3 Numbers: Use Number for type casting and parseInt always with a radix for parsing strings. eslint: radix

```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);

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