Airbnb JavaScript 风格指南

本文已将原英文文档转写为双语格式，并对可能的难点提供了相应注解。

阿里巴巴JS规范官方文档地址： GitHub - Airbnb/javascript： JavaScript 风格指南

一、变量类型（Types）

1.1 基元：当您访问基元类型时，您可以直接处理其值。Primitives: When you access a primitive type you work directly on its value.
- string
- number
- boolean
- null
- undefined
- symbol
- bigint
JavaScript 复制代码
```
const foo = 1;
let bar = foo;

bar = 9;

console.log(foo, bar); // => 1, 9
```
- Symbols 和 BigInts无法忠实地进行多填充，因此在定位到本身不支持它们的浏览器/环境时，不应使用它们。
1.2 复杂类型：当您访问复杂类型时，您需要处理对其值的引用。Complex: When you access a complex type you work on a reference to its value.
- object
- array
- function
JavaScript 复制代码
```
const foo = [1, 2];
const bar = foo;

bar[0] = 9;

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

二、变量声明（Reference）

2.1 固定变量避免用var声明，请用const。Useconst for all of your references; avoid usingvar. (.eslint: prefer-const, no-const-assign)

为什么？这样可以确保无法重新分配引用.
JavaScript 复制代码
```
// bad
var a = 1;
var b = 2;

// good
const a = 1;
const b = 2;
```
2.2 如果必须重新指定引用，请使用let 代替var 。If you must reassign references, uselet instead ofvar.（.eslint：no-var）

为什么？是块范围的，而不是函数范围的，如。let``var
JavaScript 复制代码
```
// bad
var count = 1;
if (true) {
  count += 1;
}

// good, use the let.
let count = 1;
if (true) {
  count += 1;
}
```
2.3 请注意，let和const都是块范围的，而var是函数范围的。Note that both letandconst are block-scoped, whereas varis function-scoped.
JavaScript 复制代码
```
// const and let only exist in the blocks they are defined in.
{
  let a = 1;
  const b = 1;
  var c = 1;
}
console.log(a); // ReferenceError
console.log(b); // ReferenceError
console.log(c); // Prints 1
```
在上面的代码中，您可以看到引用a and b将产生一个 ReferenceError，而c包含数字。这是因为a 和b 是块范围，而 c是包含函数的范围。

三、Object

3.1 请使用文字语法创建对象。 Use the literal syntax for object creation. eslint: no-new-object
JavaScript 复制代码
```
// bad
const item = new Object();

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

3.2 请使用动态属性名称创建对象时，使用计算属性名称。 Use computed property names when creating objects with dynamic property names.

Why? They allow you to define all the properties of an object in one place.（在一个位置定义对象所有属性。）
JavaScript 复制代码
```
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 请使用简写定义对象的方法属性。 Use object method shorthand. eslint: object-shorthand

JavaScript 复制代码

// 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 请使用简写定义属性值。 Use property value shorthand. eslint: object-shorthand

Why? It is shorter and descriptive. （短且描述性更强。）
JavaScript 复制代码
```
const lukeSkywalker = 'Luke Skywalker';

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

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

3.5 请将简写属性写在一处。 Group your shorthand properties at the beginning of your object declaration.

Why? It's easier to tell which properties are using the shorthand.

JavaScript 复制代码

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 引号只加在属于'非法标识符'的属性名上。 Only quote properties that are invalid identifiers. eslint: quote-props

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

为什么？一般来说，我们认为它主观上更容易阅读。它改进了语法高亮，并且也更容易被许多 JS 引擎优化。

JavaScript 复制代码

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

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

解释：标识符（Identifier）是JavaScript的重要概念，包括变量名、函数名、参数名、属性名、类名等等。

合法的标识符 有以下强制规则：

1. 第一个字符，可以是任意 Unicode 字母（包括英文字母和其他语言的字母），以及美元符号$和下划线_。不能是数字。

2. 第二个字符及后面的字符，除了 Unicode 字母、美元符号和下划线，还可以用数字0-9。

3. 标识符不能包含空格。

4. 标识符名称不能是 JavaScript 关键字或 JavaScript 保留字。

5. 标识符区分大小写。

例如：在上文代码示例中，foo和 bar是合法标识符，而data-blah 是非法标识符。

3.7 请不要直接通过对象实例 调用Object原型 上的方法。 Do not call Object.prototype methods directly, such as, hasOwnProperty,propertyIsEnumerable and isPrototypeOf. （eslint: no-prototype-builtins）

