Airbnb JavaScript Style Guide() {

A mostly reasonable approach to JavaScript

Other Style Guides

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

Types 类型

1.1 Primitives: When you access a primitive type you work directly on its value. 基础类型，直接赋值
- string
- number
- boolean
- null
- undefined
```
const foo = 1;
let bar = foo;

bar = 9;

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

1.2 Complex: When you access a complex type you work on a reference to its value. 对象类型，使用的都是值的指针，所以要特别注意
- object
- array
- function
```
const foo = [1, 2];
const bar = foo;

bar[0] = 9;

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

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References 引用

2.1 Use const for all of your references; avoid using var. eslint: prefer-const, no-const-assign 所有的引用，如果你不打算修改它的值的话，最好使用const来修饰

Why? This ensures that you can't reassign your references, which can lead to bugs and difficult to comprehend code. 防止被修改造成bug
```
// bad
var a = 1;
var b = 2;

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

2.2 If you must reassign references, use let instead of var. eslint: no-var jscs: disallowVar如果你一定要修改一个引用，最好使用let，而不是var。

Why? let is block-scoped rather than function-scoped like var. let是ES6中新增加的修饰符，在变量提升、作用域等和var区别较大，建议ES6中全部用let。
```
javascript
// bad
var count = 1;
if (true) {
  count += 1;
}

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

2.3 Note that both let and const are block-scoped.记得let 和 const都是块级作用域。

所谓的块级作用域是{}，相对的是函数作用域，var就是函数作用域。

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

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Objects 对象

3.1 Use the literal syntax for object creation. eslint: no-new-object 使用字面值创造对象，这个应该是看起来更简约
```
// bad
const item = new Object();

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

3.2 Use computed property names when creating objects with dynamic property names.使用动态的名称定义一个对象的属性时，使用计算好的。

Why? They allow you to define all the properties of an object in one place.这样一次就定义你的所有属性名，防止后面再出现变化，代码也更易读
```
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. eslint: object-shorthand jscs: requireEnhancedObjectLiterals对象的方法，使用速记的

我理解，下面的用法是简写的，亲测可用。

// 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. eslint: object-shorthand jscs: requireEnhancedObjectLiterals对象的属性的key和value一致时，可以直接定义key，则其value和key一样。

Why? It is shorter to write and descriptive. 这个应该也是为了简约。
```
const lukeSkywalker = 'Luke Skywalker';

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

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

3.5 Group your shorthand properties at the beginning of your object declaration.所有速记用法写的属性，都放到最上面.

Why? It's easier to tell which properties are using the shorthand. 方便看出来哪些是速记的，避免忘记等。

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

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

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

3.6 Only quote properties that are invalid identifiers. eslint: quote-props jscs: disallowQuotedKeysInObjects属性的key尽量直接用name，不需要加单引号或者双引号.

Why? In general we consider it subjectively easier to read. It improves syntax highlighting, and is also more easily optimized by many JS engines.这样不仅阅读放快捷，而且在许多引擎上面处理速度回更快。data-blah如果不加引号会报错。
```
// bad
const bad = {
  'foo': 3,
  'bar': 4,
  'data-blah': 5,
};

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

3.7 Do not call Object.prototype methods directly, such as hasOwnProperty, propertyIsEnumerable, and isPrototypeOf.不要直接使用Object.prototype方法，比如hasOwnProperty ，propertyIsEnumerable ，isPrototypeOf 。

Why? These methods may be shadowed by properties on the object in question - consider { hasOwnProperty: false } - or, the object may be a null object (Object.create(null)).防止hasOwnProperty 被对象的属性覆盖，或者对象为null等情况
```
// bad
console.log(object.hasOwnProperty(key));

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

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

3.8 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. 需要新对象而不想要某些属性的时候，不要使用Object.assign来直接修改对象，使用...来获取剩余的属性。

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

// 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 }

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Arrays 数组

4.1 Use the literal syntax for array creation. eslint: no-array-constructor 直接使用字面值类初始化数组
```
// bad
const items = new Array();

