Airbnb JavaScript Style Guide() {

A mostly reasonable approach to JavaScript

Note: this guide assumes you are using Babel, and requires that you use babel-preset-airbnb or the equivalent. It also assumes you are installing shims/polyfills in your app, with airbnb-browser-shims or the equivalent.

This guide is available in other languages too. See Translation

Other Style Guides

Types
References
Objects
Arrays
Destructuring
Strings
Functions
Arrow Functions
Classes & Constructors
Modules
Iterators and Generators
Properties
Variables
Hoisting
Comparison Operators & Equality
Blocks
Control Statements
Comments
Whitespace
Commas
Semicolons
Type Casting & Coercion
Naming Conventions
Accessors
Events
jQuery
ECMAScript 5 Compatibility
ECMAScript 6+ (ES 2015+) Styles
Standard Library
Testing
Performance
Resources
In the Wild
Translation
The JavaScript Style Guide Guide
Chat With Us About JavaScript
Contributors
License
Amendments

Types

1.1 Primitives: When you access a primitive type you work directly on its value.
- string
- number
- boolean
- null
- undefined
- symbol
- bigint
```
const foo = 1;
let bar = foo;

bar = 9;

console.log(foo, bar); // => 1, 9
```
- Symbols and BigInts cannot be faithfully polyfilled, so they should not be used when targeting browsers/environments that don’t support them natively.

1.2 Complex: When you access a complex type you work on a reference to its value.

object
array
function

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

bar[0] = 9;

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

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References

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

Perchè? This ensures that you can’t reassign your references, which can lead to bugs and difficult to comprehend code.
```
// male
var a = 1;
var b = 2;

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

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

Perchè? let is block-scoped rather than function-scoped like var.

// male
var count = 1;
if (true) {
  count += 1;
}

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

2.3 Note that both let and const are block-scoped, whereas var is function-scoped.
```
// const and let only exist in the blocks they are defined in.
{
  let a = 1;
  const b = 1;
  var c = 1;
}
console.log(a); // ReferenceError
console.log(b); // ReferenceError
console.log(c); // Prints 1
```
In the above code, you can see that referencing a and b will produce a ReferenceError, while c contains the number. This is because a and b are block scoped, while c is scoped to the containing function.

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Objects

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

// male
const item = new Object();

// bene
const item = {};

3.2 Use computed property names when creating objects with dynamic property names.

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

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

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

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

3.3 Use object method shorthand. eslint: object-shorthand

// male
const atom = {
  value: 1,

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

// bene
const atom = {
  value: 1,

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

3.4 Use property value shorthand. eslint: object-shorthand

Perchè? It is shorter and descriptive.

const lukeSkywalker = 'Luke Skywalker';

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

// bene
const obj = {
  lukeSkywalker,
};

3.5 Group your shorthand properties at the beginning of your object declaration.

Perchè? It’s easier to tell which properties are using the shorthand.

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

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

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

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

Perchè? In general we consider it subjectively easier to read. It improves syntax highlighting, and is also more easily optimized by many JS engines.
```
// male
const bad = {
  'foo': 3,
  'bar': 4,
  'data-blah': 5,
};

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

3.7 Do not call Object.prototype methods directly, such as hasOwnProperty, propertyIsEnumerable, and isPrototypeOf. eslint: no-prototype-builtins

Perchè? 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)).

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

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

// best
const has = Object.prototype.hasOwnProperty; // cache the lookup once, in module scope.
console.log(has.call(object, key));
/* or */
import has from 'has'; // https://www.npmjs.com/package/has
console.log(has(object, key));

3.8 Prefer the object spread syntax over Object.assign to shallow-copy objects. Use the object rest parameter syntax to get a new object with certain properties omitted. eslint: prefer-object-spread

// very bad
const original = { a: 1, b: 2 };
const copy = Object.assign(original, { c: 3 }); // this mutates `original` ಠ_ಠ
delete copy.a; // so does this

// male
const original = { a: 1, b: 2 };
const copy = Object.assign({}, original, { c: 3 }); // copy => { a: 1, b: 2, c: 3 }

// bene
const original = { a: 1, b: 2 };
const copy = { ...original, c: 3 }; // copy => { a: 1, b: 2, c: 3 }

const { a, ...noA } = copy; // noA => { b: 2, c: 3 }

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Arrays

4.1 Use the literal syntax for array creation. eslint: no-array-constructor
```
// male
const items = new Array();

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

4.2 Use Array#push instead of direct assignment to add items to an array.

const someStack = [];

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

// bene
someStack.push('abracadabra');

4.3 Use array spreads ... to copy arrays.

// male
const len = items.length;
const itemsCopy = [];
let i;

for (i = 0; i < len; i += 1) {
  itemsCopy[i] = items[i];
}

// bene
const itemsCopy = [...items];

4.4 To convert an iterable object to an array, use spreads ... instead of Array.from

const foo = document.querySelectorAll('.foo');

// bene
const nodes = Array.from(foo);

// best
const nodes = [...foo];

4.5 Use Array.from for converting an array-like object to an array.

const arrLike = { 0: 'foo', 1: 'bar', 2: 'baz', length: 3 };

// male
const arr = Array.prototype.slice.call(arrLike);

// bene
const arr = Array.from(arrLike);

4.6 Use Array.from instead of spread ... for mapping over iterables, because it avoids creating an intermediate array.
```
// male
const baz = [...foo].map(bar);

// bene
const baz = Array.from(foo, bar);
```

4.7 Use return statements in array method callbacks. It’s ok to omit the return if the function body consists of a single statement returning an expression without side effects, following 8.2. eslint: array-callback-return

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

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

// male - no returned value means `acc` becomes undefined after the first iteration
[[0, 1], [2, 3], [4, 5]].reduce((acc, item, index) => {
  const flatten = acc.concat(item);
});

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

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

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

  return false;
});

4.8 Use line breaks after open and before close array brackets if an array has multiple lines

// male
const arr = [
  [0, 1], [2, 3], [4, 5],
];

const objectInArray = [{
  id: 1,
}, {
  id: 2,
}];

const numberInArray = [
  1, 2,
];

// bene
const arr = [[0, 1], [2, 3], [4, 5]];

const objectInArray = [
  {
    id: 1,
  },
  {
    id: 2,
  },
];

const numberInArray = [
  1,
  2,
];

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Destructuring

5.1 Use object destructuring when accessing and using multiple properties of an object. eslint: prefer-destructuring

Perchè? Destructuring saves you from creating temporary references for those properties, and from repetitive access of the object. Repeating object access creates more repetitive code, requires more reading, and creates more opportunities for mistakes. Destructuring objects also provides a single site of definition of the object structure that is used in the block, rather than requiring reading the entire block to determine what is used.
```
// male
function getFullName(user) {
  const firstName = user.firstName;
  const lastName = user.lastName;

  return `${firstName} ${lastName}`;
}

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

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

5.2 Use array destructuring. eslint: prefer-destructuring

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

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

// bene
const [first, second] = arr;

5.3 Use object destructuring for multiple return values, not array destructuring.

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

// male
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);

// bene
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

// male
const name = "Capt. Janeway";

// male - template literals should contain interpolation or newlines
const name = `Capt. Janeway`;

// bene
const name = 'Capt. Janeway';

6.2 Strings that cause the line to go over 100 characters should not be written across multiple lines using string concatenation.

