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 类型
References 引用
Objects 对象
Arrays 数组
Destructuring
Strings 字符串
Functions 函数
Arrow Functions 箭头函数
Classes & Constructors 类与构造器
Modules 模块化
Iterators and Generators 迭代器与生成器
Properties 属性
Variables 变量
Hoisting 吊装
Comparison Operators & Equality 比较运算符与等式
Blocks 块
Control Statements 控制声明
Comments 注释
Whitespace 空白
Commas 逗号
Semicolons 分号
Type Casting & Coercion 类型转换与强制
Naming Conventions 命名约定
Accessors 访问器
Events 事件
jQuery jQuery
ECMAScript 5 Compatibility
ECMAScript 6+ (ES 2015+) Styles
Standard Library 标准库
Testing 测试
Performance 表现
Resources 资源
In the Wild
Translation
The JavaScript Style Guide Guide JS Style Guide
Chat With Us About JavaScript 跟我们聊一聊🤞🏼JS
Contributors 贡献者
License 执照🪪
Amendments 修正

Types

1.1 Primitives: When you access a primitive type you work directly on its value. 访问JS的基本数据类型，直接作用于它的值
- string 字符串
- number 数字
- boolean 布尔运算
- null 空
- undefined 未定义
- symbol 一种基本数据类型primitive data type静态属性和静态方法，是不完整的构造函数，不支持"new Symbol()"
- bigint 内置对象，提供一种方法表示大于2^53 - 1的整数。这原本是JS中能够用number表示的最大数字，BigInt可以表示任意大的整数
```
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.访问复杂类型时，您可以处理对其值的引用
- object 对象
- array 数组
- function 函数
```
const foo = [1, 2];
const bar = foo;

bar[0] = 9;

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

⬆ back to top

References

2.1 Use const for all of your references;对所有的引用统一使用const avoid using var.避免使用var eslint: prefer-const, no-const-assign

Why? This ensures that you can’t reassign your references, which can lead to bugs and difficult to comprehend code. 对于不会重新赋值的引用，会导致bugs&很难理解代码
```
// bad👎👎👎
var a = 1;
var b = 2;

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

2.2 If you must reassign references,如果一定需要重新对引用赋值 use let instead of var.使用let而不是var eslint: no-var

Why? let is block-scoped rather than function-scoped like var.let是块-范围，而不像var是函数-范围
```
// bad👎👎👎
var count = 1;
if (true) {
  count += 1;
}

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

2.3 Note that both let and const are block-scoped,区块范围 whereas var is function-scoped.函数范围
```
// const and let only exist in the blocks they are defined in. const & let只存在他们定义的blocks中
{
  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,引用a& b会产生参照错误 while c contains the number. This is because a and b are block scoped, while c is scoped to the containing function.

⬆ back to top

Objects对象

3.1 Use the literal syntax for object creation创建对象的文本语法. eslint: no-new-object

// bad👎👎👎
const item = new Object();

// good👍👍👍
const item = {};

3.2 Use computed property names使用计算的属性名 when creating objects with dynamic property names.用动态属性名称创建对象

Why? They allow you to define all the properties of an object in one place. 在一个地方定义对象的所有属性

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

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

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

3.3 Use object method shorthand.对象方法速记ECMAScript6 简洁的定义对象文本方式和属性的简洁形式 eslint: object-shorthand

// 不简洁写法！ES5 syntax👇
// Properties 属性
var foo = {
  x = x,
  y = y,
  z = z,
};
// Methods 方法
var foo = {
  a: function(){},
  b: function(){}
};

// 简洁写法，ES6 syntax👇
// Properties 属性
var foo = {x, y, z};
// methods 方法
var foo = {
a() {},
b() {}
};

//--------------------

// bad👎👎👎
const atom = {
  value: 1,

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

// good👍👍👍
const atom = {
  value: 1,

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

3.4 Use property value shorthand.属性值简写 eslint: object-shorthand

Why? It is shorter and descriptive.更短且具有描述性

const dreamShake = 'Dream Shake';

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

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

3.5 Group your shorthand properties at the beginning of your object declaration. 编组简写属性，在对象声明前

Why? It’s easier to tell which properties are using the shorthand. 哪个属性在使用简写

const chrisBosh = 'Chris Bosh';
const chrisWebber = 'Chris Webber';

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

// good👍👍👍
const obj = {
  chrisBosh,
  chrisWebber,
  episodeOne: 1,
  twoJediWalkIntoACantina: 2,
  episodeThree: 3,
  mayTheFourth: 4,
};
//even tho i still cannot tell why the good is good, the bad is bad

3.6 Only quote properties that are invalid identifiers.只有引用属性是无效的标识符 eslint: quote-props

Why? In general we consider it subjectively easier to read.通常,我们主观地认为这样更可读 It improves syntax highlighting, and is also more easily optimized by many JS engines.改进了语法的突出显示，并且更容易被许多js引擎优化
```
// bad👎👎👎
const bad = {
  'foo': 3,//不要 quote属性
  'bar': 4,//不要 quote属性
  'data-blah': 5,
};