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

4.2 Use Array#push instead of direct assignment to add items to an array. 数组增加新的对象时，不要直接修改数组，而是使用push
```
const someStack = [];

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

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

4.3 Use array spreads ... to copy arrays. 使用拓展运算符...来拷贝数组

// 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 array-like object to an array, use Array.from. 拷贝类数组的对象为一个数组时，使用Array.from，这是ES6中新增加的语法
```
const foo = document.querySelectorAll('.foo');
const nodes = Array.from(foo);
```

4.5 Use return statements in array method callbacks. It's ok to omit the return if the function body consists of a single statement following 8.2. eslint: array-callback-return 数据方法的callback中，尽量使用return，如果只有一个状态，也可以省略return

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

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

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

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

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

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

  return false;
});

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4.6 Use line breaks after open and before close array brackets if an array has multiple lines. 如果数组有多个对象，换行来区分不同的对象

// 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,
];

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Destructuring 解构

5.1 Use object destructuring when accessing and using multiple properties of an object. jscs: requireObjectDestructuring 多使用解构来获取属性值.

Why? Destructuring saves you from creating temporary references for those properties. 解构是ES6特性

// bad
function getFullName(user) {
  const firstName = user.firstName;
  const lastName = user.lastName;

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

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

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

5.2 Use array destructuring. jscs: requireArrayDestructuring 使用数组的解构，ES6特性.

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

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

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

5.3 Use object destructuring for multiple return values, not array destructuring. jscs: disallowArrayDestructuringReturn 使用对象的解构，ES6特性

Why? 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);

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Strings 字符串

6.1 Use single quotes '' for strings. eslint: quotes jscs: validateQuoteMarks 单个的字符，使用单引号

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

// bad - template literals should contain interpolation or newlines
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.

Why? 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.';

问题很多的情况，不要使用拼接。这样处理起来很痛苦，而且不好搜索。

6.3 When programmatically building up strings, use template strings instead of concatenation. eslint: prefer-template template-curly-spacing jscs: requireTemplateStrings 尽量少使用+等来拼接，而是用ES6提供的模板字符串

Why? 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}?`;
}
```

6.4 Never use eval() on a string, it opens too many vulnerabilities. eslint: no-eval 不要在字符串中使用eval()函数，太脆弱。

6.5 Do not unnecessarily escape characters in strings. eslint: no-useless-escape 除非必要，尽量少使用反斜杠

Why? Backslashes harm readability, thus they should only be present when necessary. 易读性差
```
// bad
const foo = '\'this\' \i\s \"quoted\"';

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

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Functions 方法

7.1 Use named function expressions instead of function declarations. eslint: func-style jscs: disallowFunctionDeclarations 使用已命名的方法而不是方法实现

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 name the expression - anonymous functions can make it harder to locate the problem in an Error's call stack. (Discussion) 。函数声明会被提升，所以非常非常容易在它被定义前引用，这会破坏易读性和可维护性，如果你觉得一个函数的定义太大或者太复杂，那也许是时候把它封装成一个模块了。不要忘记命名，匿名函数会让问题在错误的栈中难以定位。
```
// bad
function foo() {
  // ...
}

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

// good
const foo = function bar() {
  // ...
};
```

7.2 Wrap immediately invoked function expressions in parentheses. eslint: wrap-iife jscs: requireParenthesesAroundIIFE 立刻要调用的函数，在外面使用圆括号包起来。

Why? An immediately invoked function expression is a single unit - wrapping both it, and its invocation parens, in parens, cleanly expresses this. Note that in a world with modules everywhere, you almost never need an IIFE. 这种用法基本不太会使用。
```
// immediately-invoked function expression (IIFE)
(function () {
  console.log('Welcome to the Internet. Please follow me.');
}());
```

7.3 Never declare a function in a non-function block (if, while, etc). Assign the function to a variable instead. Browsers will allow you to do it, but they all interpret it differently, which is bad news bears. eslint: no-loop-func 不要在判断语句中定义函数，而是使用变量引用函数

7.4 Note: ECMA-262 defines a block as a list of statements. A function declaration is not a statement. Read ECMA-262's note on this issue. ECMA-262定义block为一组状态，一个方法的声明不是一个状态
```
// bad
if (currentUser) {
  function test() {
    console.log('Nope.');
  }
}

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

7.5 Never name a parameter arguments. This will take precedence over the arguments object that is given to every function scope. 永远不要使用arguments来作为参数，这样会把函数的argument替换掉，使用...args代替
```
// bad
function foo(name, options, arguments) {
  // ...
}

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

7.6 Never use arguments, opt to use rest syntax ... instead. eslint: prefer-rest-params 永远不要使用arguments来作为参数。

Why? ... is explicit about which arguments you want pulled. Plus, rest arguments are a real Array, and not merely Array-like like arguments. 这样会把函数的argument替换掉，使用...args代替
```
// bad
function concatenateAll() {
  const args = Array.prototype.slice.call(arguments);
  return args.join('');
}