Perchè? Broken strings are painful to work with and make code less searchable.

// male
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.';

// male
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.';

// bene
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.';

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

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

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

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

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

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

6.4 Never use eval() on a string, it opens too many vulnerabilities. eslint: no-eval

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

Perchè? Backslashes harm readability, thus they should only be present when necessary.
```
// male
const foo = '\'this\' \i\s \"quoted\"';

// bene
const foo = '\'this\' is "quoted"';
const foo = `my name is '${name}'`;
```

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Functions

7.1 Use named function expressions instead of function declarations. eslint: func-style

Perchè? Function declarations are hoisted, which means that it’s easy - too easy - to reference the function before it is defined in the file. This harms readability and maintainability. If you find that a function’s definition is large or complex enough that it is interfering with understanding the rest of the file, then perhaps it’s time to extract it to its own module! Don’t forget to explicitly name the expression, regardless of whether or not the name is inferred from the containing variable (which is often the case in modern browsers or when using compilers such as Babel). This eliminates any assumptions made about the Error’s call stack. (Discussion)
```
// male
function foo() {
  // ...
}

// male
const foo = function () {
  // ...
};

// bene
// lexical name distinguished from the variable-referenced invocation(s)
const short = function longUniqueMoreDescriptiveLexicalFoo() {
  // ...
};
```

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

Perchè? 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.
```
// 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.

// male
if (currentUser) {
  function test() {
    console.log('Nope.');
  }
}

// bene
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.

// male
function foo(name, options, arguments) {
  // ...
}

// bene
function foo(name, options, args) {
  // ...
}

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

Perchè? ... is explicit about which arguments you want pulled. Plus, rest arguments are a real Array, and not merely Array-like like arguments.
```
// male
function concatenateAll() {
  const args = Array.prototype.slice.call(arguments);
  return args.join('');
}

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

7.7 Use default parameter syntax rather than mutating function arguments.

// 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 = {};
  }
  // ...
}

// bene
function handleThings(opts = {}) {
  // ...
}

7.8 Avoid side effects with default parameters.

Perchè? They are confusing to reason about.

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

7.9 Always put default parameters last. eslint: default-param-last

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

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

7.10 Never use the Function constructor to create a new function. eslint: no-new-func

Perchè? Creating a function in this way evaluates a string similarly to eval(), which opens vulnerabilities.
```
// male
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. eslint: space-before-function-paren space-before-blocks

Perchè? Consistency is good, and you shouldn’t have to add or remove a space when adding or removing a name.
```
// male
const f = function(){};
const g = function (){};
const h = function() {};

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

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

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

// male
function f1(obj) {
  obj.key = 1;
}

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

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

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

// male
function f1(a) {
  a = 1;
  // ...
}

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

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

function f4(a = 1) {
  // ...
}

7.14 Prefer the use of the spread syntax ... to call variadic functions. eslint: prefer-spread

Perchè? It’s cleaner, you don’t need to supply a context, and you can not easily compose new with apply.

// male
const x = [1, 2, 3, 4, 5];
console.log.apply(console, x);

// bene
const x = [1, 2, 3, 4, 5];
console.log(...x);

// male
new (Function.prototype.bind.apply(Date, [null, 2016, 8, 5]));

// bene
new Date(...[2016, 8, 5]);

7.15 Functions with multiline signatures, or invocations, should be indented just like every other multiline list in this guide: with each item on a line by itself, with a trailing comma on the last item. eslint: function-paren-newline

// male
function foo(bar,
             baz,
             quux) {
  // ...
}

// bene
function foo(
  bar,
  baz,
  quux,
) {
  // ...
}

// male
console.log(foo,
  bar,
  baz);

// bene
console.log(
  foo,
  bar,
  baz,
);

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Arrow Functions

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

Perchè? 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 named function expression.
```
// male
[1, 2, 3].map(function (x) {
  const y = x + 1;
  return x * y;
});

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

8.2 If the function body consists of a single statement returning an expression without side effects, omit the braces and use the implicit return. Otherwise, keep the braces and use a return statement. eslint: arrow-parens, arrow-body-style

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

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

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

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

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

// No implicit return with side effects
function foo(callback) {
  const val = callback();
  if (val === true) {
    // Do something if callback returns true
  }
}

let bool = false;

// male
foo(() => bool = true);

// bene
foo(() => {
  bool = true;
});

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

Perchè? It shows clearly where the function starts and ends.

// male
['get', 'post', 'put'].map((httpMethod) => Object.prototype.hasOwnProperty.call(
    httpMagicObjectWithAVeryLongName,
    httpMethod,
  )
);

// bene
['get', 'post', 'put'].map((httpMethod) => (
  Object.prototype.hasOwnProperty.call(
    httpMagicObjectWithAVeryLongName,
    httpMethod,
  )
));

8.4 Always include parentheses around arguments for clarity and consistency. eslint: arrow-parens

Perchè? Minimizes diff churn when adding or removing arguments.

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

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

// male
[1, 2, 3].map(number => (
  `A long string with the ${number}. It’s so long that we don’t want it to take up space on the .map line!`
));

// bene
[1, 2, 3].map((number) => (
  `A long string with the ${number}. It’s so long that we don’t want it to take up space on the .map line!`
));

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

// bene
[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

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

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

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

// bene
const itemHeight = (item) => {
  const { height, largeSize, smallSize } = item;
  return height <= 256 ? largeSize : smallSize;
};

8.6 Enforce the location of arrow function bodies with implicit returns. eslint: implicit-arrow-linebreak

// male
(foo) =>
  bar;

(foo) =>
  (bar);

// bene
(foo) => bar;
(foo) => (bar);
(foo) => (
   bar
)

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Classes & Constructors

9.1 Always use class. Avoid manipulating prototype directly.

Perchè? class syntax is more concise and easier to reason about.

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

// bene
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.

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

// male
const inherits = require('inherits');
function PeekableQueue(contents) {
  Queue.apply(this, contents);
}
inherits(PeekableQueue, Queue);
PeekableQueue.prototype.peek = function () {
  return this.queue[0];
};

// bene
class PeekableQueue extends Queue {
  peek() {
    return this.queue[0];
  }
}

9.3 Methods can return this to help with method chaining.

// male
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

// bene
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.

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

// male
class Jedi {
  constructor() {}

  getName() {
    return this.name;
  }
}

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

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

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

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

// male
class Foo {
  bar() { return 1; }
  bar() { return 2; }
}

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

// bene
class Foo {
  bar() { return 2; }
}

9.7 Class methods should use this or be made into a static method unless an external library or framework requires using specific non-static methods. Being an instance method should indicate that it behaves differently based on properties of the receiver. eslint: class-methods-use-this

// male
class Foo {
  bar() {
    console.log('bar');
  }
}

// bene - this is used
class Foo {
  bar() {
    console.log(this.bar);
  }
}

// bene - constructor is exempt
class Foo {
  constructor() {
    // ...
  }
}

// bene - static methods aren't expected to use this
class Foo {
  static bar() {
    console.log('bar');
  }
}

⬆ back to top

Modules

10.1 Always use modules (import/export) over a non-standard module system. You can always transpile to your preferred module system.

Perchè? Modules are the future, let’s start using the future now.

// male
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.

Perchè? This makes sure you have a single default export.

// male
import * as AirbnbStyleGuide from './AirbnbStyleGuide';

// bene
import AirbnbStyleGuide from './AirbnbStyleGuide';

10.3 And do not export directly from an import.

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

// male
// filename es6.js
export { es6 as default } from './AirbnbStyleGuide';

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

10.4 Only import from a path in one place. eslint: no-duplicate-imports

Perchè? Having multiple lines that import from the same path can make code harder to maintain.

// male
import foo from 'foo';
// … some other imports … //
import { named1, named2 } from 'foo';

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

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

10.5 Do not export mutable bindings. eslint: import/no-mutable-exports

Perchè? 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.
```
// male
let foo = 3;
export { foo };