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

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

Why? These methods may be shadowed by properties on the object in question这些方法可能被相关对象的属性遮蔽 - consider { hasOwnProperty: false } - or, the object may be a null object (Object.create(null)).或者，这个对象可能是一个null对象
```
// 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));
```

3.8 Prefer the object spread syntax over首选对象传播语法 Object.assign to shallow-copy objects.浅层-复制对象 Use the object rest parameter syntax对象重置参数语法 to get a new object with certain properties omitted.获取省略了某些属性的新对象 eslint: prefer-object-spread

// very bad👎👎👎👎
const original = { a: 1, b: 2 };
const copy = Object.assign(original, { c: 3 }); // this mutates `original` ಠ_ಠ 这变异了`original`
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 }

⬆ back to top

Arrays阵列

4.1 Use the literal syntax for array creation.使用文字语法[] 生成阵列 eslint: no-array-constructor
```
// bad👎👎👎
const items = new Array();

// good👍👍👍
const items = [];
```

4.2 Use [Array#push] (https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_Objects/Array/push)使用[Array#push] instead of direct assignment to add items to an array. 而不是直接赋值添加物件到数列中
```
const someStack = [];

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

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

4.3 Use array spreads ... to copy arrays. 使用数列传播...复制数列的方法

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

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

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

4.4 To convert an iterable object to an array,转换可迭代对象到数列中 use spreads ... instead of Array.from

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

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

// best👍👍👍👍
const nodes = [...foo];

从array-like 到arrays

4.5 Use Array.from for converting an array-like object to an array. 转换一个array-like 对象to一个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);

阵列的映射

4.6 Use Array.from instead of spread ... for mapping over iterables,在可迭代对象上映射 because it avoids creating an intermediate array.避免创建一个中间阵列
```
// bad👎👎👎
const baz = [...foo].map(bar);

// good👍👍👍
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

// 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;
});

数组新的一行

4.8 Use line breaks after open and before close array brackets if an array has multiple lines 如果一个数组有很多行

// 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,
];

⬆ back to top

Destructuring破坏

5.1 Use object destructuring破坏对象 when accessing and using multiple properties of an object.访问或者使用同一个对象的多个属性 eslint: prefer-destructuring

Why? Destructuring saves you from creating temporary references for those properties,为那些属性创建临时引用 and from repetitive access of the object.对对象的重复访问 Repeating object access creates more repetitive code,重复地访问对象会产生更多重复的代码 requires more reading, and creates more opportunities for mistakes.需要更多的阅读更多产生错误的机会 Destructuring objects also provides a single site of definition of the object structure that is used in the block,破坏对象同时提供一个在块里的单一site的definition的对象破坏 rather than requiring reading the entire block to determine what is used.而不是要求去读整个block来定义它是如何被使用的
```
// bad👎👎👎
function getFullName(user) {
  const firstName = user.firstName;
  const lastName = user.lastName;

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

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

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

5.2 Use array destructuring.使用阵列破坏 eslint: prefer-destructuring

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

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

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

5.3 Use object destructuring for multiple return values,对多个回传的数值使用对象破坏 not array destructuring.而不是阵列的破坏

Why? You can add new properties over time or change the order of things without breaking call sites.为什么？可以添加新的属性或者改变某物的顺序而不破坏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);

⬆ back to top

Strings字符串

6.1 Use single quotes '' for strings.对数组使用'' eslint: quotes

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

// bad - template literals should contain interpolation or newlines模版字符串(temporal literals)需要包含插值或者换行
const name = `Capt. Janeway`;

// good👍👍👍
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. 字符串导致line超过100characters不应该使用字符串超过多行

Why? Broken strings are painful to work with and make code less searchable.字符串断点are painful to work with and 让代码更不可搜索🔍

// bad👎👎👎
const errorMessage = 'This is a super long error that was thrown because \
of Batman. When you stop to think about how Batman had anything to do \
with this, you would get nowhere \
fast.';

// bad👎👎👎
const errorMessage = 'This is a super long error that was thrown because ' +
  'of Batman. When you stop to think about how Batman had anything to do ' +
  'with this, you would get nowhere fast.';

// good👍👍👍
const errorMessage = 'This is a super long error that was thrown because of Batman. When you stop to think about how Batman had anything to do with this, you would get nowhere fast.';

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

Why? Template strings give you a readable, concise syntax with proper newlines and string interpolation features.模版字符串提供可读，准确语法字符串插值功能

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

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

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

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

6.4 Never use eval() on a string, it opens too many vulnerabilities.不要在字符串中使用eval()导致很多漏洞，因为eval()function evaluate javascript code represented as a string eslint: no-eval

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

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

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

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

⬆ back to top

Functions 函数

7.1 Use named function expressions instead of function declarations.使用命名的函数表达式而不是简单的函数声明 eslint: func-style

Why? 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)
```
// bad👎👎👎
function foo() {
  // ...
}