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)). In modern browsers that support ES2022, or with a polyfill such as npmjs.com/object.haso..., Object.hasOwncan also be used as an alternative to Object.prototype.hasOwnProperty.call.
解释：一开始使用Object.create(null)创建的对象实例是没有prototype的，所以没有任何原型方法，这种情况下调用原型方法只会得到undefined。而且对象实例的原型方法也可能会被{ hasOwnProperty: false } 这类的指令隐藏。

所以要使用Object.原型对象而不是对象实例.原型方法。

即使用Object.prototype.hasOwnProperty.call(object, key)或者Object.hasOwn(object, key)可以确保调用到正确的方法。
JavaScript 复制代码
```
// object为一个对象实例
// bad
console.log(object.hasOwnProperty(key));

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

// better
const has = Object.prototype.hasOwnProperty; // cache the lookup once, in module scope.
console.log(has.call(object, key));

// best
console.log(Object.hasOwn(object, key)); // only supported in browsers that support ES2022

/* or */
import has from 'has'; // https://www.npmjs.com/package/has
console.log(has(object, key));
/* or */
console.log(Object.hasOwn(object, key)); // https://www.npmjs.com/package/object.hasown
```

3.8 对象浅拷贝时，首选...展开运算符而非Object.assign。 Prefer the object spread syntax over Object.assign to shallow-copy objects. Use the object rest parameter syntax to get a new object with certain properties omitted. eslint: prefer-object-spread

浅拷贝时，使用 Object.assign()合并的对象与生成的对象之间可能会互相影响。而使用...相对稳定，不会互相影响。

JavaScript 复制代码

// 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 }

四、Array

4.1 请使用字面量语法创建数组，而非new Array()。 Use the literal syntax for array creation. eslint: no-array-constructor
js 复制代码
```
// bad
const items = new Array();

// good
const items = [];
```
4.2 请使用.push()而非直接赋值将项目添加到数组中。Use Array#push instead of direct assignment to add items to an array.
js 复制代码
```
const someStack = [];

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

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

4.3 请使用...浅拷贝数组。Use array spreads ... to copy arrays.

js 复制代码

// 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 要将可迭代对象转为数组，最好使用...代替Array.from。 To convert an iterable object to an array, use spreads ... instead of Array.from.
js 复制代码
```
const foo = document.querySelectorAll('.foo');

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

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

4.5 将类数组对象转换为数组，请使用Array.from。 Use Array.from for converting an array-like object to an array.

js 复制代码

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

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

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

4.6使用Array.from而非...+.map()，可避免创建中间数组。Use Array.from instead of spread ... for mapping over iterables, because it avoids creating an intermediate array.
js 复制代码
```
const foo=[1,3,5,2,6];
const bar=()=>{return 7;}
// bad
const baz = [...foo].map(bar);// [7,7,7,7,7]

// good
const baz = Array.from(foo, bar);// [7,7,7,7,7]
```

4.7请在书组回调方法中使用return。如果函数体由一条返回一个表达式的语句组成，并且这个表达式没有副作用，这个时候可以忽略return，遵循 8.2。 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）

js 复制代码

// 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;
});

4.8 如果一个数组有很多行，在数组的 [后和 ] 前换行。 Use line breaks after opening array brackets and before closing array brackets, if an array has multiple lines

js 复制代码

// bad
const arr = [
  [0, 1], [2, 3], [4, 5],
];
// bad
const objectInArray = [{
  id: 1,
}, {
  id: 2,
}];
// bad
const numberInArray = [
  1, 2,
];

// good
const arr = [[0, 1], [2, 3], [4, 5]];
// good
const objectInArray = [
  {
    id: 1,
  },
  {
    id: 2,
  },
];
// good
const numberInArray = [
  1,
  2,
];

五、解构

5.1在访问和使用对象的多个属性时请首选对象解构 。 Use object destructuring when accessing and using multiple properties of an object. （eslint: prefer-destructuring）

Why? Destructuring saves you from creating temporary references for those properties, and from repetitive access of the object. Repeating object access creates more repetitive code, requires more reading, and creates more opportunities for mistakes. Destructuring objects also provides a single site of definition of the object structure that is used in the block, rather than requiring reading the entire block to determine what is used.

