yield*
试一试
语法
yield* expression
参数
expression
可选-
一个可迭代对象。
返回值
当迭代器关闭时(当 done
为 true
时),返回该迭代器返回的值。
描述
yield*
表达式迭代操作数并生成它返回的每个值。它将当前生成器的迭代委托给底层迭代器——我们分别将其称为“生成器”和“迭代器”。yield*
首先通过调用后者的 [Symbol.iterator]()
方法从操作数获取迭代器。然后,每次调用生成器的 next()
方法时,yield*
都会调用迭代器的 next()
方法,并将生成器的 next()
方法接收到的参数传递给它(对于第一次调用,始终为 undefined
),并生成与迭代器的 next()
方法返回的结果对象相同的结果对象。如果迭代器结果具有 done: true
,则 yield*
表达式停止执行并返回该结果的 value
。
yield*
运算符也将当前生成器的 throw()
和 return()
方法转发到底层迭代器。如果当前生成器通过其中一种方法过早关闭,则会通知底层迭代器。如果调用生成器的 throw()
/return()
方法,则使用相同的参数调用底层迭代器的 throw()
/return()
方法。throw()
/return()
的返回值像 next()
方法的结果一样处理,如果该方法抛出异常,则异常将从 yield*
表达式传播。
如果底层迭代器没有 return()
方法,则 yield*
表达式将变成 return
语句,就像调用挂起的 yield
表达式的 return()
一样。
如果底层迭代器没有 throw()
方法,这会导致 yield*
抛出一个 TypeError
——但在抛出错误之前,如果存在,则会调用底层迭代器的 return()
方法。
示例
委托给另一个生成器
在以下代码中,g1()
生成的值像 g2()
生成的值一样从 next()
调用中返回。
function* g1() {
yield 2;
yield 3;
yield 4;
}
function* g2() {
yield 1;
yield* g1();
yield 5;
}
const gen = g2();
console.log(gen.next()); // {value: 1, done: false}
console.log(gen.next()); // {value: 2, done: false}
console.log(gen.next()); // {value: 3, done: false}
console.log(gen.next()); // {value: 4, done: false}
console.log(gen.next()); // {value: 5, done: false}
console.log(gen.next()); // {value: undefined, done: true}
其他可迭代对象
除了生成器对象之外,yield*
还可以生成其他类型的可迭代对象(例如,数组、字符串或 arguments
对象)。
function* g3(...args) {
yield* [1, 2];
yield* "34";
yield* args;
}
const gen = g3(5, 6);
console.log(gen.next()); // {value: 1, done: false}
console.log(gen.next()); // {value: 2, done: false}
console.log(gen.next()); // {value: "3", done: false}
console.log(gen.next()); // {value: "4", done: false}
console.log(gen.next()); // {value: 5, done: false}
console.log(gen.next()); // {value: 6, done: false}
console.log(gen.next()); // {value: undefined, done: true}
yield* 表达式本身的值
yield*
是一个表达式,而不是一个语句,因此它会计算出一个值。
function* g4() {
yield* [1, 2, 3];
return "foo";
}
function* g5() {
const g4ReturnValue = yield* g4();
console.log(g4ReturnValue); // 'foo'
return g4ReturnValue;
}
const gen = g5();
console.log(gen.next()); // {value: 1, done: false}
console.log(gen.next()); // {value: 2, done: false}
console.log(gen.next()); // {value: 3, done: false} done is false because g5 generator isn't finished, only g4
console.log(gen.next()); // {value: 'foo', done: true}
方法转发
当前生成器的 next()
、throw()
和 return()
方法都转发到底层迭代器。
const iterable = {
[Symbol.iterator]() {
let count = 0;
return {
next(v) {
console.log("next called with", v);
count++;
return { value: count, done: false };
},
return(v) {
console.log("return called with", v);
return { value: "iterable return value", done: true };
},
throw(v) {
console.log("throw called with", v);
return { value: "iterable thrown value", done: true };
},
};
},
};
function* gf() {
yield* iterable;
return "gf return value";
}
const gen = gf();
console.log(gen.next(10));
// next called with undefined; the argument of the first next() call is always ignored
// { value: 1, done: false }
console.log(gen.next(20));
// next called with 20
// { value: 2, done: false }
console.log(gen.return(30));
// return called with 30
// { value: 'iterable return value', done: true }
console.log(gen.next(40));
// { value: undefined, done: true }; gen is already closed
const gen2 = gf();
console.log(gen2.next(10));
// next called with undefined
// { value: 1, done: false }
console.log(gen2.throw(50));
// throw called with 50
// { value: 'gf return value', done: true }
console.log(gen.next(60));
// { value: undefined, done: true }; gen is already closed
如果底层迭代器的 return()
/throw()
方法返回 done: false
,则当前生成器继续执行,并且 yield*
继续委托到底层迭代器。
const iterable = {
[Symbol.iterator]() {
let count = 0;
return {
next(v) {
console.log("next called with", v);
count++;
return { value: count, done: false };
},
return(v) {
console.log("return called with", v);
return { value: "iterable return value", done: false };
},
};
},
};
function* gf() {
yield* iterable;
return "gf return value";
}
const gen = gf();
console.log(gen.next(10));
// next called with undefined
// { value: 1, done: false }
console.log(gen.return(20));
// return called with 20
// { value: 'iterable return value', done: false }
console.log(gen.next(30));
// { value: 2, done: false }; gen is not closed
如果底层迭代器没有 throw()
方法并且调用了生成器的 throw()
,则 yield*
会抛出错误。
const iterable = {
[Symbol.iterator]() {
let count = 0;
return {
next(v) {
count++;
return { value: count, done: false };
},
};
},
};
function* gf() {
yield* iterable;
return "gf return value";
}
const gen = gf();
gen.next(); // First next() starts the yield* expression
gen.throw(20); // TypeError: The iterator does not provide a 'throw' method.
规范
规范 |
---|
ECMAScript 语言规范 # sec-generator-function-definitions-runtime-semantics-evaluation |
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