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

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

7.2 Wrap immediately invoked function expressions in parentheses.把调用的函数表达式用{}包裹起来 eslint: wrap-iife

Why? An immediately invoked function expression is a single unit 调用的函数表达式是一个单一组 - 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.在模块化到处都有的地方，几乎不需要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永远不要在非函数块中声明一个函数 (if, while, etc). Assign the function to a variable instead.把函数赋给一个变量 Browsers will allow you to do it, but they all interpret it differently, which is bad news bears.浏览器会允许你这样做，但是他们则会有不一样的解释，虽然这很不好 eslint: no-loop-func

7.4 Note: ECMA-262 defines a block as a list of statements. A function declaration is not a statement.一个函数的声明并不是语句

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

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

7.5 Never name a parameter arguments.永远不要用arguments命名一个参数 This will take precedence over the arguments object that is given to every function scope. 这会优先于给到每个函数的范围
```
// bad👎👎👎
function foo(name, options, arguments) {
  // ...
}

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

7.6 Never use arguments, opt to use rest syntax ... instead.永远不要使用arguments，选择使用reset语法 eslint: prefer-rest-params

Why? ... is explicit about which arguments you want pulled....将明确哪一个要拉取的参数 Plus, rest arguments are a real Array, and not merely Array-like like arguments.重设参数是一个真实的数组，而不是像数组的实际arguments
```
// bad👎👎👎
function concatenateAll() {
  const args = Array.prototype.slice.call(arguments);
  return args.join('');
}

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

7.7 Use default parameter syntax使用默认参数语法 rather than mutating function arguments.而不是修改函数的参数

// really bad👎👎👎
function handleThings(opts) {
  // No! We shouldn’t mutate function arguments.不要!!变异函数的(参数)
  // Double bad: if opts is falsy it'll be set to an object which may 如果opts是false
  // be what you want but it can introduce subtle bugs.
  opts = opts || {};
  // ...
}

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

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

7.8 Avoid side effects with default parameters.避免默认参数所带来的副作用

Why? They are confusing to reason about.令人困惑的推理

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

7.9 Always put default parameters last.永远把默认的参数放在最后 eslint: default-param-last

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

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

7.10 Never use the Function constructor to create a new function.不要用函数构造器来创建函数 eslint: no-new-func

Why? Creating a function in this way evaluates a string以这种创建函数计算字符串的方式 similarly to eval(),类似 which opens vulnerabilities.开放漏洞
```
// bad👎👎👎
var add = new Function('a', 'b', 'return a + b');

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

7.11 Spacing in a function signature.函数签名留白: 函数名 () {} eslint: space-before-function-paren space-before-blocks

Why? Consistency is good, and you shouldn’t have to add or remove a space when adding or removing a name.保持一致性很重要，去掉函数名的同时不应该去掉或者添加 🈳️格
```
// bad👎👎👎
const f = function(){};
const g = function (){};
const h = function() {};

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

7.12 Never mutate parameters.不要变异参数 eslint: no-param-reassign

Why? Manipulating objects passed in as parameters 操作作为参数的对象 can cause unwanted variable side effects 会导致不必要的变量副作用 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;
}
```

7.13 Never reassign parameters.重新赋予参数 eslint: no-param-reassign

Why? Reassigning parameters can lead to unexpected behavior, especially when accessing the arguments object.访问参数对象 It can also cause optimization issues, especially in V8.在V8引擎中造成一些优化问题
```
// 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) {
  // ...
}
```

7.14 Prefer the use of the spread syntax ... to call variadic functions.可变参数函数 eslint: prefer-spread

Why? It’s cleaner,更干净 you don’t need to supply a context,无须提供上下文 and you can not easily compose new with apply.你不能简单地组合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]);

7.15 Functions with multiline signatures, or invocations,带有多行签名的函数或者调用函数 should be indented just like every other multiline list in this guide应该像其他多行列表一样缩进: with each item on a line by itself,每一件物品单独只占一行 with a trailing comma on the last item.并在最后一项后用尾随逗号 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,
);
```