通过解构，您可以避免为这些属性创建临时引用。
js 复制代码
```
// 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 首选数组解构。 Use array destructuring. eslint: prefer-destructuring

js 复制代码

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

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

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

5.3 对多个返回值使用对象解构，而不是数组解构。 Use object destructuring for multiple return values, not array destructuring.

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

不依赖于返回值的顺序，更可读

js 复制代码

// 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);

六、String

6.1 String类型请统一用单引号 ''。 Use single quotes '' for strings. eslint: quotes

js 复制代码

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

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

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

6.2 一个字符串不要通过连接符换行，超长也写在一行。 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.

不好用，且可读性差。

js 复制代码

// 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 用字符串模板而不是 + 来拼接字符串。 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.
js 复制代码
```
// 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 永远不要在字符串中用eval()，漏洞太多。 Never use eval() on a string; it opens too many vulnerabilities. （eslint: no-eval）

6.5 不要使用不必要的转义字符。Do not unnecessarily escape characters in strings. （eslint: no-useless-escape）

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

反斜线可读性差，只在必要时使用
js 复制代码
```
// bad
const foo = ''this' \i\s "quoted"';

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

七、Function

7.1 用命名函数表达式而不是函数声明。 Use named function expressions instead of function declarations. （eslint: func-style, func-names）

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)

函数声明作用域会提升，降低了代码可读性和可维护性。如果你发现一个函数又大又复杂，这个函数妨碍这个文件其他部分的理解性，这可能就是时候把这个函数单独抽成一个模块了。
js 复制代码
```
// bad 函数声明
function foo() {
  // ...
}

// bad 匿名函数表达式
const foo = function () {
  // ...
};

// good 命名函数表达式
// 函数表达式名和声明的函数名是不一样的
const short = function longUniqueMoreDescriptiveLexicalFoo() {
  // ...
};
```
7.2 将立即执行函数包裹在圆括号里。Wrap immediately invoked function expressions in parentheses. eslint: wrap-iife

日常开发中，很少用到立即执行函数
js 复制代码
```
// immediately-invoked function expression (IIFE)
(function () {
  console.log('Welcome to the Internet. Please follow me.');
}());
```
7.3不要在非函数块（if、while等等）内声明函数。而是把这个函数分配给一个变量。浏览器会允许你这样做，但浏览器解析方式不同，结果也许会有差异。 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）

js 复制代码

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

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

7.4注意: 在ECMA-262中 [块 block] 的定义是：一系列的语句；但是函数声明不是一个语句。函数表达式是一个语句。 Note: ECMA-262 defines a block as a list of statements. A function declaration is not a statement.

7.5 永远不要用arguments命名参数。它的优先级高于每个函数作用域自带的 arguments 对象，所以会导致函数自带的 arguments 值被覆盖。 Never name a parameter arguments. This will take precedence over the arguments object that is given to every function scope.
js 复制代码
```
// bad
function foo(name, options, arguments) {
  // ...
}

// good
function foo(name, options, args) {
  // ...
}
```
7.6优先使用rest语法...，而不是 arguments。 Never use arguments, opt to use rest syntax ... instead. （eslint: prefer-rest-params）

Why? ... is explicit about which arguments you want pulled. Plus, rest arguments are a real Array, and not merely Array-like like arguments.

... 更明确你想用哪些参数。
js 复制代码
```
// bad
function concatenateAll() {
  const args = Array.prototype.slice.call(arguments);
  return args.join('');
}

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

7.7 使用默认参数语法，而不是在函数里对参数重新赋值改变函数参数。Use default parameter syntax rather than mutating function arguments.

js 复制代码

// 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.
  // 虽然你想这么写， 但是这个会带来一些细微的bug
  // 如果 opts 的值为 false, 它会被赋值为 {}
  
  opts = opts || {};
  // ...
}

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

// good 默认参数语法
function handleThings(opts = {}) {
  // ...
}

7.8 使用默认参数时，需要避免副作用。 Avoid side effects with default parameters.

避免使用令人困惑的默认参数。
js 复制代码
```
let b = 1;
// bad
function count(a = b++) {
  console.log(a);
}
count();  // 1
count();  // 2
count(3); // 3
count();  // 3
```

7.9 始终将默认参数放在最后。 Always put default parameters last. （eslint: default-param-last）
js 复制代码
```
// bad
function handleThings(opts = {}, name) {
  // ...
}

// good
function handleThings(name, opts = {}) {
  // ...
}
```
7.10 不要用 Function 创建函数。Never use the Function constructor to create a new function. （eslint: no-new-func）