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

7.7 Use default parameter syntax rather than mutating function arguments. 使用默认参数值，而不是在函数中计算或者判断。此为ES6新增加的方法，建议全部使用这种方式。

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

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

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

7.8 Avoid side effects with default parameters. 注意不要影响默认参数值

Why? 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

7.9 Always put default parameters last. 永远把带默认值的参数放到最后

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

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

7.10 Never use the Function constructor to create a new function. eslint: no-new-func 不要使用函数的构造器来初始化一个函数，使用字面值

Why? 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');
```

7.11 Spacing in a function signature. eslint: space-before-function-paren space-before-blocks 函数中使用空格来区分不同的部分，这样比较统一

Why? 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() {};
```

7.12 Never mutate parameters. eslint: no-param-reassign 不要修改参数值

Why? 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;
}
```

7.13 Never reassign parameters. eslint: no-param-reassign 不要修改参数

Why? 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) {
  // ...
}

7.14 Prefer the use of the spread operator ... to call variadic functions. eslint: prefer-spread 使用...来调用可变参数函数

Why? 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]);

7.15 Functions with multiline signatures, or invocations, should be indented just like every other multiline list in this guide: with each item on a line by itself, with a trailing comma on the last item. 多参数的时候，每个参数各自占一行

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

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

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

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

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Arrow Functions 箭头函数

8.1 When you must use function expressions (as when passing an anonymous function), use arrow function notation. eslint: prefer-arrow-callback, arrow-spacing jscs: requireArrowFunctions 如果你需要声明一个方法，最好使用箭头函数

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. 因为它会创造一个含有this的上下文

Why not? If you have a fairly complicated function, you might move that logic out into its own function declaration. 如果你有一个比较复杂的方法，你可以把视线部分转移到函数声明上
```
// bad
[1, 2, 3].map(function (x) {
  const y = x + 1;
  return x * y;
});

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

8.2 If the function body consists of a single expression, omit the braces and use the implicit return. Otherwise, keep the braces and use a return statement. eslint: arrow-parens, arrow-body-style jscs: disallowParenthesesAroundArrowParam, requireShorthandArrowFunctions 如果一个函数的主体只有一个表达式，可以省略return，否则要使用。

Why? Syntactic sugar. It reads well when multiple functions are chained together. 这是一个语法糖，并且链式函数中容易阅读
```
// bad
[1, 2, 3].map(number => {
  const nextNumber = number + 1;
  `A string containing the ${nextNumber}.`;
});

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

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

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

8.3 In case the expression spans over multiple lines, wrap it in parentheses for better readability. 如果一个函数有多行，用圆括号包起来，方便阅读

Why? 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,
  )
));

8.4 If your function takes a single argument and doesn’t use braces, omit the parentheses. Otherwise, always include parentheses around arguments for clarity and consistency. Note: it is also acceptable to always use parentheses, in which case use the "always" option for eslint or do not include disallowParenthesesAroundArrowParam for jscs. eslint: arrow-parens jscs: disallowParenthesesAroundArrowParam 如果函数只有一个参数，不要使用圆括号，否则永远使用圆括号。

Why? Less visual clutter. 这样阅读起来比较清晰。
```
// bad
[1, 2, 3].map((x) => x * x);

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

// good
[1, 2, 3].map(number => (
  `A long string with the ${number}. It’s so long that we 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;
});
```

8.5 Avoid confusing arrow function syntax (=>) with comparison operators (<=, >=). eslint: no-confusing-arrow 避免在箭头函数中使用<=,>=等符号让函数变得更复杂。

// 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;
};

⬆ back to top

Classes & Constructors 类和构造器

9.1 Always use class. Avoid manipulating prototype directly. 永远要使用class，避免直接修改类的属性

Why? 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;
  }
}

9.2 Use extends for inheritance. 继承永远使用extends

Why? It is a built-in way to inherit prototype functionality without breaking instanceof . 因为这是内置的继承方法，并且不会破坏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];
  }
}

9.3 Methods can return this to help with method chaining. 方法可以返回this来方便链式调用

// bad
Jedi.prototype.jump = function () {
  this.jumping = true;
  return true;
};

Jedi.prototype.setHeight = function (height) {
  this.height = height;
};

const luke = new Jedi();
luke.jump(); // => true
luke.setHeight(20); // => undefined

// good
class Jedi {
  jump() {
    this.jumping = true;
    return this;
  }

  setHeight(height) {
    this.height = height;
    return this;
  }
}

const luke = new Jedi();

luke.jump()
  .setHeight(20);

9.4 It's okay to write a custom toString() method, just make sure it works successfully and causes no side effects. 可以实现自定义的toString()方法，只要确保它可以工作并且不会产生副作用

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 类有默认的构造方法，如果没有特殊需求，不用实现一个空的构造方法。

// 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 避免重复定义类的成员变量

Why? Duplicate class member declarations will silently prefer the last one - having duplicates is almost certainly a bug. 重复的类成员定义，系统会默认选择最后一个。重复的定义经常会造成bug。
```
// bad
class Foo {
  bar() { return 1; }
  bar() { return 2; }
}