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

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

Perchè? To encourage more files that only ever export one thing, which is better for readability and maintainability.
```
// male
export function foo() {}

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

10.7 Put all imports above non-import statements. eslint: import/first

Perchè? Since imports are hoisted, keeping them all at the top prevents surprising behavior.
```
// male
import foo from 'foo';
foo.init();

import bar from 'bar';

// bene
import foo from 'foo';
import bar from 'bar';

foo.init();
```

10.8 Multiline imports should be indented just like multiline array and object literals. eslint: object-curly-newline

Perchè? The curly braces follow the same indentation rules as every other curly brace block in the style guide, as do the trailing commas.
```
// male
import {longNameA, longNameB, longNameC, longNameD, longNameE} from 'path';

// bene
import {
  longNameA,
  longNameB,
  longNameC,
  longNameD,
  longNameE,
} from 'path';
```

10.9 Disallow Webpack loader syntax in module import statements. eslint: import/no-webpack-loader-syntax

Perchè? Since using Webpack syntax in the imports couples the code to a module bundler. Prefer using the loader syntax in webpack.config.js.
```
// male
import fooSass from 'css!sass!foo.scss';
import barCss from 'style!css!bar.css';

// bene
import fooSass from 'foo.scss';
import barCss from 'bar.css';
```

10.10 Do not include JavaScript filename extensions eslint: import/extensions

Perchè? Including extensions inhibits refactoring, and inappropriately hardcodes implementation details of the module you're importing in every consumer.
```
// male
import foo from './foo.js';
import bar from './bar.jsx';
import baz from './baz/index.jsx';

// bene
import foo from './foo';
import bar from './bar';
import baz from './baz';
```

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Iterators and Generators

11.1 Don’t use iterators. Prefer JavaScript’s higher-order functions instead of loops like for-in or for-of. eslint: no-iterator no-restricted-syntax

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

Use map() / every() / filter() / find() / findIndex() / reduce() / some() / ... to iterate over arrays, and Object.keys() / Object.values() / Object.entries() to produce arrays so you can iterate over objects.

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

// male
let sum = 0;
for (let num of numbers) {
  sum += num;
}
sum === 15;

// bene
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;

// male
const increasedByOne = [];
for (let i = 0; i < numbers.length; i++) {
  increasedByOne.push(numbers[i] + 1);
}

// bene
const increasedByOne = [];
numbers.forEach((num) => {
  increasedByOne.push(num + 1);
});

// best (keeping it functional)
const increasedByOne = numbers.map((num) => num + 1);

11.2 Don’t use generators for now.

Perchè? 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

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

// male
function * foo() {
  // ...
}

// male
const bar = function * () {
  // ...
};

// male
const baz = function *() {
  // ...
};

// male
const quux = function*() {
  // ...
};

// male
function*foo() {
  // ...
}

// male
function *foo() {
  // ...
}

// very bad
function
*
foo() {
  // ...
}

// very bad
const wat = function
*
() {
  // ...
};

// bene
function* foo() {
  // ...
}

// bene
const foo = function* () {
  // ...
};

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Properties

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

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

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

// bene
const isJedi = luke.jedi;

12.2 Use bracket notation [] when accessing properties with a variable.

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

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

const isJedi = getProp('jedi');

12.3 Use exponentiation operator ** when calculating exponentiations. eslint: no-restricted-properties.
```
// male
const binary = Math.pow(2, 10);

// bene
const binary = 2 ** 10;
```

⬆ back to top

Variables

13.1 Always use const or let 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. eslint: no-undef prefer-const
```
// male
superPower = new SuperPower();

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

13.2 Use one const or let declaration per variable or assignment. eslint: one-var

Perchè? 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.

// male
const items = getItems(),
    goSportsTeam = true,
    dragonball = 'z';

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

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

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

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

// male
let i, len, dragonball,
    items = getItems(),
    goSportsTeam = true;

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

// bene
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.

Perchè? let and const are block scoped and not function scoped.

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

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

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

  return name;
}

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

  const name = getName();

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

  return name;
}

13.5 Don’t chain variable assignments. eslint: no-multi-assign

Perchè? Chaining variable assignments creates implicit global variables.

// male
(function example() {
  // JavaScript interprets this as
  // let a = ( b = ( c = 1 ) );
  // The let keyword only applies to variable a; variables b and c become
  // global variables.
  let a = b = c = 1;
}());

console.log(a); // throws ReferenceError
console.log(b); // 1
console.log(c); // 1

// bene
(function example() {
  let a = 1;
  let b = a;
  let c = a;
}());

console.log(a); // throws ReferenceError
console.log(b); // throws ReferenceError
console.log(c); // throws ReferenceError

// the same applies for `const`

13.6 Avoid using unary increments and decrements (++, --). eslint no-plusplus

Perchè? Per the eslint documentation, unary increment and decrement statements are subject to automatic semicolon insertion and can cause silent errors with incrementing or decrementing values within an application. It is also more expressive to mutate your values with statements like num += 1 instead of num++ or num ++. Disallowing unary increment and decrement statements also prevents you from pre-incrementing/pre-decrementing values unintentionally which can also cause unexpected behavior in your programs.
```
// male

const array = [1, 2, 3];
let num = 1;
num++;
--num;

let sum = 0;
let truthyCount = 0;
for (let i = 0; i < array.length; i++) {
  let value = array[i];
  sum += value;
  if (value) {
    truthyCount++;
  }
}

// bene

const array = [1, 2, 3];
let num = 1;
num += 1;
num -= 1;

const sum = array.reduce((a, b) => a + b, 0);
const truthyCount = array.filter(Boolean).length;
```

13.7 Avoid linebreaks before or after = in an assignment. If your assignment violates max-len, surround the value in parens. eslint operator-linebreak.

Perchè? Linebreaks surrounding = can obfuscate the value of an assignment.

// male
const foo =
  superLongLongLongLongLongLongLongLongFunctionName();

// male
const foo
  = 'superLongLongLongLongLongLongLongLongString';

// bene
const foo = (
  superLongLongLongLongLongLongLongLongFunctionName()
);

// bene
const foo = 'superLongLongLongLongLongLongLongLongString';

13.8 Disallow unused variables. eslint: no-unused-vars

Perchè? Variables that are declared and not used anywhere in the code are most likely an error due to incomplete refactoring. Such variables take up space in the code and can lead to confusion by readers.

