yield*
表达式用于委托给另一个generator
或可迭代对象。
yield* [[expression]];
expression
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)。
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 }
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"
Specification | Status | Comment |
---|---|---|
ECMAScript 2015 (6th Edition, ECMA-262) Yield |
Standard | Initial definition. |
ECMAScript Latest Draft (ECMA-262) Yield |
Draft |
Desktop | Mobile | Server | ||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
yield* | Chrome Full support Yes | Edge Full support Yes | Firefox
Full support
27
| IE No support No | Opera Full support Yes | Safari Full support 10 | WebView Android Full support Yes | Chrome Android Full support Yes | Edge Mobile Full support Yes | Firefox Android
Full support
27
| Opera Android Full support Yes | Safari iOS Full support 10 | Samsung Internet Android Full support Yes | nodejs
Full support
4.0.0
|
yield
表达式的解析已被更新以符合ES2015规范(bug 981599):
yield
和 *
之间不允许有换行符。如下代码会抛出SyntaxError
:
function* foo() {
yield
*[];
}