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Javascript Style Guide

A guide for writing consistent and aesthetically pleasing JavaScript.

Inspired by other popular styles such as eslint:recommended, and Airbnb.

Formatting​

Tabs for Indentation​

Use tabs for indenting your code. With tabs users can choose their desired width. This has positive implications for accessibility and screenreaders, putting people in control of how they want to view the code.

Newlines​

Use UNIX-style newlines (\n), and a newline character as the last character of a file. Windows-style newlines (\r\n) are forbidden inside any repository.

No trailing whitespace​

Always clean up any trailing whitespace in your .js files before committing.

Use semicolons​

According to scientific research, the usage of semicolons is a core value of our community. Consider the points of the opposition, but be a traditionalist when it comes to abusing error correction mechanisms for cheap syntactic pleasures.

100 characters per line​

Limit your lines to 100 characters.

Use single quotes​

Use single quotes, unless you are writing JSON. This helps you separate your objects' strings from normal strings.

// bad 👎
var foo = "bad";

// good 👍
var foo = 'bar';

Opening braces go on the same line, Your opening braces go on the same line as the statement.

// bad 👎
if (true)
{
    console.log('losing');
}

// good 👍
if (true) {
    console.log('winning');
}

Also, notice the use of whitespace before and after the condition statement. What if you want to write 'else' or 'else if' along with your 'if'...

// bad 👎
if (true)
{
    console.log('losing');
}
else if (false)
{
    console.log('this is bad');
}
else
{
    console.log('not good');
}

// good 👍
if (true) {
    console.log('winning');
} else if (false) {
    console.log('this is good');
} else {
    console.log('finally');
}

Declare one variable per var statement​

Declare one variable per var statement, it makes it easier to re-order the lines.

// bad 👎
var keys = ['foo', 'bar'],
values = [23, 42],
object = {},
key;

// good 👍
var keys = ['foo', 'bar'];
var values = [23, 42];
var object = {};

Whitespace​

Use tabs​

eslint: indent

// bad 👎
function foo() {
∙∙let name;
}

// bad 👎
function bar() {
∙let name;
}

// good 👍
function baz() {
∙ let name;
}

Space Before Blocks​

Place 1 space before the leading brace.

eslint: space-before-blocks

// 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',
});

Keyword Spacing​

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

// bad 👎
if(isJedi) {
    fight ();
}

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

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

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

Space Around Infix Operators​

Set off operators with spaces.

eslint: space-infix-ops

// bad 👎
const x=y+5;

// good 👍
const x = y + 5;

End of File​

End files with a single newline character.

eslint: eol-last

// bad 👎
import { es6 } from './AirbnbStyleGuide';
    // ...
export default es6;
// bad 👎
import { es6 } from './AirbnbStyleGuide';
    // ...
export default es6;↵
↵
// good 👍
import { es6 } from './AirbnbStyleGuide';
    // ...
export default es6;↵

Chained Calls​

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 eslint: no-whitespace-before-property

// 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);

Padding​

Blank Lines​

Leave a blank line after blocks and before the next statement.

// 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;

Padded Blocks​

Do not pad your blocks with blank lines.

eslint: padded-blocks

// bad 👎
function bar() {

  console.log(foo);

}

// bad 👎
if (baz) {

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

}

// bad 👎
class Foo {

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

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

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

No Multiple Blank Lines​

Do not use multiple blank lines to pad your code.

eslint: no-multiple-empty-lines

// bad 👎
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) {
    // ..
  }
}

// good 👍
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) {
    // ..
  }
}

Spacing​

Parentheses Spacing​

Do not add spaces inside parentheses.

eslint: space-in-parens

// bad 👎
function bar( foo ) {
  return foo;
}

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

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

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

Inside Brackets​

Do not add spaces inside brackets.

eslint: array-bracket-spacing

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

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

Inside Curly Braces​

Add spaces inside curly braces.

eslint: object-curly-spacing

// bad 👎
const foo = {clark: 'kent'};

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

Max Length​

Avoid having lines of code that are longer than 100 characters (including whitespace). Note: long strings are exempt from this rule, and should not be broken up.

eslint: max-len

Why? This ensures readability and maintainability.