// male

var some_unused_var = 42;

// Write-only variables are not considered as used.
var y = 10;
y = 5;

// A read for a modification of itself is not considered as used.
var z = 0;
z = z + 1;

// Unused function arguments.
function getX(x, y) {
    return x;
}

// bene

function getXPlusY(x, y) {
  return x + y;
}

var x = 1;
var y = a + 2;

alert(getXPlusY(x, y));

// 'type' is ignored even if unused because it has a rest property sibling.
// This is a form of extracting an object that omits the specified keys.
var { type, ...coords } = data;
// 'coords' is now the 'data' object without its 'type' property.

⬆ back to top

Hoisting

14.1 var declarations get hoisted to the top of their closest enclosing function 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.

// 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.

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.

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.

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

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

For more information refer to JavaScript Scoping & Hoisting by Ben Cherry.

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Operatori di Confronto e Uguaglianza

15.1 Preferire === e !== a == e !=. eslint: eqeqeq

15.2 Istruzioni condizionali tipo if valutano la propria espressione usando la coercizione con il metodo astratto ToBoolean e seguono sempre queste semplici regoleù:
- Objects è valutato come true
- Undefined è valutato come false
- Null è valutato come false
- Booleans è valutato come il valore del boolean
- Numbers è valutato come false se +0, -0, o NaN, altrimenti true
- Strings è valutato come false se stringa vuota '', altrimenti true
```
if ([0] && []) {
  // true
  // un array (anche se vuoto) è un oggetto, gli oggetti vengono valutati come true
}
```

15.3 Usare le scorciatoie per booleani, ma confronti espliciti per stringhe e numeri.

// male
if (isValid === true) {
  // ...
}

// bene
if (isValid) {
  // ...
}

// male
if (name) {
  // ...
}

// bene
if (name !== '') {
  // ...
}

// male
if (collection.length) {
  // ...
}

// bene
if (collection.length > 0) {
  // ...
}

15.4 Per ulteriori informazioni, vedere Truth Equality and JavaScript di Angus Croll.

15.5 Usare parentesi graffe per creare blocchi in clausole case e default che contengono dichiarazioni lessicali (es. let, const, function, e class). eslint: no-case-declarations

Perchè? Le dichiarazioni lessicali sono visibili nell'intero blocco switch ma vengono inizializzate solo quando assegnate, cosa che si verifica solo quando viene raggiunto il suo case. Ciò provoca problemi quando multipleclausole case tentano di definire la stessa cosa.

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

// bene
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 I ternari non dovrebbero essere annidati e generalmente dovrebbero essere espressioni su singola riga. eslint: no-nested-ternary

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

// dividere in 2 espressioni ternarie separate
const maybeNull = value1 > value2 ? 'baz' : null;

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

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

15.7 Evitare istruzioni ternarie non necessarie. eslint: no-unneeded-ternary

// male
const foo = a ? a : b;
const bar = c ? true : false;
const baz = c ? false : true;

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

15.8 Quando si mescolano gli operatori, racchiuderli tra parentesi. L'unica eccezione sono gli operatori aritmetici standard: +, -, e ** Poiché la loro precedenza è ampiamente compresa. Si consiglia di racchiudere / e * tra parentesi Perché la loro precedenza può essere ambigua quando sono mescolati. eslint: no-mixed-operators

Perchè? Ciò migliora la leggibilità e chiarisce l'intenzione dello sviluppatore.

// male
const foo = a && b < 0 || c > 0 || d + 1 === 0;

// male
const bar = a ** b - 5 % d;

// male
// ci si potrebbe confondere pensando (a || b) && c
if (a || b && c) {
  return d;
}

// male
const bar = a + b / c * d;

// bene
const foo = (a && b < 0) || c > 0 || (d + 1 === 0);

// bene
const bar = a ** b - (5 % d);

// bene
if (a || (b && c)) {
  return d;
}

// bene
const bar = a + (b / c) * d;

⬆ back to top

Blocks

16.1 Usare parentesi graffe per tutti i blocchi su più righe. eslint: nonblock-statement-body-position

// male
if (test)
  return false;

// bene
if (test) return false;

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

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

// bene
function bar() {
  return false;
}

16.2 Se si usano blocchi su più righe con if ed else, inserire else nella stessa riga della parentesi di chiusura del blocco if. eslint: brace-style

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

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

16.3 Se un blocco if esegue sempre una istruzione return, il seguente blocco else non è necessario. Un return in un blocco else if che segue un blcco if che contiene un return può essere separato in più blocchi if. eslint: no-else-return

// male
function foo() {
  if (x) {
    return x;
  } else {
    return y;
  }
}

// male
function cats() {
  if (x) {
    return x;
  } else if (y) {
    return y;
  }
}

// male
function dogs() {
  if (x) {
    return x;
  } else {
    if (y) {
      return y;
    }
  }
}

// bene
function foo() {
  if (x) {
    return x;
  }

  return y;
}

// bene
function cats() {
  if (x) {
    return x;
  }

  if (y) {
    return y;
  }
}

// bene
function dogs(x) {
  if (x) {
    if (z) {
      return y;
    }
  } else {
    return z;
  }
}

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Istruzioni di Controllo

17.1 Nel caso che l'istruzione di controllo (if, while ecc.) sia troppo lunga e supere la lunghezza massima di riga, ciascuna condizione (raggruppata) potrebbe essere inserita in una nuova riga. L'operatore logico dovrebbe iniziare la riga.

Perchè? Richiedere gli operatori all'inizio della riga mantiene gli operatori allineati e segue uno schema simile al concatenamento dei metodi. Ciò migliora anche la leggibilità rendendo più facile seguire visivamente una logica complessa.

// male
if ((foo === 123 || bar === 'abc') && doesItLookGoodWhenItBecomesThatLong() && isThisReallyHappening()) {
  thing1();
}

// male
if (foo === 123 &&
  bar === 'abc') {
  thing1();
}

// male
if (foo === 123
  && bar === 'abc') {
  thing1();
}

// male
if (
  foo === 123 &&
  bar === 'abc'
) {
  thing1();
}

// bene
if (
  foo === 123
  && bar === 'abc'
) {
  thing1();
}

// bene
if (
  (foo === 123 || bar === 'abc')
  && doesItLookGoodWhenItBecomesThatLong()
  && isThisReallyHappening()
) {
  thing1();
}

// bene
if (foo === 123 && bar === 'abc') {
  thing1();
}

17.2 Non usare operatori di selezione al posto di istruzioni di controllo.

// male
!isRunning && startRunning();

// bene
if (!isRunning) {
  startRunning();
}

⬆ back to top

Comments

18.1 Usare /** ... */ per commenti su più righe.

// male
// make() restituisce un nuovo elemento
// basato sul nome del tag ricevuto
//
// @param {String} tag
// @return {Element} element
function make(tag) {

  // ...

  return element;
}

// bene
/**
 * make() restituisce un nuovo elemento
 * basato sul nome del tag ricevuto
 */
function make(tag) {

  // ...

  return element;
}

18.2 Usare // per commenti su riga singola. Piazzare i commenti su riga singola su una nuova riga sopra l'oggetto del commento. Inserire una riga vuota prime del commento a mno che non sia la prima riga di un blocco.