⬆ back to top

Arrow Functions箭头➡️函数

8.1 When you must use an anonymous function (as when passing an inline callback),当你一定要使用一个匿名函数就像传递内联回调一样 use arrow function notation.使用箭头函数作为标注 eslint: prefer-arrow-callback, arrow-spacing

Why? It creates a version of the function that executes in the context of this, which is usually what you want, and is a more concise syntax. 创建了一个函数的版本，执行this的语境，通常情况下是你想要的，而且是一个更简洁的句法

Why not? If you have a fairly complicated function, you might move that logic out into its own named function expression.如果你有一个很复杂的函数，你可能需要把逻辑移动出来到它自命名的函数表达式
```
// bad👎👎👎
[1, 2, 3].map(function (x) {
  const y = x + 1;
  return x * y;
});

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

8.2 If the function body consists of a single statement returning an [expression]如果函数的主体由一个单一的声明回传一个表达式组成而没有副作用(https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Expressions_and_Operators#Expressions) without side effects, omit the braces and use the implicit return. Otherwise, keep the braces and use a return statement. 省略大括号使用隐式返回，否则，保留大括号使用return语句eslint: arrow-parens, arrow-body-style

Why? Syntactic sugar.Syntactic sugar是一种一种句法设计来让编程语言更可读更易表达 It reads well when multiple functions are chained together.尤其是当多个函数链接🔗在一起

// 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 不含蓄回传没有副作用
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;
});

8.3 In case the expression spans over multiple lines, wrap it in parentheses for better readability.如果表达式跨越多行，请将其括在括号中以提高可读性

Why? It shows clearly where the function starts and ends.清晰展现函数从哪里开始在哪里结束

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

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

箭头函数--一个--arg-

8.4 Always include parentheses around arguments for clarity and consistency.在参数周围包含()为了清晰度与持续性 eslint: arrow-parens

Why? Minimizes diff churn when adding or removing arguments.最小化差异流失当添加或移除参数

// 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;
});

箭头--混淆

8.5 Avoid confusing arrow function syntax避免混淆箭头函数句法与比较句法 (=>) 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;
};

空白--含蓄-箭头函数-换行

8.6 Enforce the location of arrow function bodies with implicit returns.使用隐式返回强制实施箭头函数体的地址 eslint: implicit-arrow-linebreak
```
// bad👎👎👎
(foo) =>
  bar;

(foo) =>
  (bar);

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

⬆ back to top

Classes & Constructors类型&构造器

构造器--使用-类

9.1 Always use class. Avoid manipulating prototype directly.永远使用class.避免直接操作prototype

Why? class syntax is more concise and easier to reason about.class语句更准确且容易归因

// 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;
  }
}

构造器--扩建

9.2 Use extends for inheritance.使用extends做继承

Why? It is a built-in way to inherit prototype functionality without breaking instanceof.内建方式继承原型功能且不破坏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];
  }
}

构造器--链

9.3 Methods can return this to help with method chaining.方法会回传this助于方法链

// 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);

构造--牙绳

9.4 It’s okay to write a custom toString() method, just make sure it works successfully and causes no side effects. 习惯性地写一个toString()确保工作成功不会造成副作用

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

// bad👎👎👎
class Jedi {
  constructor() {}

  getName() {
    return this.name;
  }
}

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

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

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

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

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

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

// good
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

// 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');
  }
}

⬆ 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.

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;

10.2 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';

10.3 And do not export directly from an import.

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

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

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

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

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

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

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

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

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

Why? Mutation should be avoided in general, but in particular when exporting mutable bindings. While this technique may be needed for some special cases, in general, only constant references should be exported.
```
// bad
let foo = 3;
export { foo };

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

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

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

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

10.7 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();

10.8 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';
```

10.9 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';
```

10.10 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';
```

⬆ back to top

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

Why? 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);

11.2 Don’t use generators for now.

Why? They don’t transpile well to ES5.

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

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

// 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* () {
  // ...
};

⬆ back to top

Properties

12.1 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;

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

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

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

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

Why? It’s easier to add new variable declarations this way, and you never have to worry about swapping out a ; for a , or introducing punctuation-only diffs. You can also step through each declaration with the debugger, instead of jumping through all of them at once.

