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    yield*

    This is an experimental technology, part of the ECMAScript 6 (Harmony) proposal.
    Because this technology's specification has not stabilized, check the compatibility table for usage in various browsers. Also note that the syntax and behavior of an experimental technology is subject to change in future version of browsers as the spec changes.

    The yield* expression is used to delegate to another generator or iterable object.

    Syntax

     yield* [[expression]];
    expression
    The expression which returns an iterable object.

    Description

    The yield* expression iterates over the operand and yields each value returned by it.

    The value of yield* expression 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* 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 6 (ECMA-262)
    The definition of 'Yield' in that specification.
    Release Candidate Initial definition.

    Browser compatibility

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

    Document Tags and Contributors

    Contributors to this page: ziyunfei, fscholz, arai
    Last updated by: fscholz,
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