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

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

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

Block Spacing​

Require consistent spacing inside an open block token and the next token on the same line. This rule also enforces consistent spacing inside a close block token and previous token on the same line.

eslint: block-spacing

// bad 👎
function foo() {return true;}
if (foo) { bar = 0;}

// good 👍
function foo() { return true; }
if (foo) { bar = 0; }

Comma Spacing​

Avoid spaces before commas and require a space after commas.

eslint: comma-spacing

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

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

Computed Property Spacing​

Enforce spacing inside of computed property brackets.

eslint: computed-property-spacing

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

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

Function Spacing​

Avoid spaces between functions and their invocations.

eslint: func-call-spacing

// bad 👎
func ();

func
();

// good 👍
func();

Key Spacing​

Enforce spacing between keys and values in object literal properties.

eslint: key-spacing

// bad 👎
var obj = { foo : 42 };
var obj2 = { foo:42 };

// good 👍
var obj = { foo: 42 };

Trailing Spaces​

Avoid trailing spaces at the end of lines.

eslint: no-trailing-spaces

Multiple Empty Lines​

Avoid multiple empty lines, only allow one newline at the end of files, and avoid a newline at the beginning of files.

eslint: no-multiple-empty-lines

// bad 👎 - multiple empty lines
var x = 1;


var y = 2;

// bad 👎 - 2+ newlines at end of file
var x = 1;
var y = 2;


// bad 👎 - 1+ newline(s) at beginning of file

var x = 1;
var y = 2;

// good 👍
var x = 1;
var y = 2;

Types​

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.

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

References​

Use const for all of your references:​

avoid using var. eslint: prefer-const, no-const-assign

IMPORTANCE: This ensures that you can't reassign your references, which can lead to bugs and difficult to comprehend code.

// bad 👎
var a = 1;
var b = 2;

// good 👍
const a = 1;
const b = 2;

Use let instead of var.​

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

IMPORTANCE: let is block-scoped rather than function-scoped like var.

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

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

Block Scoped​

Note that both let and const are block-scoped.

// 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

Objects​

No New Object​

Use the literal syntax for object creation.

eslint: no-new-object

// bad 👎
const item = new Object();

// good 👍
const item = {};

Computed Property Names​

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

IMPORTANCE: They allow you to define all the properties of an object in one place.

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

// bad 👎
const obj = {
  id: 5,
  name: 'Lorem Ipsum',
};
obj[getKey('enabled')] = true;

// good 👍
const obj = {
  id: 5,
  name: 'Lorem Ipsum',
  [getKey('enabled')]: true,
};

Object Method Shorthand​

Use object method shorthand.

eslint: object-shorthand

// bad 👎
const atom = {
  value: 1,

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

// good 👍
const atom = {
  value: 1,

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

Property Value Shorthand​

Use property value shorthand.

eslint: object-shorthand

IMPORTANCE: It is shorter and descriptive.

const lukeSkywalker = 'Lorem Ipsum';

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

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

Shorthand Property Grouping​

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

IMPORTANCE: It's easier to tell which properties are using the shorthand.

const newyorkSubway = 'Newyork Subway';
const newjerseyTransit = 'NewJersy Transit';

// bad 👎
const obj = {
  episodeOne: 1,
  twoJediWalkIntoACantina: 2,
  newjerseyTransit,
  episodeThree: 3,
  mayTheFourth: 4,
  newyorkSubway,
};

// good 👍
const obj = {
  newjerseyTransit,
  newyorkSubway,
  episodeOne: 1,
  twoJediWalkIntoACantina: 2,
  episodeThree: 3,
  mayTheFourth: 4,
};

Quote Props​

Only quote properties that are invalid identifiers.

eslint: quote-props

IMPORTANCE: 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,
};

Object.prototype Methods​

Do not call Object.prototype methods directly,

such as hasOwnProperty, propertyIsEnumerable, and isPrototypeOf. eslint: no-prototype-builtins

IMPORTANCE: 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.
console.log(has.call(object, key));
/* or */
import has from 'has'; // https://www.npmjs.com/package/has
console.log(has(object, key));

Object Spread Operator​

Prefer the object spread operator.

over Object.assign to shallow-copy objects. Use the object rest operator to get a new object with certain properties omitted.