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

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

⬆ back to top

Modules 模块

10.1 Always use modules (import/export) over a non-standard module system. You can always transpile to your preferred module system. 永远使用模块而不是其他的非模块标准，你可以编译你喜欢的模块。

Why? Modules are the future, let's start using the future now. 模块是未来，让我们享受未来。
```
// 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.不要使用通配符import

Why? This makes sure you have a single default export. 这样能保证你只有一个默认的export
```
// bad
import * as AirbnbStyleGuide from './AirbnbStyleGuide';

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

10.3 And do not export directly from an import. 不要直接从一个import上直接export。

Why? Although the one-liner is concise, having one clear way to import and one clear way to export makes things consistent. 让import和export各司其职
```
// 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 从一个地址import只写在一个地方

Why? Having multiple lines that import from the same path can make code harder to maintain. 从多出import同一个地址的模块，不易维护
```
// 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. 不要export可变的绑定 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. 应该避免变化，尤其是export可变的绑定，有时候会引起特殊bug，只有不可变的引用可以被export
```
// 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.如果只有一个export，尽量使用export default eslint: import/prefer-default-export
```
// bad
export function foo() {}

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

10.7 Put all imports above non-import statements. 把import写在其他的上面 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.多个import 应该想数据一样，分行写

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 在import的语句中，不要使用构建工具语法。

Why? Since using Webpack syntax in the imports couples the code to a module bundler. Prefer using the loader syntax in webpack.config.js. 这句话的含义是使用css和sass打包foo.scss中的fooSass
```
// 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';
```

⬆ back to top

Iterators and Generators 迭代器和生成器

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不要使用迭代器，最好使用更高级的方法，而不是 for-in for-of等

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. 这加强了我们不可变的规则，这样可以不必考虑其他影响

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.
现在不要使用生成器

Why? They don't transpile well to ES5. 翻译到ES5会有问题

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

如果一定要使用生成器，一定要确保命名的空格是正确的

Why? function and * are part of the same conceptual keyword - * is not a modifier for function, function* is a unique construct, different from function. 因为*也是一个函数关键词，function*是一个特殊的构造器，和 function不同

// 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 属性

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

当需要用属性时，使用.来读取

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.

当使用变量来读取属性时，使用[]读取

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

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

const isJedi = getProp('jedi');

⬆ back to top

Variables 变量

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

永远使用const或者let声明变量，否则会导致其变成全局变量

// bad
superPower = new SuperPower();

// good
const superPower = new SuperPower();

13.2 Use one const or let declaration per variable. eslint: one-var jscs: disallowMultipleVarDecl
声明变量的时候，一个变量用一个const或者let