// male
const active = true;  // tab corrente

// bene
// tab corrente
const active = true;

// male
function getType() {
  console.log('fetching type...');
  // imposta il tipo predefinito a  'no type'
  const type = this.type || 'no type';

  return type;
}

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

  // imposta il tipo predefinito a  'no type'
  const type = this.type || 'no type';

  return type;
}

// also good
function getType() {
  // imposta il tipo predefinito a  'no type'
  const type = this.type || 'no type';

  return type;
}

18.3 Iniziare tutti i commenti con uno spazio per facilitarne la lettura.. eslint: spaced-comment

// male
//tab corrente
const active = true;

// bene
// tab corrente
const active = true;

// male
/**
 *make() ritorna un nuovo elemento
 *in base al nome del tag ricevuto
 */
function make(tag) {

  // ...

  return element;
}

// bene
/**
 * make() ritorna un nuovo elemento
 * in base al nome del tag ricevuto
 */
function make(tag) {

  // ...

  return element;
}

18.4 Prefissare i commenti con FIXME o TODO aiuta gli altri svilupptori a comprendere velocemente che si sta evidenziando un problema che deve essere rivisto, oppure che si sta suggerendo una soluzione a un problema che deve essere implementata. Questi sono diversi dai normali commenti in quanto richiedono una attuazione. Le azioni sono FIXME: -- è da capire o TODO: -- deve essere implementato.

18.5 Usare // FIXME: per annotare problemi.

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

    // FIXME: non si dovrebbe usare un global qui
    total = 0;
  }
}

18.6 Usare // TODO: per annotare soluzioni a problemi.

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

    // TODO: total dovrebbe essere configurabile da un parametro di opzione
    this.total = 0;
  }
}

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Spazi bianchi

19.1 Usare soft tab (carattere spazio) impostato a 2 spazi. eslint: indent

// male
function foo() {
∙∙∙∙let name;
}

// male
function bar() {
∙let name;
}

// bene
function baz() {
∙∙let name;
}

19.2 Inserire 1 spazio prima della parentesi graffa di apertura. eslint: space-before-blocks

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

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

// male
dog.set('attr',{
  age: '1 year',
  breed: 'Bernese Mountain Dog',
});

// bene
dog.set('attr', {
  age: '1 year',
  breed: 'Bernese Mountain Dog',
});

19.3 Inserire 1 spazio prima delle parentesi di apertura nelle istruzioni di controllo (if, while ecc.). Non inserire spazi tra l'elenco degli argomenti e il nome della funzione in chiamate a funzione e dichiarazioni. eslint: keyword-spacing
```
// male
if(isJedi) {
  fight ();
}

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

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

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

19.4 Isolare gli operatori con spazi. eslint: space-infix-ops
```
// male
const x=y+5;

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

19.5 Terminare i file con un singolo carattere di ritorno a capo. eslint: eol-last

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

// male
import { es6 } from './AirbnbStyleGuide';
  // ...
export default es6;↵
↵

// bene
import { es6 } from './AirbnbStyleGuide';
  // ...
export default es6;↵

19.6 Usare indentazione quando si hanno lunghi concatenamenti di metodi (più di 2). Usare un punto iniale, che enfatizza che la riga è una chiamata a un metodo, non una nuova istruzione. eslint: newline-per-chained-call no-whitespace-before-property

// male
$('#items').find('.selected').highlight().end().find('.open').updateCount();

// male
$('#items').
  find('.selected').
    highlight().
    end().
  find('.open').
    updateCount();

// bene
$('#items')
  .find('.selected')
    .highlight()
    .end()
  .find('.open')
    .updateCount();

// male
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);

// bene
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);

// bene
const leds = stage.selectAll('.led').data(data);
const svg = leds.enter().append('svg:svg');
svg.classed('led', true).attr('width', (radius + margin) * 2);
const g = svg.append('svg:g');
g.attr('transform', `translate(${radius + margin},${radius + margin})`).call(tron.led);

19.7 Lasciare una riga vuota dopo ogni blocco e prima dell'istruzione successiva.

// male
if (foo) {
  return bar;
}
return baz;

// bene
if (foo) {
  return bar;
}

return baz;

// male
const obj = {
  foo() {
  },
  bar() {
  },
};
return obj;

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

  bar() {
  },
};

return obj;

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

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

  function bar() {
  },
];

return arr;

19.8 Non inserire righe vuote prima e dopo i blocchi. eslint: padded-blocks

// male
function bar() {

  console.log(foo);

}

// male
if (baz) {

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

}

// male
class Foo {

  constructor(bar) {
    this.bar = bar;
  }
}

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

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

19.9 Non usare più righe vuote per distanziare il proprio codice. eslint: no-multiple-empty-lines

// male
class Person {
  constructor(fullName, email, birthday) {
    this.fullName = fullName;


    this.email = email;


    this.setAge(birthday);
  }


  setAge(birthday) {
    const today = new Date();


    const age = this.getAge(today, birthday);


    this.age = age;
  }


  getAge(today, birthday) {
    // ..
  }
}

// bene
class Person {
  constructor(fullName, email, birthday) {
    this.fullName = fullName;
    this.email = email;
    this.setAge(birthday);
  }

  setAge(birthday) {
    const today = new Date();
    const age = getAge(today, birthday);
    this.age = age;
  }

  getAge(today, birthday) {
    // ..
  }
}

19.10 Non aggiungere spazi all'interno delle parentesi. eslint: space-in-parens

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

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

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

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

19.11 Non aggiungere spazi all'interno di parentesi quadre. eslint: array-bracket-spacing

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

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

19.12 Aggiungere spazi all'interno delle parentesi graffe. eslint: object-curly-spacing

// male
const foo = {clark: 'kent'};

// bene
const foo = { clark: 'kent' };

19.13 Evitare di avere righe di codice che sono più lunghe di 100 caratteri (compresi gli spazi bianchi). Nota: per quanto esposto sopra, le stringhe lunghe sono esentate da questa regola e non dovrebbere essere spezzate. eslint: max-len

Perchè? Ciò assicura la leggibilità e la manutenibilità.

// male
const foo = jsonData && jsonData.foo && jsonData.foo.bar && jsonData.foo.bar.baz && jsonData.foo.bar.baz.quux && jsonData.foo.bar.baz.quux.xyzzy;

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

// bene
const foo = jsonData
  && jsonData.foo
  && jsonData.foo.bar
  && jsonData.foo.bar.baz
  && jsonData.foo.bar.baz.quux
  && jsonData.foo.bar.baz.quux.xyzzy;

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

19.14 Richiedere consistenza di spaziatura all'interno di un token di blocco di apertura e il token successivo sulla stessa riga. Questa regola impone una consistente spaziatura all'interno di un token di blocco di chiusura e il token precedente sulla stessa riga. eslint: block-spacing
```
// male
function foo() {return true;}
if (foo) { bar = 0;}

// bene
function foo() { return true; }
if (foo) { bar = 0; }
```

19.15 Evitare gli spazi prima delle virgole e richiedere uno spazio dopo le virgole. eslint: comma-spacing

// male
var foo = 1,bar = 2;
var arr = [1 , 2];

// bene
var foo = 1, bar = 2;
var arr = [1, 2];