// very bad 👎👎
const original = { a: 1, b: 2 };
const copy = Object.assign(original, { c: 3 }); // this mutates `original` ಠ_à²
delete copy.a; // so does this

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

// good 👍
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 }

Arrays​

No Array Constructor​

Use the literal syntax for array creation.

eslint: no-array-constructor

// bad 👎
const items = new Array();

// good 👍
const items = [];

Use Arraypush​

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

const someStack = [];

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

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

Array Spreads​

Use array spreads ... to copy arrays.

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

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

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

Use Spreads for Iterable Object​

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

Array.from.

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

// good 👍
const nodes = Array.from(foo);

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

Using Array.from​

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

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

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

// good 👍
const arr = Array.from(arrLike);

Mapping over Iterables​

Use Array.from instead of spread ... for mapping over iterables, because it avoids creating an intermediate array.

// bad 👎
const baz = [...foo].map(bar);

// good 👍
const baz = Array.from(foo, bar);

Array Callback Return​

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

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

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

// bad 👎 - 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);
});

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

Line Breaks for Array Brackets​

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

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

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

const numberInArray = [
  1, 2
];

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

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

const numberInArray = [
  1,
  2
];

Destructuring​

Use Object Destructuring​

Use object destructuring when accessing and using multiple properties of an object.

eslint: prefer-destructuring

IMPORTANCE: Destructuring saves you from creating temporary references for those properties.

// 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}`;
}

Use Array Destructuring​

eslint: prefer-destructuring

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

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

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

Multiple Return Values​

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

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

// 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);

Strings​

Single Quotes​

Use single quotes '' for strings.

eslint: quotes

// bad 👎
const name = 'Capt. Janeway';

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

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

Broken Strings​

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

IMPORTANCE: Broken strings are painful to work with and make code less searchable.

// 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.';

Prefer Template Strings​

When programmatically building up strings, use template strings instead of concatenation.

eslint: prefer-template template-curly-spacing

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

// 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}?`;
}

No Eval​

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

eslint: no-eval

No Useless Escapes​

Do not unnecessarily escape characters in strings.

eslint: no-useless-escape

IMPORTANCE: Backslashes harm readability, thus they should only be present when necessary.

// bad 👎
const foo = ''this' is \'quoted\'';

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

Functions​

Function Style​

Use named function expressions instead of function declarations.

eslint: func-style

IMPORTANCE: 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.

// bad 👎
function foo() {
  // ...
}

// bad 👎
const foo = function () {
  // ...
};

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

Wrap Immediately Invoked Functions​

Wrap immediately invoked function expressions in parentheses.

eslint: wrap-iife

IMPORTANCE: 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.');
})();

No Loop Functions​

Never declare a function in a non-function block

(if, while, etc). Assign the function to a variable instead.

eslint: no-loop-func

A function declaration is not a statement.​

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

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

Parameters and Arguments​

arguments​

Never name a parameter arguments. This will take precedence over the arguments object that is given to every function scope.

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

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

Prefer Rest Params​

Never use arguments, opt to use rest syntax ... instead.

eslint: prefer-rest-params

IMPORTANCE: ... is explicit about which arguments you want pulled.

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

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

Default Parameter Syntax​

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 = {};
  }
  // ...
}

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

Parameter Side Effects​

Avoid side effects with default parameters.

IMPORTANCE: They are confusing to reason about.