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. 这样就不用担心用错;和,啦
```
// 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.

let 和 const 都分类放好

Why? 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;

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

在合理的地方，修改你的变量

Why? let and const are block scoped and not function scoped. let和const是块级作用域

// 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.

不要使用链式声明

Why? Chaining variable assignments creates implicit global variables. 因为链式修改会制造隐式的全局变量

// 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); // undefined
console.log(b); // 1
console.log(c); // 1

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

console.log(a); // undefined
console.log(b); // undefined
console.log(c); // undefined

// the same applies for `const`

13.6 Avoid using unary increments and decrements (++, --). eslint no-plusplus
避免使用++ --等操作

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. 不推荐使用，防止出现意外的错误 ,关于这个问题的讨论也可以看下githubairbnb#540
```
// 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;
```

⬆ back to top

Hoisting 提升

14.1 var declarations get hoisted to the top of their scope, their assignment does not. const and let declarations are blessed with a new concept called Temporal Dead Zones (TDZ). It's important to know why typeof is no longer safe.

var声明的变量会出现变量提升，尽量使用let，具体的可以看下我的blog 《ES6入门》读书笔记之let和const命令

// 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.

匿名函数会提升他们的变量名，但是不会提升变量实现，所以尽量不要用匿名函数，上面讲函数的时候已经讲过。

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

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

  var anonymous = function () { //function bar () is better
    console.log('anonymous function expression');
  };
}

14.3 Named function expressions hoist the variable name, not the function name or the function body.

命名函数会提升变量name，但不会提升函数方法名或者方法主体

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.

方法声明提升他们的name和函数主体

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 比较和相等

15.1 Use === and !== over == and !=. eslint: eqeqeq
使用===和!==而不是==和!=

区别看下列子，如果类型不一样，==会转换类型：

let a = 3; 
let b = "3"; 
console.log(a == b); //true
console.log(a === b); //false

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:
以下是各种类型在if判断中的值
- 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.

布尔值的判断使用简单写法，但是字符串和数字要使用明确的写法

// 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).

使用{}来包裹case和default中的内容

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. 这里是因为ES6中let是块作用域，如果case 2中再定义x，则会报错，所以要加上大括号

eslint rules: no-case-declarations.

// 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.

不要嵌套三元运算，三元运算应该是阅读更清晰，而不是更复杂

eslint rules: no-nested-ternary.

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

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

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

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

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

15.7 Avoid unneeded ternary statements.

避免不需要的三元运算、建议使用||等，更简洁、高效

eslint rules: 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;

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Blocks 代码块

16.1 Use braces with all multi-line blocks.

所有的多行代码块，都使用{}

// bad
if (test)
  return false;

// good
if (test) return false;

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

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

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

16.2 If you're using multi-line blocks with if and else, put else on the same line as your if block's closing brace. eslint: brace-style jscs: disallowNewlineBeforeBlockStatements

如果使用了if 和 else、else 要在第一个{}结束的同一行

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

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

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Control Statements 控制状态

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. It's up to you whether the logical operator should begin or end the line.

为了防止控制状态太长，每个情况都要单独放一行

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

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

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

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

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

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

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

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Comments 注释

18.1 Use /** ... */ for multi-line comments.

多行注释，使用/** ... */

// 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.

单行注释使用//，将注释卸载要注释语句的上一行，第一个单词前面加个空格

// 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

所有注释前都加个空格，提高易读性

// 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.
如果是bug或者待做事项，前面加个FIXME或者TODO

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.

使用TODO来注释一个问题的解决方案

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

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

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Whitespace 空格

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

把tab设置为两个空间（默认应该是4个空间）

// 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 jscs: requireSpaceBeforeBlockStatements

大括号前留一个空格

// 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 jscs: requireSpaceAfterKeywords

if function等后面留一个空格，方法名后面不留空格

// 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 jscs: requireSpaceBeforeBinaryOperators, requireSpaceAfterBinaryOperators

操作之间留空格

// bad
const x=y+5;

// good
const x = y + 5;

19.5 End files with a single newline character. eslint: eol-last

文件末尾留一行

// 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 函数链留一个标志来标明这是一个函数调用，在开头使用.

// 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);

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

在代码块后面留一个空行

// 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 jscs: disallowPaddingNewlinesInBlocks

代码块里面不要留空行

// bad
function bar() {

  console.log(foo);

}

// also bad
if (baz) {

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

}

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

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

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

不要在()内留空格

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

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

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

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

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

不要在中括号里面留空格

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

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

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

在大括号里留空格

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

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

19.12 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 jscs: maximumLineLength

避免一行代码超过100个字符

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.'));

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Commas 逗号

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

逗号放在末尾

// 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 jscs: requireTrailingComma

每行结尾如果可以，都写上逗号

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. 这样git diff的时候会更清晰，另外babel会溢出多余的逗号，所以你不用担心trailing comma problem

// 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
);

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Semicolons 分号

21.1 Yup. eslint: semi jscs: requireSemicolons

使用分号，每行都要用！

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

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

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

Type Casting & Coercion

22.1 Perform type coercion at the beginning of the statement.
在语句开始时，使用类型转换

22.2 Strings:

// => this.reviewScore = 9;

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

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

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

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

如果因为某些原因 parseInt 成为你所做的事的瓶颈而需要使用位操作解决性能问题时，留个注释说清楚原因和你的目的。

// 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:
小心使用位操作运算符。数字会被当成 64 位值，但是位操作运算符总是返回 32 位的整数（参考）。位操作处理大于 32 位的整数值时还会导致意料之外的行为。关于这个问题的讨论。最大的 32 位整数是 2,147,483,647：
```
2147483647 >> 0; // => 2147483647
2147483648 >> 0; // => -2147483648
2147483649 >> 0; // => -2147483647
```

22.6 Booleans:

const age = 0;

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

// good
const hasAge = Boolean(age);

// best
const hasAge = !!age;

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Naming Conventions 命名规则

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

避免单字母命名。命名应具备描述性。

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

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

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

使用驼峰式命名对象、函数和实例。

// 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 jscs: requireCapitalizedConstructors

使用帕斯卡式命名构造函数或类。

// 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 jscs: disallowDanglingUnderscores
不要使用下划线 _ 结尾或开头来命名属性和方法。

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';
```