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

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

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

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

Why? This is helpful when later on you might need to assign a variable depending on one of the previously 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;

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

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

// 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;
}

13.5 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`

13.6 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;
```

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

Why? 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';

13.8 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.

⬆ 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.

⬆ back to top

Comparison Operators & Equality

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

15.2 Conditional statements such as the if statement evaluate their expression using coercion with the ToBoolean abstract method and always follow these simple rules:
- Objects evaluate to true
- Undefined evaluates to false
- Null evaluates to false
- Booleans evaluate to the value of the boolean
- Numbers evaluate to false if +0, -0, or NaN, otherwise true
- Strings evaluate to false if an empty string '', otherwise true
```
if ([0] && []) {
  // true
  // an array (even an empty one) is an object, objects will evaluate to true
}
```

15.3 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) {
  // ...
}

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

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

15.6 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;

15.7 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;

15.8 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

Why? 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;

⬆ back to top

Blocks

16.1 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;
}

16.2 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();
}

16.3 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;
  }
}

⬆ back to top

Control Statements

17.1 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.

Why? Requiring operators at the beginning of the line keeps the operators aligned and follows a pattern similar to method chaining. 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();
}

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

// bad
!isRunning && startRunning();

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

⬆ back to top

Comments

18.1 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;
}

18.2 Use // for single line comments. Place single line comments on a newline above the subject of the comment. Put an empty line before the comment unless it’s on the first line of a block.

// 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;
}

18.3 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;
}

18.4 Prefixing your comments with FIXME or TODO helps other developers quickly understand if you’re pointing out a problem that needs to be revisited, or if you’re suggesting a solution to the problem that needs to be implemented. These are different than regular comments because they are actionable. The actions are FIXME: -- need to figure this out or TODO: -- need to implement.

18.5 Use // FIXME: to annotate problems.

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

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

18.6 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;
  }
}

⬆ back to top

Whitespace

19.1 Use soft tabs (space character) set to 2 spaces. eslint: indent

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

// bad
function bar() {
∙let name;
}

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

19.2 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',
});

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

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

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

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

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

19.4 Set off operators with spaces. eslint: space-infix-ops
```
// bad
const x=y+5;

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

19.5 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;↵

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

// 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);
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 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;

19.8 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);
}

19.9 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) {
    // ..
  }
}

19.10 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);
}

19.11 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]);

19.12 Add spaces inside curly braces. eslint: object-curly-spacing

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

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

19.13 Avoid having lines of code that are longer than 100 characters (including whitespace). Note: per above, 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.'));

19.14 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; }
```

19.15 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];

19.16 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]]

19.17 Avoid spaces between functions and their invocations. eslint: func-call-spacing
```
// bad
func ();

func
();

// good
func();
```

19.18 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 };

19.19 Avoid trailing spaces at the end of lines. eslint: no-trailing-spaces

19.20 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;

⬆ back to top

Commas

20.1 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',
};

20.2 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
);

⬆ back to top

Semicolons

21.1 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 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

22.1 Perform type coercion at the beginning of the statement.

22.2 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);

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

Why? 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';

// 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);
```

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.
```
// 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;
```

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;

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

// good
const hasAge = Boolean(age);

// best
const hasAge = !!age;

⬆ back to top

Naming Conventions

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

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

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

23.2 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() {}

23.3 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',
});

23.4 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');
```

23.5 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);
  };
}

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
// 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

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.

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 = [
  // ...
];

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.

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'
};
```

⬆ back to top

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').

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

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

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

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

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

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

// good
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];
  }
}

⬆ back to top

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:
```
// 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
});
```

⬆ back to top

jQuery

26.1 Prefix jQuery object variables with a $.

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

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

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

26.2 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',
  });
}

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

26.4 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();

⬆ back to top

ECMAScript 5 Compatibility

27.1 Refer to Kangax’s ES5 compatibility table.

⬆ back to top

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.

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.

⬆ back to top

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

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
```

29.2 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
```

⬆ back to top

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.

⬆ back to top

Performance

⬆ back to top

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

⬆ back to top

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

⬆ back to top

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.

⬆ back to top

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.

Name		Name	Last commit message	Last commit date
Latest commit History 1,936 Commits
.github/workflows		.github/workflows
css-in-javascript		css-in-javascript
linters		linters
packages		packages
react		react
.editorconfig		.editorconfig
.gitignore		.gitignore
.npmrc		.npmrc
LICENSE.md		LICENSE.md
README.md		README.md
package.json		package.json

License

ChenxingWang93/javascript

Folders and files

Latest commit

History

Repository files navigation

Airbnb JavaScript Style Guide() {

Table of Contents

Types

References

Objects对象

Arrays阵列

Destructuring破坏

Strings字符串

Functions 函数

Arrow Functions箭头➡️函数

Classes & Constructors类型&构造器

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

};

About

Resources

License

Stars

Watchers

Forks

Releases

Packages 0

Languages

Packages