19.16 Imporre spaziatura all'interno di parentesi quadre di proprietà calcolate. eslint: computed-property-spacing

// male
obj[foo ]
obj[ 'foo']
var x = {[ b ]: a}
obj[foo[ bar ]]

// bene
obj[foo]
obj['foo']
var x = { [b]: a }
obj[foo[bar]]

19.17 Evitare spazi tra le funzioni e le loro invocazioni. eslint: func-call-spacing
```
// male
func ();

func
();

// bene
func();
```

19.18 Imporre spaziatura tra chiavi e valori nelle proprietà di oggetti letterali. eslint: key-spacing

// male
var obj = { foo : 42 };
var obj2 = { foo:42 };

// bene
var obj = { foo: 42 };

19.19 Evitare spazi prima della fine delle righe. eslint: no-trailing-spaces

19.20 Evitare più righe vuote, consentire solo una riga vuota alla fine dei file, ed evitare righe vuote all'inizio dei file. eslint: no-multiple-empty-lines

// male - più righe vuote
var x = 1;


var y = 2;

// male - 2+ righe vuote alla fine del file
var x = 1;
var y = 2;


// male - 1+ righe vuote all'inizio del file

var x = 1;
var y = 2;

// bene
var x = 1;
var y = 2;

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VIrgole

20.1 Leading commas: Nope. eslint: comma-style

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

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

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

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

20.2 Additional trailing comma: Yup. eslint: comma-dangle

Perchè? 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.

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

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

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

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

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

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

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

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

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

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

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

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

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Semicolons

21.1 Yup. eslint: semi

Perchè? When JavaScript encounters a line break without a semicolon, it uses a set of rules called Automatic Semicolon Insertion to determine whether it should regard that line break as the end of a statement, and (as the name implies) place a semicolon into your code before the line break if it thinks so. ASI contains a few eccentric behaviors, though, and your code will break if JavaScript misinterprets your line break. These rules will become more complicated as new features become a part of JavaScript. Explicitly terminating your statements and configuring your linter to catch missing semicolons will help prevent you from encountering issues.

// male - raises exception
const luke = {}
const leia = {}
[luke, leia].forEach((jedi) => jedi.father = 'vader')

// male - raises exception
const reaction = "No! That’s impossible!"
(async function meanwhileOnTheFalcon() {
  // handle `leia`, `lando`, `chewie`, `r2`, `c3p0`
  // ...
}())

// male - returns `undefined` instead of the value on the next line - always happens when `return` is on a line by itself because of ASI!
function foo() {
  return
    'search your feelings, you know it to be foo'
}

// bene
const luke = {};
const leia = {};
[luke, leia].forEach((jedi) => {
  jedi.father = 'vader';
});

// bene
const reaction = "No! That’s impossible!";
(async function meanwhileOnTheFalcon() {
  // handle `leia`, `lando`, `chewie`, `r2`, `c3p0`
  // ...
}());

// bene
function foo() {
  return 'search your feelings, you know it to be foo';
}

Type Casting & Coercion

22.1 Perform type coercion at the beginning of the statement.

22.2 Strings: eslint: no-new-wrappers

// => this.reviewScore = 9;

// male
const totalScore = new String(this.reviewScore); // typeof totalScore is "object" not "string"

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

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

// bene
const totalScore = String(this.reviewScore);

22.3 Numbers: Use Number for type casting and parseInt always with a radix for parsing strings. eslint: radix no-new-wrappers

Perchè? The parseInt function produces an integer value dictated by interpretation of the contents of the string argument according to the specified radix. Leading whitespace in string is ignored. If radix is undefined or 0, it is assumed to be 10 except when the number begins with the character pairs 0x or 0X, in which case a radix of 16 is assumed. This differs from ECMAScript 3, which merely discouraged (but allowed) octal interpretation. Many implementations have not adopted this behavior as of 2013. And, because older browsers must be supported, always specify a radix.
```
const inputValue = '4';

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

// male
const val = +inputValue;

// male
const val = inputValue >> 0;

// male
const val = parseInt(inputValue);

// bene
const val = Number(inputValue);

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

22.4 If for whatever reason you are doing something wild and parseInt is your bottleneck and need to use Bitshift for performance reasons, leave a comment explaining why and what you’re doing.
```
// bene
/**
 * 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;
```

22.5 Note: Be careful when using bitshift operations. Numbers are represented as 64-bit values, but bitshift operations always return a 32-bit integer (source). Bitshift can lead to unexpected behavior for integer values larger than 32 bits. Discussion. Largest signed 32-bit Int is 2,147,483,647:
```
2147483647 >> 0; // => 2147483647
2147483648 >> 0; // => -2147483648
2147483649 >> 0; // => -2147483647
```

22.6 Booleans: eslint: no-new-wrappers

const age = 0;

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

// bene
const hasAge = Boolean(age);

// best
const hasAge = !!age;

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Naming Conventions

23.1 Avoid single letter names. Be descriptive with your naming. eslint: id-length

// male
function q() {
  // ...
}

// bene
function query() {
  // ...
}

23.2 Use camelCase when naming objects, functions, and instances. eslint: camelcase

// male
const OBJEcttsssss = {};
const this_is_my_object = {};
function c() {}

// bene
const thisIsMyObject = {};
function thisIsMyFunction() {}

23.3 Use PascalCase only when naming constructors or classes. eslint: new-cap

// male
function user(options) {
  this.name = options.name;
}

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

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

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

23.4 Do not use trailing or leading underscores. eslint: no-underscore-dangle

Perchè? JavaScript does not have the concept of privacy in terms of properties or methods. Although a leading underscore is a common convention to mean “private”, in fact, these properties are fully public, and as such, are part of your public API contract. This convention might lead developers to wrongly think that a change won’t count as breaking, or that tests aren’t needed. tl;dr: if you want something to be “private”, it must not be observably present.
```
// male
this.__firstName__ = 'Panda';
this.firstName_ = 'Panda';
this._firstName = 'Panda';

// bene
this.firstName = 'Panda';

// bene, in environments where WeakMaps are available
// see https://kangax.github.io/compat-table/es6/#test-WeakMap
const firstNames = new WeakMap();
firstNames.set(this, 'Panda');
```

23.5 Don’t save references to this. Use arrow functions or Function#bind.

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

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

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

23.6 A base filename should exactly match the name of its default export.

// file 1 contents
class CheckBox {
  // ...
}
export default CheckBox;

// file 2 contents
export default function fortyTwo() { return 42; }

// file 3 contents
export default function insideDirectory() {}

// in some other file
// male
import CheckBox from './checkBox'; // PascalCase import/export, camelCase filename
import FortyTwo from './FortyTwo'; // PascalCase import/filename, camelCase export
import InsideDirectory from './InsideDirectory'; // PascalCase import/filename, camelCase export

// male
import CheckBox from './check_box'; // PascalCase import/export, snake_case filename
import forty_two from './forty_two'; // snake_case import/filename, camelCase export
import inside_directory from './inside_directory'; // snake_case import, camelCase export
import index from './inside_directory/index'; // requiring the index file explicitly
import insideDirectory from './insideDirectory/index'; // requiring the index file explicitly

// bene
import CheckBox from './CheckBox'; // PascalCase export/import/filename
import fortyTwo from './fortyTwo'; // camelCase export/import/filename
import insideDirectory from './insideDirectory'; // camelCase export/import/directory name/implicit "index"
// ^ supports both insideDirectory.js and insideDirectory/index.js

23.7 Use camelCase when you export-default a function. Your filename should be identical to your function’s name.
```
function makeStyleGuide() {
  // ...
}