23.5 Don't save references to this. Use arrow functions or Function#bind. jscs: disallowNodeTypes

别保存 this 的引用。使用箭头函数或 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.

如果你的文件只输出一个类，那你的文件名必须和类名完全保持一致。

// 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.
当你导出默认的函数时使用驼峰式命名。你的文件名必须和函数名完全保持一致。
```
function makeStyleGuide() {
  // ...
}

export default makeStyleGuide;
```

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

当你导出单例、函数库、空对象时使用帕斯卡式命名。

const AirbnbStyleGuide = {
  es6: {
  },
};

export default AirbnbStyleGuide;

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

缩写和缩略词应该永远大写，或者全称

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 = [
  // ...
];

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

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

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Accessors

24.1 Accessor functions for properties are not required.
属性的存取函数不是必须的

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').

不要使用JS的getter和setter函数，因为他们可能引起副作用。如果你想要使用存取函数，使用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().

布尔值存取函数使用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.

可以创建get() 和 set()函数，但是要保持一致

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

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

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

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Events

25.1 When attaching data payloads to events (whether DOM events or something more proprietary like Backbone events), pass 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:

当给事件附加数据时（无论是 DOM 事件还是私有事件），传入一个哈希而不是原始值。这样可以让后面的贡献者增加更多数据到事件数据而无需找出并更新事件的每一个处理器

// 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
});

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jQuery

26.1 Prefix jQuery object variables with a $. jscs: requireDollarBeforejQueryAssignment

使用JQuery对象时，在前面加$

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

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

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

26.2 Cache jQuery lookups.

缓存jquery

// 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();

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ECMAScript 5 Compatibility

27.1 Refer to Kangax's ES5 compatibility table.

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ECMAScript 6+ (ES 2015+) Styles

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

Arrow Functions
Classes
Object Shorthand
Object Concise
Object Computed Properties
Template Strings
Destructuring
Default Parameters
Rest
Array Spreads
Let and Const
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.

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Testing

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

29.2 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 at Airbnb. tape is also used occasionally for small, separate modules.
- 100% test coverage is a good goal to strive for, even if it's not always practical to reach it.
- Whenever you fix a bug, write a regression test. A bug fixed without a regression test is almost certainly going to break again in the future.

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Performance

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Resources

Learning ES6

Read This

Standard ECMA-262

Tools

Code Style Linters
- ESlint - Airbnb Style .eslintrc
- JSHint - Airbnb Style .jshintrc
- JSCS - Airbnb Style Preset (Deprecated, please use ESlint)
Neutrino preset - neutrino-preset-airbnb-base