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

类似于 eval() ，可能会产生漏洞，加大XSS攻击风险。
js 复制代码
```
// bad
const add = new Function('a', 'b', 'return a + b');

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

7.11 函数签名部分要有空格。Spacing in a function signature. （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.

可保证一致性，在添加或删除名称时不必添加或删除空格。
js 复制代码
```
// bad
const f = function(){};
const g = function (){};
const h = function() {};

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

7.12 永远不要改参数。Never mutate parameters. （eslint: no-param-reassign）

Why? Manipulating objects passed in as parameters can cause unwanted variable side effects in the original caller.
js 复制代码
```
// bad
function f1(obj) {
  obj.key = 1;
}

// good
function f2(obj) {
  const key = Object.prototype.hasOwnProperty.call(obj, 'key') ? obj.key : 1;
}
```
7.13 不要对参数重新赋值。Never reassign parameters. eslint: no-param-reassign

Why? Reassigning parameters can lead to unexpected behavior, especially when accessing the arguments object. It can also cause optimization issues, especially in V8.
js 复制代码
```
// 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 首选 ...。Prefer the use of the spread syntax ... to call variadic functions. （eslint: prefer-spread）

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

js 复制代码

// 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 具有多行签名或调用的函数应像本指南中的其他多行列表一样缩进：每个项目单独在一行上，最后一项上有一个逗号。Functions with multiline signatures, or invocations, should be indented just like every other multiline list in this guide: with each item on a line by itself, with a trailing comma on the last item. （eslint: function-paren-newline）

js 复制代码

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

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

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

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

八、箭头函数

8.1 果要用匿名函数做回调，最好使用箭头函数。When you must use an anonymous function (as when passing an inline callback), use arrow function notation. （eslint: prefer-arrow-callback, arrow-spacing）

简洁
js 复制代码
```
// 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 如果函数体由一个没有副作用的单个语句组成，去掉大括号和 return。否则，保留大括号且使用 return 语句。If the function body consists of a single statement returning an expression without side effects, omit the braces and use the implicit return. Otherwise, keep the braces and use a return statement. （eslint: arrow-parens, arrow-body-style）

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

语法糖，可读性更好。

js 复制代码

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

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

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

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

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

let bool = false;

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

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

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

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

它更清楚地显示了函数的开始和结束位置。

js 复制代码

// 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 为了清晰和一致，始终在参数周围加上括号。Always include parentheses around arguments for clarity and consistency. （eslint: arrow-parens）

Why? Minimizes diff churn when adding or removing arguments.

为什么？在添加或删除参数时最大限度地减少差异改动。

js 复制代码

// 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;
});

8.5 避免箭头函数语法 => 和比较操作符 <=, >= 混淆。Avoid confusing arrow function syntax (=>) with comparison operators (<=, >=). （eslint: no-confusing-arrow）

js 复制代码

// 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;
};

8.6 使用隐式返回时强制约束函数体在箭头后面。Enforce the location of arrow function bodies with implicit returns. （eslint: implicit-arrow-linebreak）
js 复制代码
```
// bad
(foo) =>
  bar;

(foo) =>
  (bar);

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

九、Classes & Constructors

9.1 始终用class，避免直接操作prototype。Always use class. Avoid manipulating prototype directly.

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

语法更简洁可读

js 复制代码

// 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 使用extends实现继承。Use extends for inheritance.

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

它是一种在不破坏instanceof的情况下继承原型功能的内置方法。

js 复制代码

// 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 方法可以返回this来实现方法链。Methods can return this to help with method chaining.

js 复制代码

// 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 允许写一个自定义的 toString() 方法，但是要保证它是可以正常工作且没有副作用。It's okay to write a custom toString() method, just make sure it works successfully and causes no side effects.
js 复制代码
```
class Jedi {
  constructor(options = {}) {
    this.name = options.name || 'no name';
  }

  getName() {
    return this.name;
  }

  toString() {
    return `Jedi - ${this.getName()}`;
  }
}
```
9.5 如果没有特殊说明，类有默认的构造方法。不用特意写一个空的构造函数或只是代表父类的构造函数。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）
js 复制代码
```
// 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 避免重复的类成员。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.

