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yield* expression используется для делегирования generator или итерируемого объекта.

Синтаксис

 yield* [[expression]];
expression
Выражение, которое возвращает итерируемый объект.

Описание

The yield* выражение итерирует операнды и присваивает каждому, возвращаемые им значения.

Значени yield* выражения itself is the value returned by that iterator when it's closed (i.e., when done is true).

Examples

Delegating to another generator

In following code, values yielded by g1() are returned from next() calls just like those which are yielded by g2().

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 }

Other Iterable objects

Besides generator objects, yield* can also yield other kinds of iterables, e.g. arrays, strings or arguments objects.

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

The value of yield* expression itself

yield* is an expression, not a statement, so it evaluates to a value.

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 }, 
                              // g4() returned { value: "foo", done: true } at this point

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

Specifications

Specification Status Comment
ECMAScript 2015 (6th Edition, ECMA-262)
Определение 'Yield' в этой спецификации.
Стандарт Initial definition.
ECMAScript Latest Draft (ECMA-262)
Определение 'Yield' в этой спецификации.
Живой стандарт  

Browser compatibility

Feature Chrome Firefox (Gecko) Internet Explorer Opera Safari (WebKit)
Basic support (Да) 27.0 (27.0) ? ? ?
Feature Android Firefox Mobile (Gecko) IE Mobile Opera Mobile Safari Mobile
Basic support (Да) 27.0 (27.0) ? ? ?

Firefox-specific notes

  • Starting with Gecko 33 (Firefox 33 / Thunderbird 33 / SeaMonkey 2.30), the parsing of the yield expression has been updated to conform with the latest ES6 specification (баг 981599):
    • The line terminator restriction is now implemented. No line terminator between "yield" and "*" is allowed. Code like the following will throw a SyntaxError:
      function* foo() {
        yield
        *[];
      }

See also

Метки документа и участники

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