`function*` 这种声明方式(`function`关键字后跟一个星号）会定义一个生成器函数 (generator function)，它返回一个 `Generator` 对象。

The source for this interactive example is stored in a GitHub repository. If you'd like to contribute to the interactive examples project, please clone https://github.com/mdn/interactive-examples and send us a pull request.

你也可以使用构造函数 `GeneratorFunction` 或 `function* expression` 定义生成器函数 。

## 语法

    function* name([param[, param[, ... param]]]) { statements }

`name`
  函数名

`param`
  要传递给函数的一个参数的名称，一个函数最多可以有255个参数。

`statements`
  普通JS语句。

## 描述

生成器函数在执行时能暂停，后面又能从暂停处继续执行。

调用一个生成器函数并不会马上执行它里面的语句，而是返回一个这个生成器的 迭代器 （iterator ）对象。当这个迭代器的 `next()` 方法被首次（后续）调用时，其内的语句会执行到第一个（后续）出现`yield`的位置为止，`yield` 后紧跟迭代器要返回的值。或者如果用的是 `yield*`（多了个星号），则表示将执行权移交给另一个生成器函数（当前生成器暂停执行）。

`next()`方法返回一个对象，这个对象包含两个属性：value 和 done，value 属性表示本次 `yield` 表达式的返回值，done 属性为布尔类型，表示生成器后续是否还有`yield` 语句，即生成器函数是否已经执行完毕并返回。

调用 `next()`方法时，如果传入了参数，那么这个参数会作为上一条执行的 yield 语句的返回值，例如：

    function *gen(){
        yield 10;
        y=yield 'foo';
        yield y;
    }

    var gen_obj=gen();
    console.log(gen_obj.next());// 执行 yield 10，返回 10
    console.log(gen_obj.next());// 执行 yield 'foo'，返回 'foo'
    console.log(gen_obj.next(10));// 将 10 赋给上一条 yield 'foo' 的左值，即执行 y=10，返回 10
    console.log(gen_obj.next());// 执行完毕，value 为 undefined，done 为 true

当在生成器函数中显式 `return` 时，会导致生成器立即变为完成状态，即调用 `next()` 方法返回的对象的 `done` 为 `true`。如果 `return` 后面跟了一个值，那么这个值会作为当前调用 `next()` 方法返回的 value 值。

## 示例

### 简单示例

    function* idMaker(){
      var index = 0;
      while(index<3)
        yield index++;
    }

    var gen = idMaker();
    console.log(gen.next().value); // 0
    console.log(gen.next().value); // 1
    console.log(gen.next().value); // 2
    console.log(gen.next().value); // undefined

### 生成器也可以接收参数：

    function* idMaker(){
        var index = arguments[0] || 0;
        while(true)
            yield index++;
    }

    var gen = idMaker(5);
    console.log(gen.next().value); // 5
    console.log(gen.next().value); // 6

### yield*的示例

    function* anotherGenerator(i) {
      yield i + 1;
      yield i + 2;
      yield i + 3;
    }

    function* generator(i){
      yield i;
      yield* anotherGenerator(i);// 移交执行权
      yield i + 10;
    }

    var gen = generator(10);

    console.log(gen.next().value); // 10
    console.log(gen.next().value); // 11
    console.log(gen.next().value); // 12
    console.log(gen.next().value); // 13
    console.log(gen.next().value); // 20

### 传递参数

    function *createIterator() {
        let first = yield 1;
        let second = yield first + 2; // 4 + 2
                                      // first =4 是next(4)将参数赋给上一条的
        yield second + 3;             // 5 + 3
    }

    let iterator = createIterator();

    console.log(iterator.next());    // "{ value: 1, done: false }"
    console.log(iterator.next(4));   // "{ value: 6, done: false }"
    console.log(iterator.next(5));   // "{ value: 8, done: false }"
    console.log(iterator.next());    // "{ value: undefined, done: true }"

### 显式返回

    function* yieldAndReturn() {
      yield "Y";
      return "R";//显式返回处，可以观察到 done 也立即变为了 true
      yield "unreachable";// 不会被执行了
    }

    var gen = yieldAndReturn()
    console.log(gen.next()); // { value: "Y", done: false }
    console.log(gen.next()); // { value: "R", done: true }
    console.log(gen.next()); // { value: undefined, done: true }

### 生成器函数不能当构造器使用

    function* f() {}
    var obj = new f; // throws "TypeError: f is not a constructor"