重复类成员会默默的执行最后一个，有重复肯定就是一个错误
js 复制代码
```
// bad
class Foo {
  bar() { return 1; }
  bar() { return 2; }
}

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

// good
class Foo {
  bar() { return 2; }
}
```
9.7 类方法应使用this或编写为静态方法，除非外部库或框架要求使用特定的非静态方法。作为实例方法，应表明它的行为基于接收器的属性而有所不同。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）
js 复制代码
```
// 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');
  }
}
```

十、Modules

10.1 在非标准模块系统上使用(import/export)。或者随时换成其他的首选模块系统。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.
js 复制代码
```
// 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 不要用 import * 这种通配符。Do not use wildcard imports.

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

这样可以确保您有一个默认导出。
js 复制代码
```
// bad
import * as AirbnbStyleGuide from './AirbnbStyleGuide';

// good
import AirbnbStyleGuide from './AirbnbStyleGuide';
```
10.3 不要直接从import中export。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.

虽然单行很简洁，但有一种明确的导入方式和一种明确的导出方式可以使代码风格保持一致。
js 复制代码
```
// bad
// filename es6.js
export { es6 as default } from './AirbnbStyleGuide';

// good
// filename es6.js
import { es6 } from './AirbnbStyleGuide';
export default es6;
```
10.4 一个入口只 import 一次。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.

从同一个路径下import多行会使代码难以维护
js 复制代码
```
// 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 不要导出可变的绑定，尽量导出常量。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. 虽然在某些场景下可能需要这种技术，但总的来说应该尽量导出常量。
js 复制代码
```
// bad
let foo = 3;
export { foo };

// good
const foo = 3;
export { foo };
```
10.6 在只有一个导出的模块里，用 export default 更好。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.

鼓励使用更多文件，每个文件只做一件事情并导出，这样可读性和可维护性更好。
js 复制代码
```
// bad
export function foo() {}

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

10.7 import 放在其他所有语句之前。Put all imports above non-import statements. eslint: import/first

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

防止意外行为。
js 复制代码
```
// bad
import foo from 'foo';
foo.init();

import bar from 'bar';

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

foo.init();
```
10.8 多行 import 应该缩进，就像多行数组和对象字面量。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.
js 复制代码
```
// bad
import {longNameA, longNameB, longNameC, longNameD, longNameE} from 'path';

// good
import {
  longNameA,
  longNameB,
  longNameC,
  longNameD,
  longNameE,
} from 'path';
```
10.9 在 import 语句里不允许 Webpack loader 语法。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.

最好是在webpack.config.js里写
js 复制代码
```
// 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 导入应不包括 JavaScript 文件扩展名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.

包含扩展名会抑制重构，并且不恰当地对要导入的模块的实现细节进行硬编码。
js 复制代码
```
// 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';
```

十一、Iterators and Generators

11.1 不要用迭代器。用 JavaScript 高阶函数代替for-in、 for-of。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.

js 复制代码

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 现在不要用 generator。Don't use generators for now.

Why? They don't transpile well to ES5.

兼容性不好

11.3 如果你一定要用generator，或者你忽略我们的建议, 请确保它们的函数签名之间的空格是正确的。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.

function 和 * 是同一概念，关键字 *不是function的修饰符，function*是一个和function不一样的独特结构
js 复制代码
```
// 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* () {
  // ...
};
```

十二、Properties

12.1 访问属性时使用点符号。Use dot notation when accessing properties. （eslint: dot-notation）
js 复制代码
```
const luke = {
  jedi: true,
  age: 28,
};

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

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

12.2 获取的属性是变量时用方括号。Use bracket notation [] when accessing properties with a variable.

js 复制代码

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

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

const isJedi = getProp('jedi');

12.3 做幂运算时用幂操作符 ** 。Use exponentiation operator ** when calculating exponentiations. （eslint: prefer-exponentiation-operator.）
js 复制代码
```
// bad
const binary = Math.pow(2, 10);

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

十三、Variables

13.1 始终用 const 或 let 声明变量。如果你不想遇到变量提升、全局变量的 bug 的话。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）
js 复制代码
```
// bad
superPower = new SuperPower();

// good
const superPower = new SuperPower();
```
13.2 每个变量单独用一个 const 或 let。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.