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

Default Parameters Last​

Always put default parameters last.

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

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

Never Mutate Parameters.​

eslint: no-param-reassign

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

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

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

Never Reassign Parameters.​

eslint: no-param-reassign

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

// 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) {
  // ...
}

No New Function​

Never use the Function constructor to create a new function.

eslint: no-new-func

IMPORTANCE: Creating a function in this way evaluates a string similarly to eval(), which opens vulnerabilities.

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

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

Function Spacing​

Spacing in a function signature.

eslint: space-before-function-paren space-before-blocks

IMPORTANCE: Consistency is good, and you shouldn't have to add or remove a space when adding or removing a name.

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

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

Prefer Spread​

Prefer the use of the spread operator ... to call variadic functions.

eslint: prefer-spread

IMPORTANCE: It's cleaner, you don't need to supply a context, and you can not easily compose new with apply.

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

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

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

// good 👍
new Date(...[2016, 8, 5]);

Function Parentheses Newline​

Functions with multiline signatures, or invocations, should be indented just like every other multiline.

eslint: function-paren-newline

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

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

// bad 👎
console.log(foo,
  bar,
  baz);

// good 👍
console.log(
  foo,
  bar,
  baz,
);

Arrow Functions​

Prefer Arrow Callback​

When you must use an anonymous function (as when passing an inline callback), use arrow function notation.

eslint: prefer-arrow-callback, arrow-spacing

IMPORTANCE: 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.

// 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;
});

Arrow Styling​

If the function body consists of a single statement, omit the braces and use the implicit return. Otherwise, keep the braces and use a return statement.

eslint: arrow-parens, arrow-body-style

IMPORTANCE: It reads well when multiple functions are chained together.

// 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 + 1}.`);

// 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,
}));

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

let bool = false;

// bad 👎
foo(() => (bool = true));

// good 👍
foo(() => {
  bool = true;
});

Arrow Readability​

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

IMPORTANCE: It shows clearly where the function starts and ends.

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

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

Arrow Parentheses​

Always include parentheses around arguments for clarity and consistency.

eslint: arrow-parens

IMPORTANCE: Minimizes diff churn when adding or removing arguments.

// bad 👎
[1, 2, 3].map(x => x * x);

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

// bad 👎
[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!`
));

// good 👍
[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!`
));

// 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;
});

No Confusing Arrow​

Avoid confusing arrow function syntax (=>) with comparison operators (<=, >=).

eslint: no-confusing-arrow

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

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

Implicit Arrow Linebreak​

Enforce the location of arrow function bodies with implicit returns.

eslint: implicit-arrow-linebreak

// bad 👎
(foo) =>
  bar;

(foo) =>
  (bar);

// good 👍
(foo) => bar;
(foo) => (bar);
(foo) => (
  bar
)

Classes & Constructors​

Class​

Always use class.

IMPORTANCE: class syntax is more concise and easier to reason about.

// 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;
  }
}

Extends​

Use extends for inheritance.

IMPORTANCE: It is a built-in way to inherit prototype functionality without breaking instanceof.

// 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];
  }
}

Method Chaining​

Methods can return this to help with method chaining.

// 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);

toString()​

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()}`;
  }
}

No Useless Constructor​

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

// 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';
  }
}

No Duplicate Class Members​

Avoid duplicate class members.

eslint: no-dupe-class-members

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

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

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

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

Class Methods Use this​

Class methods should use this or be made into a static method unless an external library or framework requires to use 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

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

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

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

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

Modules​

Import/Export​

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.

// 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;

Wildcards​

Do not use wildcard imports.

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

// bad 👎
import * as AirbnbStyleGuide from './AirbnbStyleGuide';

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

Export From Import​

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.

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

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

No Duplicate Imports​

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.

// 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';

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.

// bad 👎
let foo = 3;
export { foo };

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

Named and Default Export​

There are two different types of export, named and default. You can have multiple named exports per module but only one default export.

