yield*

yield* 表达式用于委托给另一个generator 或可迭代对象。

语法

 yield* [[expression]];

expression: 返回一个可迭代对象的表达式。

描述

yield* 表达式迭代操作数，并产生它返回的每个值。

yield* 表达式本身的值是当迭代器关闭时返回的值（即done为true时）。

示例

委托给其他生成器

以下代码中，g1() yield 出去的每个值都会在 g2() 的 next() 方法中返回，就像那些 yield 语句是写在 g2() 里一样。

function* g1() {
  yield 2;
  yield 3;
  yield 4;
}

function* g2() {
  yield 1;
  yield* g1();
  yield 5;
}

var iterator = g2();

console.log(iterator.next()); // { value: 1, done: false }
console.log(iterator.next()); // { value: 2, done: false }
console.log(iterator.next()); // { value: 3, done: false }
console.log(iterator.next()); // { value: 4, done: false }
console.log(iterator.next()); // { value: 5, done: false }
console.log(iterator.next()); // { value: undefined, done: true }

委托给其他可迭代对象

除了生成器对象这一种可迭代对象，yield* 还可以 yield 其它任意的可迭代对象，比如说数组、字符串、arguments 对象等等。

function* g3() {
  yield* [1, 2];
  yield* "34";
  yield* arguments;
}

var iterator = g3(5, 6);

console.log(iterator.next()); // { value: 1, done: false }
console.log(iterator.next()); // { value: 2, done: false }
console.log(iterator.next()); // { value: "3", done: false }
console.log(iterator.next()); // { value: "4", done: false }
console.log(iterator.next()); // { value: 5, done: false }
console.log(iterator.next()); // { value: 6, done: false }
console.log(iterator.next()); // { value: undefined, done: true }

`yield*` 表达式的值

yield* 是一个表达式，不是语句，所以它会有自己的值。

function* g4() {
  yield* [1, 2, 3];
  return "foo";
}

var result;

function* g5() {
  result = yield* g4();
}

var iterator = g5();

console.log(iterator.next()); // { value: 1, done: false }
console.log(iterator.next()); // { value: 2, done: false }
console.log(iterator.next()); // { value: 3, done: false }
console.log(iterator.next()); // { value: undefined, done: true }, 
                              // 此时 g4() 返回了 { value: "foo", done: true }

console.log(result);          // "foo"

规范

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

浏览器兼容

Update compatibility data on GitHub

	Desktop						Mobile						Server
	Chrome	Edge	Firefox	Internet Explorer	Opera	Safari	Android webview	Chrome for Android	Firefox for Android	Opera for Android	Safari on iOS	Samsung Internet	Node.js
`yield*`	Chrome Full support 39	Edge Full support 12	Firefox Full support 27 Notes Full support 27 Notes Notes Starting with Firefox 33, the parsing of the `yield` expression has been updated to conform with the ES2015 specification.	IE No support No	Opera Full support 26	Safari Full support 10	WebView Android Full support 39	Chrome Android Full support 39	Firefox Android Full support 27 Notes Full support 27 Notes Notes Starting with Firefox 33, the parsing of the `yield` expression has been updated to conform with the ES2015 specification.	Opera Android Full support 26	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.

Legend

Full support: Full support
No support: No support
See implementation notes.: See implementation notes.
User must explicitly enable this feature.: User must explicitly enable this feature.

Firefox 特别提示

从 Gecko 33 (Firefox 33 / Thunderbird 33 / SeaMonkey 2.30) 开始，yield表达式的解析已被更新以符合ES2015规范（bug 981599）：
- 现在有行结束符限制。在 yield 和 * 之间不允许有换行符。如下代码会抛出SyntaxError：
```
function* foo() {
  yield
  *[];
}
```