方便调试，可以使用调试器逐步完成每个声明，而不是一次跳过所有声明。
js 复制代码
```
// 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 const放一起，let放一起。 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.

js 复制代码

// 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 变量声明放在合理的位置（在需要的时候分配）。Assign variables where you need them, but place them in a reasonable place.

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

js 复制代码

// 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
// 在需要的时候分配
function checkName(hasName) {
  if (hasName === 'test') {
    return false;
  }

  const name = getName();

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

  return name;
}

13.5 不要使用链式变量分配。Don't chain variable assignments. eslint: no-multi-assign

Why? Chaining variable assignments creates implicit global variables.

链式变量分配会创建隐式全局变量。

js 复制代码

// bad
(function example() {
  let a = b = c = 1;
// JavaScript 将其解释为
// let a = ( b = ( c = 1 ) );
// let 只对变量 a 起作用; 变量 b 和 c 都变成了全局变量

}());

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 不要使用一元递增递减运算符（++， --）。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.

根据 eslint 文档，一元递增和递减语句受到自动分号插入的影响，并且可能会导致应用程序中的值递增或递减的静默错误。使用num += 1 而不是 num++ 或代替语句来改变你的值也更具表现力。禁止一元递增和递减语句也会阻止您无意中预先递增/预递减值，从而减少程序出现意外行为。
js 复制代码
```
// 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 避免在 = 前/后换行。如果你的语句超出 max-len，那就用()把这个值包起来再换行。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. =周围的换行符可能会混淆赋的值。
js 复制代码
```
// bad
const foo =
  superLongLongLongLongLongLongLongLongFunctionName();

// bad
const foo
  = 'superLongLongLongLongLongLongLongLongString';

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

// good
const foo = 'superLongLongLongLongLongLongLongLongString';
```

13.8 不允许有未使用的变量。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.

在代码中声明但未在代码中的任何位置使用的变量很可能是由于重构不完整而导致的错误。此类变量会占用代码中的空间，并可能导致读者混淆。

js 复制代码

// bad

const some_unused_var = 42;

// 定义却未使用
let y = 10;
y = 5;

// 用于修改自身的读取不算已使用
let z = 0;
z = z + 1;

// 参数定义了却未使用
function getX(x, y) {
    return x;
}

// good

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

const x = 1;
const 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.
const { type, ...coords } = data;
// 'coords' is now the 'data' object without its 'type' property.

十四、Hoisting

14.1 var 声明被提升。const 和 let 声明被赋予一个所谓的新概念Temporal Dead Zones (TDZ)。重要的是要知道为什么 typeof不再安全。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.

js 复制代码

// 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 匿名函数表达式和 var 情况相同。Anonymous function expressions hoist their variable name, but not the function assignment.

js 复制代码

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

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

  var anonymous = function () {
    console.log('anonymous function expression');
  };
}

14.3 已命名的函数表达式提升他的变量名，而不是函数名或函数体。Named function expressions hoist the variable name, not the function name or the function body.

js 复制代码

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 函数声明则提升了函数名和函数体Function declarations hoist their name and the function body.
js 复制代码
```
function example() {
  superPower(); // => Flying

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

14.5 变量、类和函数在使用之前应先定义。Variables, classes, and functions should be defined before they can be used. （eslint: no-use-before-define）

Why? When variables, classes, or functions are declared after being used, it can harm readability since a reader won't know what a thing that's referenced is. It's much clearer for a reader to first encounter the source of a thing (whether imported from another module, or defined in the file) before encountering a use of the thing.

当变量、类或函数在使用后被声明时，它可能会损害可读性，因为读者不会知道所引用的事物是什么。

js 复制代码

// bad

// Variable a is being used before it is being defined.
console.log(a); // this will be undefined, since while the declaration is hoisted, the initialization is not
var a = 10;

// Function fun is being called before being defined.
fun();
function fun() {}

// Class A is being used before being defined.
new A(); // ReferenceError: Cannot access 'A' before initialization
class A {
}

// `let` and `const` are hoisted, but they don't have a default initialization.
// The variables 'a' and 'b' are in a Temporal Dead Zone where JavaScript
// knows they exist (declaration is hoisted) but they are not accessible
// (as they are not yet initialized).

console.log(a); // ReferenceError: Cannot access 'a' before initialization
console.log(b); // ReferenceError: Cannot access 'b' before initialization
let a = 10;
const b = 5;


// good

var a = 10;
console.log(a); // 10

function fun() {}
fun();

class A {
}
new A();

let a = 10;
const b = 5;
console.log(a); // 10
console.log(b); // 5

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

十五、比较和相等运算符 Comparison Operators & Equality

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

15.2 if 等条件语句使用强制 ToBoolean抽象方法来评估它们的表达式，并且始终遵循以下简单规则：非值：Undefined、Null、+0、-0、NaN、''、false。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
js 复制代码
```
if ([0] && []) {
  // true
  // an array (even an empty one) is an object, objects will evaluate to true
}
```

15.3 布尔值比较可以省略，但是字符串和数字要显示比较。Use shortcuts for booleans, but explicit comparisons for strings and numbers.

js 复制代码

// 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 switch case 中，在 case 和 default 分句里用大括号创建一个块(如：let, const, function, and class).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.

词汇声明在整个 switch 块中都是可见的，但只有在分配时才会被初始化，这只有在 case 达到时才会发生。当多个 case 子句尝试定义相同的事物时，会出现问题。
js 复制代码
```
// 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 三元表达式不应该嵌套，通常是单行表达式。Ternaries should not be nested and generally be single line expressions. eslint: no-nested-ternary

js 复制代码

// 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 避免不需要的三元表达式。Avoid unneeded ternary statements. eslint: no-unneeded-ternary

js 复制代码

// bad
const foo = a ? a : b;
const bar = c ? true : false;
const baz = c ? false : true;
const quux = a != null ? a : b;