In JS modules with a single export, prefer default export over named export.

Note: this is inverted in Typescript, where we prefer named exports and avoid default export.

eslint: import/prefer-default-export

Why? To encourage more files that only ever export one thing, which is better for readability and maintainability. 

// okay 
export default function foo() {}

// good 👍
export function foo() {}

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

Why? Since imports are hoisted, keeping them all at the top prevents surprising behavior.

// bad 👎
import foo from 'foo';
foo.init();

import bar from 'bar';

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

foo.init();

Object Curly Newline​

Multiline imports should be indented just like multiline array and object literals.

eslint: object-curly-newline

Why? The curly braces follow the same indentation rules as every other curly brace block in the style guide, as do the trailing commas.

// bad 👎
import {longNameA, longNameB, longNameC, longNameD, longNameE} from 'path';

// good 👍
import {
    longNameA,
    longNameB,
    longNameC,
    longNameD,
    longNameE,
} from 'path';

No webpack Loader Syntax​

Disallow Webpack loader syntax in module import statements.

eslint: import/no-webpack-loader-syntax

Why? Since using Webpack syntax in the imports couples the code to a module bundler. Prefer using the loader syntax in webpack.config.js.

// bad 👎
import fooSass from 'css!sass!foo.scss';
import barCss from 'style!css!bar.css';

// good 👍
import fooSass from 'foo.scss';
import barCss from 'bar.css';

No JS Filename Extensions​

Do not include JavaScript filename extensions

eslint: import/extensions

Why? Including extensions inhibits refactoring, and inappropriately hardcodes implementation details of the module you're importing in every consumer.

// bad 👎
import foo from './foo.js';
import bar from './bar.jsx';
import baz from './baz/index.jsx';

// good 👍
import foo from './foo';
import bar from './bar';
import baz from './baz';

Iterators and Generators​

Iterators​

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

IMPORTANCE: 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];

// 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;

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

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

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

Generators​

Don’t Use Generators For Now

Why? They don’t transpile well to ES5.

Generator Spacing​

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.

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

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

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

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

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

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

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

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

// good 👍
function* foo() {
    // ...
}

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

Properties​

Dot Notation​

Use dot notation when accessing properties.

eslint: dot-notation

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

// bad 👎
const isJedi = luke['jedi'];

// good 👍
const isJedi = luke.jedi;

Bracket Notation​

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');

No Restricted Properties​

Use exponentiation operator ** when calculating exponentiations.

eslint: no-restricted-properties.

// bad 👎
const binary = Math.pow(2, 10);

// good 👍
const binary = 2 ** 10;

Variables​

Use const and let​

Always use const or let to declare variables.

'var' is a global variables. We want to avoid using the global namespace. eslint: no-undef prefer-const

// bad 👎
superPower = new SuperPower();

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

One Variable​

Use one const or let declaration per variable or assignment.

eslint: one-var

IMPORTANCE: 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.

// 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';

Grouping Variables​

Group all your const and then group all your let.

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

// 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;

Reasonably Placed Variables​

Assign variables where you need them, but place them in a reasonable place.

let and const are block scoped and not function scoped.

// 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;
}

No Multi Assign​

Don't chain variable assignments.

eslint: no-multi-assign

Why? Chaining variable assignments creates implicit global variables.

// bad 👎
(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

// good 👍
(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`

No Unary Increments​

Avoid using unary increments and decrements (++, --).

eslint no-plusplus

Why? 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.

// bad 👎
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++;
  }
}

// good 👍
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;

Operator Linebreak​

Avoid linebreaks before or after = in an assignment.

If your assignment violates max-len, surround the value in parens. eslint operator-linebreak.

Linebreaks surrounding = can obfuscate the value of an assignment.