Other Style Guides

Other Styles

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

Further Reading

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

Books

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

Blogs

Podcasts

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In the Wild

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

3blades: 3Blades/javascript
4Catalyzer: 4Catalyzer/javascript
Aan Zee: AanZee/javascript
Adult Swim: adult-swim/javascript
Airbnb: airbnb/javascript
AltSchool: AltSchool/javascript
Apartmint: apartmint/javascript
Ascribe: ascribe/javascript
Avalara: avalara/javascript
Avant: avantcredit/javascript
Axept: axept/javascript
BashPros: BashPros/javascript
Billabong: billabong/javascript
Bisk: bisk/javascript
Bonhomme: bonhommeparis/javascript
Brainshark: brainshark/javascript
CaseNine: CaseNine/javascript
Chartboost: ChartBoost/javascript-style-guide
ComparaOnline: comparaonline/javascript
Compass Learning: compasslearning/javascript-style-guide
DailyMotion: dailymotion/javascript
DoSomething: DoSomething/eslint-config
Digitpaint digitpaint/javascript
Ecosia: ecosia/javascript
Evernote: evernote/javascript-style-guide
Evolution Gaming: evolution-gaming/javascript
EvozonJs: evozonjs/javascript
ExactTarget: ExactTarget/javascript
Expensify Expensify/Style-Guide
Flexberry: Flexberry/javascript-style-guide
Gawker Media: gawkermedia/javascript
General Electric: GeneralElectric/javascript
Generation Tux: GenerationTux/javascript
GoodData: gooddata/gdc-js-style
Grooveshark: grooveshark/javascript
Honey: honeyscience/javascript
How About We: howaboutwe/javascript
Huballin: huballin/javascript
HubSpot: HubSpot/javascript
Hyper: hyperoslo/javascript-playbook
InterCity Group: intercitygroup/javascript-style-guide
Jam3: Jam3/Javascript-Code-Conventions
JeopardyBot: kesne/jeopardy-bot
JSSolutions: JSSolutions/javascript
KickorStick: kickorstick/javascript
Kinetica Solutions: kinetica/javascript
Lonely Planet: lonelyplanet/javascript
M2GEN: M2GEN/javascript
Mighty Spring: mightyspring/javascript
MinnPost: MinnPost/javascript
MitocGroup: MitocGroup/javascript
ModCloth: modcloth/javascript
Money Advice Service: moneyadviceservice/javascript
Muber: muber/javascript
National Geographic: natgeo/javascript
Nimbl3: nimbl3/javascript
Nulogy: nulogy/javascript
Orange Hill Development: orangehill/javascript
Orion Health: orionhealth/javascript
OutBoxSoft: OutBoxSoft/javascript
Peerby: Peerby/javascript
Razorfish: razorfish/javascript-style-guide
reddit: reddit/styleguide/javascript
React: facebook.github.io/react/contributing/how-to-contribute.html#style-guide
REI: reidev/js-style-guide
Ripple: ripple/javascript-style-guide
SeekingAlpha: seekingalpha/javascript-style-guide
Shutterfly: shutterfly/javascript
Sourcetoad: sourcetoad/javascript
Springload: springload/javascript
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
TheLadders: TheLadders/javascript
The Nerdery: thenerdery/javascript-standards
T4R Technology: T4R-Technology/javascript
VoxFeed: VoxFeed/javascript-style-guide
WeBox Studio: weboxstudio/javascript
Weggo: Weggo/javascript
Zillow: zillow/javascript
ZocDoc: ZocDoc/javascript

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Translation

This style guide is also available in other languages:

Brazilian Portuguese: armoucar/javascript-style-guide
Bulgarian: borislavvv/javascript
Catalan: fpmweb/javascript-style-guide
Chinese (Simplified): sivan/javascript-style-guide
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: tipjs/javascript-style-guide
Polish: mjurczyk/javascript
Russian: leonidlebedev/javascript-airbnb
Spanish: paolocarrasco/javascript-style-guide
Thai: lvarayut/javascript-style-guide
Ukrainian: ivanzusko/javascript
Vietnam: hngiang/javascript-style-guide

The JavaScript Style Guide Guide

Reference

Chat With Us About JavaScript

Find us on gitter.

Contributors

View Contributors

License

(The MIT License)

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

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

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

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Amendments

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

njafei/javascript

Airbnb JavaScript Style Guide() {

Table of Contents

Types 类型

References 引用

Objects 对象

Arrays 数组

Destructuring 解构

Strings 字符串

Functions 方法

Arrow Functions 箭头函数

Classes & Constructors 类和构造器

Modules 模块

Iterators and Generators 迭代器和生成器

Properties 属性

Variables 变量

Hoisting 提升

Comparison Operators & Equality 比较和相等

Blocks 代码块

Control Statements 控制状态

Comments 注释

Whitespace 空格

Commas 逗号

Semicolons 分号

Type Casting & Coercion

Naming Conventions 命名规则

Accessors

Events

jQuery

ECMAScript 5 Compatibility

ECMAScript 6+ (ES 2015+) Styles

Testing

Performance

Resources

In the Wild

Translation

The JavaScript Style Guide Guide

Chat With Us About JavaScript

Contributors

License

Amendments

};