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

15.8 混合操作符时，要放在 () 里，只有当它们是标准的算术运算符(+, -, *, & /)，并且它们的优先级显而易见时，可以不用。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.
js 复制代码
```
// 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;
```
15.9 无效合并运算符（??）是一个逻辑运算符，当其左侧操作数为 null 或undefined 时，它返回其右侧操作数。否则，它将返回左侧操作数。The nullish coalescing operator (??) is a logical operator that returns its right-hand side operand when its left-hand side operand is null or undefined. Otherwise, it returns the left-hand side operand.

Why? It provides precision by distinguishing null/undefined from other falsy values, enhancing code clarity and predictability.
js 复制代码
```
// bad
const value = 0 ?? 'default';
// returns 0, not 'default'

// bad
const value = '' ?? 'default';
// returns '', not 'default'

// good
const value = null ?? 'default';
// returns 'default'

// good
const user = {
  name: 'John',
  age: null
};
const age = user.age ?? 18;
// returns 18
```

十六、块（Blocks）

16.1 用大括号 {} 包裹多行代码块。Use braces with all multiline blocks. （eslint: nonblock-statement-body-position）

js 复制代码

// 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 else 和 if 的大括号保持在一行。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）
js 复制代码
```
// bad
if (test) {
  thing1();
  thing2();
}
else {
  thing3();
}

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

16.3 如果 if 语句都要用 return 返回，那后面的 else 就不用写了。如果 if 块中包含 return，它后面的 else if 块中也包含了 return，这个时候就可以把 else if 拆开。 eslint: no-else-returnIf 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

js 复制代码

// 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;
  }
}

十七、控制语句 Control Statements

17.1 当你的控制语句 if, while 等太长或者超过最大长度限制的时候，把每个判断条件放在单独一行里，逻辑运算符放在行首。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.
js 复制代码
```
// 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 不要用选择操作符代替控制语句。Don't use selection operators in place of control statements.
js 复制代码
```
// bad
!isRunning && startRunning();

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

十八、注释 Comments

18.1 多行注释用 /** ... */。Use /** ... */ for multiline comments.

js 复制代码

// 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 单行注释用//，将单行注释放在被注释区域上方。如果注释不是在第一行，就在注释前面加一个空行。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.

js 复制代码

// 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 所有注释开头加一个空格，方便阅读。Start all comments with a space to make it easier to read. *（eslint: spaced-comment）

js 复制代码

// 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 在注释前加上 FIXME 或TODO 前缀，这有助于其他开发人员快速理解你指出的问题，或者您建议的问题的解决方案。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.

js 复制代码

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

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

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

js 复制代码

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

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

十九、空格 Whitespace

19.1 Tab 使用两个空格 ~~（或者 4 个，你开心就好，但是团队统一是必须的）~~ 。Use soft tabs (space character) set to 2 spaces. （eslint: indent）
csharp 复制代码
```
// bad
function foo() {
∙∙∙∙let name;
}

// bad
function bar() {
∙let name;
}

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

19.2 在大括号 {} 前空一格。Place 1 space before the leading brace. eslint: space-before-blocks

js 复制代码

// 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 在控制语句 if, while 等的圆括号前空一格。在函数调用和定义时，函数名和圆括号之间不空格。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
js 复制代码
```
// bad
if(isJedi) {
  fight ();
}

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

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

// good
function fight() {
  console.log('Swooosh!');
}
```
19.4 用空格来隔开运算符。Set off operators with spaces. eslint: space-infix-ops
js 复制代码
```
// bad
const x=y+5;

// good
const x = y + 5;
```
19.5 文件结尾空一行。End files with a single newline character. （eslint: eol-last）

暂不采纳

js 复制代码

// 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 当出现长的方法链（一般超过两个的时候）时换行。用点开头强调该行是一个方法调用，而不是一个新的语句。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

js 复制代码

// 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 在一个代码块之后，下一条语句之前空一行。Leave a blank line after blocks and before the next statement.

