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    Generator

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    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.

    Summary

    The Generator object is returned by a generator function and it conforms to both the iterator and the Iterable protocol.

    Syntax

    function* gen() { 
      yield 1;
      yield 2;
      yield 3;
    }
    
    var g = gen(); // "Generator { }"

    Methods

    Generator.prototype.next()
    Returns a value yielded by the yield expression.
    Generator.prototype.return()
    Returns the given value and finishes the generator.
    Generator.prototype.throw()
    Throws an error to a generator.

    Example

    An infinite iterator

    function* idMaker(){
        var index = 0;
        while(true)
            yield index++;
    }
    
    var gen = idMaker(); // "Generator { }"
    
    console.log(gen.next().value); // 0
    console.log(gen.next().value); // 1
    console.log(gen.next().value); // 2
    // ...

    Legacy generator objects

    Firefox (SpiderMonkey) also implements an earlier version of generators in JavaScript 1.7. There is no need for the star (*) in the function declaration, you just use the yield keyword in the function body. However, legacy generators are deprecated. Do not use them; they are going to be removed (bug 1083482).

    Legacy generator methods

    Generator.prototype.next()
    Returns a value yielded by the yield expression. This corresponds to next() in the ES6 generator object.
    Generator.prototype.close()
    Closes the generator, so that when calling next() an StopIteration error will be thrown. This corresponds to the return() method in the ES6 generator object.
    Generator.prototype.send()
    Used to send a value to a generator. The value is returned from the yield expression, and returns a value yielded by the next yield expression. send(x) corresponds to next(x) in the ES6 generator object.
    Generator.prototype.throw()
    Throws an error to a generator. This corresponds to the throw() method in the ES6 generator object.

    Legacy generator example

    function fibonacci() {
      var a = yield 1;
      yield a * 2;
    }
    
    var it = fibonacci();
    console.log(it);          // "Generator {  }"
    console.log(it.next());   // 1
    console.log(it.send(10)); // 20
    console.log(it.close());  // undefined
    console.log(it.next());   // throws StopIteration (as the generator is now closed)
    

    Specifications

    Specification Status Comment
    ECMAScript 6 (ECMA-262)
    The definition of 'Generator objects' in that specification.
    Release Candidate Initial definition.

    Browser compatibility

    Feature Chrome Firefox (Gecko) Internet Explorer Opera Safari
    Basic support Not supported (Yes) Not supported Not supported Not supported
    Feature Android Chrome for Android Firefox Mobile (Gecko) IE Mobile Opera Mobile Safari Mobile
    Basic support Not supported Not supported (Yes) Not supported Not supported Not supported

    See also

    Legacy generators

    ES6 generators

    Document Tags and Contributors

    Contributors to this page: Sheppy, thesebas, fscholz, SphinxKnight, arai, jpmedley, chrisAnderson
    Last updated by: jpmedley,