// bad 👎
const foo =
  superLongLongLongLongLongLongLongLongFunctionName();

// bad 👎
const foo
  = 'superLongLongLongLongLongLongLongLongString';

// good 👍
const foo = (
  superLongLongLongLongLongLongLongLongFunctionName();
);

// good 👍
const foo = 'superLongLongLongLongLongLongLongLongString';

No Unused Variables​

Disallow unused variables.

eslint: no-unused-vars

Why? 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.

// bad 👎
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;
}

// good 👍
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.

Object / Array creation​

Use trailing commas and put short declarations on a single line.

// bad 👎
var a = [
  'hello', 'world'
];
var b = {'good': 'code'
        , is generally: 'pretty'
        };

// good 👍
var a = ['hello', 'world'];
var b = {
  good: 'code',
  'is generally': 'pretty',
};

Hoisting​

var​

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;
}

Anonymous Function Expressions​

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');
  };
}

Named Function Expressions​

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');
  };
}

Function Declarations​

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.

Comparison Operators & Equality​

Equals​

Must use === and !== over == and !=.

eslint: eqeqeq

if​

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
if ([0] && []) {
  // true
  // an array (even an empty one) is an object, objects will evaluate to true
}

Booleans, Strings and Numbers​

Use shortcuts for booleans, but explicit comparisons for strings and numbers.

// bad 👎
if (isValid === true) {
  // ...
}

// good 👍
if (isValid) {
  // ...
}

// bad 👎
if (name) {
  // ...
}

// good 👍
if (name !== '') {
  // ...
}

// bad 👎
if (collection.length) {
  // ...
}

// good 👍
if (collection.length > 0) {
  // ...
}

More Info​

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

No Case Declarations​

Use braces to create blocks in case and default clauses that contain lexical declarations (e.g. let, const, function, and class). eslint: no-case-declarations

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.

// 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 {}
  }
}

Ternaries​

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

eslint: no-nested-ternary

// bad 👎
const foo = maybe1 > maybe2
  ? 'bar'
  : value1 > value2 ? 'baz' : null;

// split into 2 separated ternary expressions
const maybeNull = value1 > value2 ? 'baz' : null;

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

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

Unnecessary Ternaries​

Avoid unneeded ternary statements.

eslint: no-unneeded-ternary

// 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;

Mixing Operators​

When mixing operators, enclose them in parentheses.

The only exception is the standard arithmetic operators: +, -, and ** since their precedence is broadly understood. We recommend enclosing / and * in parentheses because their precedence can be ambiguous when they are mixed. eslint: no-mixed-operators

IMPORTANCE: This improves readability and clarifies the developer's intention.

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

// bad 👎
const bar = a ** b - 5 % d;

// bad 👎
// one may be confused into thinking (a || b) && c
if (a || b && c) {
  return d;
}

// bad 👎
const bar = a + b / c * d;

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

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

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

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

Blocks​

Multiline Blocks​

Use braces with all multiline blocks.

eslint: nonblock-statement-body-position

// 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;
}

Brace Style​

If you're using multiline blocks with if and else, put else on the same line. as your if block's closing brace.

eslint: brace-style

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

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

No Else Return​

If an if block always executes a return statement, the subsequent else block is unnecessary.

A return in an else if block following an if block that contains a return can be separated into multiple if blocks. eslint: no-else-return

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

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

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

// good 👍
function foo() {
  if (x) {
    return x;
  }

  return y;
}

// good 👍
function cats() {
  if (x) {
    return x;
  }

  if (y) {
    return y;
  }
}

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

Control Statements​

New Line Condition​

In case your control statement (if, while etc.) gets too long or exceeds the maximum line length, each (grouped) condition could be put into a new line. The logical operator should begin the line.

IMPORTANCE: This also improves readability by making it easier to visually follow complex logic.

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

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

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

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

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

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

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

Selection Operators​

Don't use selection operators in place of control statements.

// bad 👎
!isRunning && startRunning();

// good 👍
if (!isRunning) {
  startRunning();
}

Comments​

Multiline Comments​

Use /** ... */ for multiline comments.