### 使用迭代器遍历二维数组并转换成一维数组：

>     function* iterArr(arr) {            //迭代器返回一个迭代器对象
>       if (Array.isArray(arr)) {         // 内节点
>           for(let i=0; i < arr.length; i++) {
>               yield* iterArr(arr[i]);   // (*)递归
>           }
>       } else {                          // 离开
>           yield arr;
>       }
>     }
>     // 使用 for-of 遍历:
>     var arr = ['a', ['b', 'c'], ['d', 'e']];
>     for(var x of iterArr(arr)) {
>             console.log(x);               // a  b  c  d  e
>      }

>     // 或者直接将迭代器展开:
>     var arr = [ 'a', ['b',[ 'c', ['d', 'e']]]];
>     var gen = iterArr(arr);
>     arr = [...gen];                        // ["a", "b", "c", "d", "e"]

## 规范

Specification | Status | Comment
--- | --- | ---
ECMAScript 2015 (6th Edition, ECMA-262) function* | Standard | Initial definition.
ECMAScript Latest Draft (ECMA-262) function* | Draft |

## 浏览器兼容性

The compatibility table on this page is generated from structured data. If you'd like to contribute to the data, please check out https://github.com/mdn/browser-compat-data and send us a pull request.

Update compatibility data on GitHub

| Desktop | Mobile | Server
| Chrome | Edge | Firefox | Internet Explorer | Opera | Safari | Android webview | Chrome for Android | Edge Mobile | Firefox for Android | Opera for Android | Safari on iOS | Samsung Internet | Node.js
`function*` | Chrome Full support 39 | Edge Full support 13 | Firefox Full support 26 | IE No support No | Opera Full support 26 | Safari Full support 10 | WebView Android Full support Yes | Chrome Android Full support 39 | Edge Mobile Full support Yes | Firefox Android Full support 26 | Opera Android Full support Yes | Safari iOS Full support 10 | Samsung Internet Android Full support 4.0 | nodejs Full support 4.0.0 Full support 4.0.0 Full support 0.12 Disabled Disabled From version 0.12: this feature is behind the `--harmony` runtime flag.
`IteratorResult` object instead of throwing | Chrome Full support 49 | Edge Full support 13 | Firefox Full support 29 | IE No support No | Opera Full support Yes | Safari Full support Yes | WebView Android Full support Yes | Chrome Android Full support Yes | Edge Mobile Full support Yes | Firefox Android Full support 29 | Opera Android Full support Yes | Safari iOS Full support Yes | Samsung Internet Android Full support Yes | nodejs Full support Yes
Not constructable with `new` (ES2016) | Chrome Full support Yes | Edge ? | Firefox Full support 43 | IE No support No | Opera Full support Yes | Safari Full support 10 | WebView Android Full support Yes | Chrome Android Full support Yes | Edge Mobile ? | Firefox Android Full support 43 | Opera Android Full support Yes | Safari iOS Full support 10 | Samsung Internet Android Full support Yes | nodejs Full support Yes
Trailing comma in parameters | Chrome ? | Edge ? | Firefox Full support 52 | IE No support No | Opera Full support Yes | Safari ? | WebView Android ? | Chrome Android ? | Edge Mobile ? | Firefox Android Full support 52 | Opera Android ? | Safari iOS ? | Samsung Internet Android ? | nodejs Full support 8.0.0

### Legend

Full support
  Full support
No support
  No support
Compatibility unknown
  Compatibility unknown
User must explicitly enable this feature.
  User must explicitly enable this feature.

## Firefox浏览器具体事项

#### Firefox 26之前的生成器和迭代器

旧版本的Firefox实施了旧版本的生成器提案。旧版中用普通的`function关键字定义`（没有星号）.

#### `IteratorResult`不再抛出错误

从Gecko 29 (Firefox 29 / Thunderbird 29 / SeaMonkey 2.26)开始，完成的生成器函数不再抛出`TypeError` "generator has already finished". 而是返回一个`IteratorResult`对象：{ value: undefined, done: true } (bug 958951)。

## 相关链接

- `function* expression`
- `GeneratorFunction` object
- 迭代器协议
- `yield`
- `yield*`
- `Function` object
- `function declaration`
- `function expression`
- `Functions and function scope`
- 其他网络资源:
  - Regenerator an ES2015 generator compiler to ES5
  - Forbes Lindesay: Promises and Generators: control flow utopia -- JSConf EU 2013
  - Hemanth.HM: The New gen of *gen(){}
  - Task.js