export default makeStyleGuide;
```

23.8 Use PascalCase when you export a constructor / class / singleton / function library / bare object.
```
const AirbnbStyleGuide = {
  es6: {
  },
};

export default AirbnbStyleGuide;
```

23.9 Acronyms and initialisms should always be all uppercased, or all lowercased.

Perchè? Names are for readability, not to appease a computer algorithm.

// male
import SmsContainer from './containers/SmsContainer';

// male
const HttpRequests = [
  // ...
];

// bene
import SMSContainer from './containers/SMSContainer';

// bene
const HTTPRequests = [
  // ...
];

// also good
const httpRequests = [
  // ...
];

// best
import TextMessageContainer from './containers/TextMessageContainer';

// best
const requests = [
  // ...
];

23.10 You may optionally uppercase a constant only if it (1) is exported, (2) is a const (it can not be reassigned), and (3) the programmer can trust it (and its nested properties) to never change.

Perchè? This is an additional tool to assist in situations where the programmer would be unsure if a variable might ever change. UPPERCASE_VARIABLES are letting the programmer know that they can trust the variable (and its properties) not to change.
- What about all const variables? - This is unnecessary, so uppercasing should not be used for constants within a file. It should be used for exported constants however.
- What about exported objects? - Uppercase at the top level of export (e.g. EXPORTED_OBJECT.key) and maintain that all nested properties do not change.
```
// male
const PRIVATE_VARIABLE = 'should not be unnecessarily uppercased within a file';

// male
export const THING_TO_BE_CHANGED = 'should obviously not be uppercased';

// male
export let REASSIGNABLE_VARIABLE = 'do not use let with uppercase variables';

// ---

// allowed but does not supply semantic value
export const apiKey = 'SOMEKEY';

// better in most cases
export const API_KEY = 'SOMEKEY';

// ---

// male - unnecessarily uppercases key while adding no semantic value
export const MAPPING = {
  KEY: 'value'
};

// bene
export const MAPPING = {
  key: 'value'
};
```

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Accessors

24.1 Accessor functions for properties are not required.

24.2 Do not use JavaScript getters/setters as they cause unexpected side effects and are harder to test, maintain, and reason about. Instead, if you do make accessor functions, use getVal() and setVal('hello').

// male
class Dragon {
  get age() {
    // ...
  }

  set age(value) {
    // ...
  }
}

// bene
class Dragon {
  getAge() {
    // ...
  }

  setAge(value) {
    // ...
  }
}

24.3 If the property/method is a boolean, use isVal() or hasVal().

// male
if (!dragon.age()) {
  return false;
}

// bene
if (!dragon.hasAge()) {
  return false;
}

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

class Jedi {
  constructor(options = {}) {
    const lightsaber = options.lightsaber || 'blue';
    this.set('lightsaber', lightsaber);
  }

  set(key, val) {
    this[key] = val;
  }

  get(key) {
    return this[key];
  }
}

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Events

25.1 When attaching data payloads to events (whether DOM events or something more proprietary like Backbone events), pass an object literal (also known as 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:
```
// male
$(this).trigger('listingUpdated', listing.id);

// ...

$(this).on('listingUpdated', (e, listingID) => {
  // do something with listingID
});
```
prefer:
```
// bene
$(this).trigger('listingUpdated', { listingID: listing.id });

// ...

$(this).on('listingUpdated', (e, data) => {
  // do something with data.listingID
});
```

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jQuery

26.1 Prefix jQuery object variables with a $.

// male
const sidebar = $('.sidebar');

// bene
const $sidebar = $('.sidebar');

// bene
const $sidebarBtn = $('.sidebar-btn');

26.2 Cache jQuery lookups.

// male
function setSidebar() {
  $('.sidebar').hide();

  // ...

  $('.sidebar').css({
    'background-color': 'pink',
  });
}

// bene
function setSidebar() {
  const $sidebar = $('.sidebar');
  $sidebar.hide();

  // ...

  $sidebar.css({
    'background-color': 'pink',
  });
}

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

26.4 Use find with scoped jQuery object queries.

// male
$('ul', '.sidebar').hide();

// male
$('.sidebar').find('ul').hide();

// bene
$('.sidebar ul').hide();

// bene
$('.sidebar > ul').hide();

// bene
$sidebar.find('ul').hide();

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ECMAScript 5 Compatibility

27.1 Refer to Kangax’s ES5 compatibility table.

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ECMAScript 6+ (ES 2015+) Styles

28.1 This is a collection of links to the various ES6+ features.

Arrow Functions
Classes
Object Shorthand
Object Concise
Object Computed Properties
Template Strings
Destructuring
Default Parameters
Rest
Array Spreads
Let and Const
Exponentiation Operator
Iterators and Generators
Modules

28.2 Do not use TC39 proposals that have not reached stage 3.

Perchè? They are not finalized, and they are subject to change or to be withdrawn entirely. We want to use JavaScript, and proposals are not JavaScript yet.

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Standard Library

The Standard Library contains utilities that are functionally broken but remain for legacy reasons.

29.1 Use Number.isNaN instead of global isNaN. eslint: no-restricted-globals

Perchè? The global isNaN coerces non-numbers to numbers, returning true for anything that coerces to NaN. If this behavior is desired, make it explicit.
```
// male
isNaN('1.2'); // false
isNaN('1.2.3'); // true

// bene
Number.isNaN('1.2.3'); // false
Number.isNaN(Number('1.2.3')); // true
```

29.2 Use Number.isFinite instead of global isFinite. eslint: no-restricted-globals

Perchè? The global isFinite coerces non-numbers to numbers, returning true for anything that coerces to a finite number. If this behavior is desired, make it explicit.
```
// male
isFinite('2e3'); // true