// bad 👎
// make() returns a new element
// based on the passed in tag name
//
// @param {String} tag
// @return {Element} element
function make(tag) {
  // ...

  return element;
}

// good 👍
/**
 * make() returns a new element
 * based on the passed-in tag name
 */
function make(tag) {
  // ...

  return element;
}

Single Line Comments​

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.

// 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;
}

Spaced Comment​

Start all comments with a space to make it easier to read.

eslint: spaced-comment

// bad 👎
//is current tab
const active = true;

// good 👍
// is current tab
const active = true;

// bad 👎
/**
 *make() returns a new element
 *based on the passed-in tag name
 */
function make(tag) {
  // ...

  return element;
}

// good 👍
/**
 * make() returns a new element
 * based on the passed-in tag name
 */
function make(tag) {
  // ...

  return element;
}

FIXME or TODO​

Prefixing your comments with FIXME or TODO helps other developers quickly understand.

Use // FIXME: to annotate problems.​

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

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

Use // TODO: to annotate solutions to problems.​

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

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

Commas​

Leading commas: Nope.​

eslint: comma-style

// 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',
};

Additional trailing comma: Yup.​

eslint: comma-dangle

Why? This leads to cleaner git diffs. Also, transpilers like Babel will remove the additional trailing comma in the transpiled code which means you don’t have to worry about the trailing comma problem in legacy browsers.

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

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

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

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

// bad 👎
createHero(
  firstName,
  lastName,
  inventorOf
);

// good 👍
createHero(
  firstName,
  lastName,
  inventorOf,
);

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

Semicolons​

Yup.​

eslint: semi

Why? When JavaScript encounters a line break without a semicolon, it uses a set of rules called Automatic Semicolon Insertion to determine whether or not 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.

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

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

// bad 👎 - 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'
}

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

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

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

Type Casting & Coercion​

Type Coercion at Start​

Perform type coercion at the beginning of the statement.

Strings​

eslint: no-new-wrappers

// => this.reviewScore = 9;

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

// 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);

Numbers​

Use Number for type casting and parseInt always with a radix for parsing strings.

eslint: radix no-new-wrappers

const inputValue = '4';

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

// bad 👎
const val = +inputValue;

// bad 👎
const val = inputValue >> 0;

// bad 👎
const val = parseInt(inputValue);

// good 👍
const val = Number(inputValue);

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

parseInt​

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.

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

Bitshift Operations​

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

Booleans​

eslint: no-new-wrappers

const age = 0;

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

// good 👍
const hasAge = Boolean(age);

// best
const hasAge = !!age;

Naming Conventions​

Naming Length​

Avoid single letter names. Be descriptive with your naming.

eslint: id-length

// bad 👎
function q() {
    // ...
}

// good 👍
function query() {
    // ...
}

camelCase​

Use camelCase when naming objects, functions, and instances.

eslint: camelcase

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

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

camelCase Export Usage​

Use camelCase when you export-default a function. Your filename should be identical to your function’s name.

function makeStyleGuide() {
    // ...
}

export default makeStyleGuide;

More camelCase Usage​

Use camelCase for variables, properties and function names

Variables, properties and function names should use camelCase. They should also be descriptive. Single character variables and uncommon abbreviations should generally be avoided.

// bad 👎
var admin_user = db.query('SELECT * FROM users ...');

// good 👍
var adminUser = db.query('SELECT * FROM users ...');

PascalCase​

Use PascalCase only when naming constructors or classes.

eslint: new-cap

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

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

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

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

PascalCase Export Usage​

Use PascalCase when you export a constructor / class / singleton / function library / bare object.

const AirbnbStyleGuide = {
  es6: {
  },
};

export default AirbnbStyleGuide;

No Underscore Dangle​

Do not use trailing or leading underscores.

eslint: no-underscore-dangle

Why? 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.