js 复制代码

// 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 不要故意留一些没必要的空白行。Do not pad your blocks with blank lines. eslint: padded-blocks

js 复制代码

// bad
function bar() {

  console.log(foo);

}

// bad
if (baz) {

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

}

// bad
class Foo {

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

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

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

19.9 不要加多行空格。Do not use multiple blank lines to pad your code. （eslint: no-multiple-empty-lines）

js 复制代码

// 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 圆括号里不要加空格。Do not add spaces inside parentheses. （eslint: space-in-parens）

js 复制代码

// 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 方括号里不要加空格。Do not add spaces inside brackets. （eslint: array-bracket-spacing）
js 复制代码
```
// bad
const foo = [ 1, 2, 3 ];
console.log(foo[ 0 ]);

// good
const foo = [1, 2, 3];
console.log(foo[0]);
```
19.12 花括号 {} 里加空格。Add spaces inside curly braces. eslint: object-curly-spacing
js 复制代码
```
// bad
const foo = {clark: 'kent'};

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

19.13 避免一行代码超过 100 个字符，需换行。（但包含空格、纯字符串就不要换行了）。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.

js 复制代码

// 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;

// better
const foo = 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 同行间距一致。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）
js 复制代码
```
// bad
function foo() {return true;}
if (foo) { bar = 0;}

// good
function foo() { return true; }
if (foo) { bar = 0; }
```
19.15 逗号前避免使用空格，逗号后要求有空格。Avoid spaces before commas and require a space after commas. （eslint: comma-spacing）
js 复制代码
```
// bad
const foo = 1,bar = 2;
const arr = [1 , 2];

// good
const foo = 1, bar = 2;
const arr = [1, 2];
```
19.16 在计算的属性括号内强制使用间距。Enforce spacing inside of computed property brackets. （eslint: computed-property-spacing）
js 复制代码
```
// bad
obj[foo ]
obj[ 'foo']
const x = {[ b ]: a}
obj[foo[ bar ]]

// good
obj[foo]
obj['foo']
const x = { [b]: a }
obj[foo[bar]]
```
19.17 避免在函数及其调用之间使用空格。Avoid spaces between functions and their invocations. eslint: func-call-spacing
js 复制代码
```
// bad
func ();

func
();

// good
func();
```
19.18 在对象的属性中，键和值之间要有空格。Enforce spacing between keys and values in object literal properties. eslint: key-spacing
js 复制代码
```
// bad
const obj = { foo : 42 };
const obj2 = { foo:42 };

// good
const obj = { foo: 42 };
```
19.19 行末不要空格。Avoid trailing spaces at the end of lines. （eslint: no-trailing-spaces）

19.20 避免出现多个空行。在文件末尾只允许空一行。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
js 复制代码
```
// bad - multiple empty lines
const x = 1;


const y = 2;

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


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

const x = 1;
const y = 2;

// good
const x = 1;
const y = 2;
```

二十、逗号 Commas

20.1 不要前置逗号。Leading commas: Nope. （eslint: comma-style）

js 复制代码

// 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 结尾额外加逗号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.

js 复制代码

// 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
);

二十一、分号 Semicolons

21.1 明确地使用分号。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.

为什么？当 JavaScript 遇到没有分号的换行符时，它会使用一组称为自动插入分号的规则来确定是否应该将该换行符视为语句的结尾，并且（顾名思义）在代码中放入分号之前，如果它认为是这样的话。但是，ASI 包含一些古怪的行为，如果 JavaScript 误解了换行符，您的代码就会中断。随着新功能成为 JavaScript 的一部分，这些规则将变得更加复杂。显式终止语句并配置 linter 以捕获缺少的分号将有助于防止遇到问题。
js 复制代码
```
// 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';
}
```

Airbnb JavaScript 风格指南-上篇

Airbnb JavaScript 风格指南

一、变量类型（Types）

二、变量声明（Reference）

三、Object

四、Array

五、解构

六、String

七、Function

八、箭头函数

九、Classes & Constructors

十、Modules

十一、Iterators and Generators

十二、Properties

十三、Variables

十四、Hoisting

十五、比较和相等运算符 Comparison Operators & Equality

十六、块 （Blocks）

十七、控制语句 Control Statements

十八、注释 Comments

十九、空格 Whitespace

二十、逗号 Commas

二十一、分号 Semicolons

十六、块（Blocks）