// bene
Number.isFinite('2e3'); // false
Number.isFinite(parseInt('2e3', 10)); // true
```

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Testing

30.1 Yup.
```
function foo() {
  return true;
}
```

30.2 No, but seriously:
- Whichever testing framework you use, you should be writing tests!
- Strive to write many small pure functions, and minimize where mutations occur.
- Be cautious about stubs and mocks - they can make your tests more brittle.
- We primarily use mocha and jest at Airbnb. tape is also used occasionally for small, separate modules.
- 100% test coverage is a good goal to strive for, even if it’s not always practical to reach it.
- Whenever you fix a bug, write a regression test. A bug fixed without a regression test is almost certainly going to break again in the future.

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Performance

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Resources

Learning ES6+

Read This

Standard ECMA-262

Tools

Code Style Linters
- ESlint - Airbnb Style .eslintrc
- JSHint - Airbnb Style .jshintrc
Neutrino Preset - @neutrinojs/airbnb

Other Style Guides

Other Styles

Naming this in nested functions - Christian Johansen
Conditional Callbacks - Ross Allen
Popular JavaScript Coding Conventions on GitHub - JeongHoon Byun
Multiple var statements in JavaScript, not superfluous - Ben Alman

Further Reading

Understanding JavaScript Closures - Angus Croll
Basic JavaScript for the impatient programmer - Dr. Axel Rauschmayer
You Might Not Need jQuery - Zack Bloom & Adam Schwartz
ES6 Features - Luke Hoban
Frontend Guidelines - Benjamin De Cock

Books

JavaScript: The Good Parts - Douglas Crockford
JavaScript Patterns - Stoyan Stefanov
Pro JavaScript Design Patterns - Ross Harmes and Dustin Diaz
High Performance Web Sites: Essential Knowledge for Front-End Engineers - Steve Souders
Maintainable JavaScript - Nicholas C. Zakas
JavaScript Web Applications - Alex MacCaw
Pro JavaScript Techniques - John Resig
Smashing Node.js: JavaScript Everywhere - Guillermo Rauch
Secrets of the JavaScript Ninja - John Resig and Bear Bibeault
Human JavaScript - Henrik Joreteg
Superhero.js - Kim Joar Bekkelund, Mads Mobæk, & Olav Bjorkoy
JSBooks - Julien Bouquillon
Third Party JavaScript - Ben Vinegar and Anton Kovalyov
Effective JavaScript: 68 Specific Ways to Harness the Power of JavaScript - David Herman
Eloquent JavaScript - Marijn Haverbeke
You Don’t Know JS: ES6 & Beyond - Kyle Simpson

Blogs

Podcasts

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In the Wild

This is a list of organizations that are using this style guide. Send us a pull request and we'll add you to the list.

123erfasst: 123erfasst/javascript
4Catalyzer: 4Catalyzer/javascript
Aan Zee: AanZee/javascript
Airbnb: airbnb/javascript
AloPeyk: AloPeyk
AltSchool: AltSchool/javascript
Apartmint: apartmint/javascript
Ascribe: ascribe/javascript
Avant: avantcredit/javascript
Axept: axept/javascript
Billabong: billabong/javascript
Bisk: bisk
Bonhomme: bonhommeparis/javascript
Brainshark: brainshark/javascript
CaseNine: CaseNine/javascript
Cerner: Cerner
Chartboost: ChartBoost/javascript-style-guide
Coeur d'Alene Tribe: www.cdatribe-nsn.gov
ComparaOnline: comparaonline/javascript
Compass Learning: compasslearning/javascript-style-guide
DailyMotion: dailymotion/javascript
DoSomething: DoSomething/eslint-config
Digitpaint digitpaint/javascript
Drupal: www.drupal.org
Ecosia: ecosia/javascript
Evernote: evernote/javascript-style-guide
Evolution Gaming: evolution-gaming/javascript
EvozonJs: evozonjs/javascript
ExactTarget: ExactTarget/javascript
Flexberry: Flexberry/javascript-style-guide
Gawker Media: gawkermedia
General Electric: GeneralElectric/javascript
Generation Tux: GenerationTux/javascript
GoodData: gooddata/gdc-js-style
GreenChef: greenchef/javascript
Grooveshark: grooveshark/javascript
Grupo-Abraxas: Grupo-Abraxas/javascript
Happeo: happeo/javascript
Honey: honeyscience/javascript
How About We: howaboutwe/javascript
HubSpot: HubSpot/javascript
Hyper: hyperoslo/javascript-playbook
InterCity Group: intercitygroup/javascript-style-guide
Jam3: Jam3/Javascript-Code-Conventions
JSSolutions: JSSolutions/javascript
Kaplan Komputing: kaplankomputing/javascript
KickorStick: kickorstick
Kinetica Solutions: kinetica/javascript
LEINWAND: LEINWAND/javascript
Lonely Planet: lonelyplanet/javascript
M2GEN: M2GEN/javascript
Mighty Spring: mightyspring/javascript
MinnPost: MinnPost/javascript
MitocGroup: MitocGroup/javascript
Muber: muber
National Geographic: natgeo
NullDev: NullDevCo/JavaScript-Styleguide
Nulogy: nulogy/javascript
Orange Hill Development: orangehill/javascript
Orion Health: orionhealth/javascript
OutBoxSoft: OutBoxSoft/javascript
Peerby: Peerby/javascript
Pier 1: Pier1/javascript
Qotto: Qotto/javascript-style-guide
React: facebook.github.io/react/contributing/how-to-contribute.html#style-guide
REI: reidev/js-style-guide
Ripple: ripple/javascript-style-guide
Sainsbury’s Supermarkets: jsainsburyplc
Shutterfly: shutterfly/javascript
Sourcetoad: sourcetoad/javascript
Springload: springload
StratoDem Analytics: stratodem/javascript
SteelKiwi Development: steelkiwi/javascript
StudentSphere: studentsphere/javascript
SwoopApp: swoopapp/javascript
SysGarage: sysgarage/javascript-style-guide
Syzygy Warsaw: syzygypl/javascript
Target: target/javascript
Terra: terra
TheLadders: TheLadders/javascript
The Nerdery: thenerdery/javascript-standards
Tomify: tomprats
Traitify: traitify/eslint-config-traitify
T4R Technology: T4R-Technology/javascript
UrbanSim: urbansim
VoxFeed: VoxFeed/javascript-style-guide
WeBox Studio: weboxstudio/javascript
Weggo: Weggo/javascript
Zillow: zillow/javascript
ZocDoc: ZocDoc/javascript

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Translation

This style guide is also available in other languages:

Brazilian Portuguese: armoucar/javascript-style-guide
Bulgarian: borislavvv/javascript
Catalan: fpmweb/javascript-style-guide
Chinese (Simplified): lin-123/javascript
Chinese (Traditional): jigsawye/javascript
French: nmussy/javascript-style-guide
German: timofurrer/javascript-style-guide
Italian: sinkswim/javascript-style-guide
Japanese: mitsuruog/javascript-style-guide
Korean: ParkSB/javascript-style-guide
Russian: leonidlebedev/javascript-airbnb
Spanish: paolocarrasco/javascript-style-guide
Thai: lvarayut/javascript-style-guide
Turkish: eraycetinay/javascript
Ukrainian: ivanzusko/javascript
Vietnam: dangkyokhoang/javascript-style-guide

The JavaScript Style Guide Guide

Reference

Chat With Us About JavaScript

Find us on gitter.

Contributors

View Contributors

License

(The MIT License)

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the 'Software'), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED 'AS IS', WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

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Amendments

We encourage you to fork this guide and change the rules to fit your team’s style guide. Below, you may list some amendments to the style guide. This allows you to periodically update your style guide without having to deal with merge conflicts.

Airbnb JavaScript Style Guide() {

Table of Contents

Types

References

Objects

Arrays

Destructuring

Strings

Functions

Arrow Functions

Classes & Constructors

Modules

Iterators and Generators

Properties

Variables

Hoisting

Operatori di Confronto e Uguaglianza

Blocks

Istruzioni di Controllo

Comments

Spazi bianchi

VIrgole

Semicolons

Type Casting & Coercion

Naming Conventions

Accessors

Events

jQuery

ECMAScript 5 Compatibility

ECMAScript 6+ (ES 2015+) Styles

Standard Library

Testing

Performance

Resources

In the Wild

Translation

The JavaScript Style Guide Guide

Chat With Us About JavaScript

Contributors

License

Amendments

};

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