// bad 👎
this.__firstName__ = 'Panda';
this.firstName_ = 'Panda';
this._firstName = 'Panda';

// good 👍
this.firstName = 'Panda';

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

No this References​

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

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

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

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

Filename Export Matching​

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
// bad 👎
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

// bad 👎
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

// good 👍
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

Acronyms and Initialisms​

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

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

// bad 👎
import SmsContainer from './containers/SmsContainer';

// bad 👎
const HttpRequests = [
    // ...
];

// good 👍
import SMSContainer from './containers/SMSContainer';

// good 👍
const HTTPRequests = [
    // ...
];

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

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

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

Optional UPPERCASE​

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.

Why? 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.
// bad 👎
const PRIVATE_VARIABLE = 'should not be unnecessarily uppercased within a file';

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

// bad 👎
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';

// ---

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

// good 👍
export const MAPPING = {
    key: 'value'
};

Use UPPERCASE for Constants​

Constants should be declared as regular variables or static class properties, using all uppercase letters.

// bad 👎
const second = 1 * 1000;
function File() {
}
File.fullPermissions = 0777;

// good 👍
var SECOND = 1 * 1000;
function File() {
}
File.FULL_PERMISSIONS = 0777;

Accessors​

Accessor Functions​

Accessor functions for properties are not required.

Getters / Setters​

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').

// bad 👎
class Dragon {
    get age() {
    // ...
    }

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

// good 👍
class Dragon {
    getAge() {
    // ...
    }

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

boolean Property​

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

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

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

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

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

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

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

Events​

Data Payloads​

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:

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

// ...

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

prefer:

// good 👍
$(this).trigger('listingUpdated', { listingID: listing.id });

// ...

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

jQuery​

jQuery Object Variables​

Prefix jQuery object variables with a $.

// bad 👎
const sidebar = $('.sidebar');

// good 👍
const $sidebar = $('.sidebar');

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

Cache jQuery lookups.​

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

  // ...

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

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

  // ...

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

DOM Queries​

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

Performance Testing Link - jsPerf

Scoped jQuery Object Queries​

Use find with scoped jQuery object queries.

// bad 👎
$('ul', '.sidebar').hide();

// bad 👎
$('.sidebar').find('ul').hide();

// good 👍
$('.sidebar ul').hide();

// good 👍
$('.sidebar > ul').hide();

// good 👍
$sidebar.find('ul').hide();

ECMAScript 5 Compatibility​

Kangax​

Refer to Kangax’s ES5 compatibility table.

ECMAScript 6+ (ES 2015+) Styles​

ES6+ Features​

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

  1. Arrow Functions

  2. Classes

  3. Object Shorthand

  4. Object Concise

  5. Object Computed Properties

  6. Template Strings

  7. Destructuring

  8. Default Parameters

  9. Rest

  10. Array Spreads

  11. Let and Const

  12. Exponentiation Operator

  13. Iterators and Generators

  14. Modules

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

    Why? 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.

Standard Library​

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

Number.isNaN​

Use Number.isNaN instead of global isNaN.

eslint: no-restricted-globals

Why? The global isNaN coerces non-numbers to numbers, returning true for anything that coerces to NaN. If this behavior is desired, make it explicit.

// bad 👎
isNaN('1.2'); // false
isNaN('1.2.3'); // true

// good 👍
Number.isNaN('1.2.3'); // false
Number.isNaN(Number('1.2.3')); // true

Number.isFinite​

Use Number.isFinite instead of global isFinite.

eslint: no-restricted-globals

Why? 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.

// bad 👎
isFinite('2e3'); // true

// good 👍
Number.isFinite('2e3'); // false
Number.isFinite(parseInt('2e3', 10)); // true

Testing​

Yup.​

function foo() {
    return true;
}

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.

Performance​

Resources​

Learning ES6+

Read This

Tools

Other Style Guides

Other Styles

Further Reading

Books

Blogs

Podcasts

Versioning​

We use Major.Minor.Batch style for styles versioning. For the versions available, see the [tags on this repository].

Authors​