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The Generator object is returned by a generator function and it conforms to both the iterable protocol and the iterator 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 (also finishes the generator, unless caught from within that 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 implemented an earlier version of generators in JavaScript 1.7, where the star (*) in the function declaration was not necessary (you just use the yield keyword in the function body). However, legacy generators support has been removed since Firefox 58 (released on January 23, 2018) (bug 1083482).

Legacy generator methods

Generator.prototype.next()
Returns a value yielded by the yield expression. This corresponds to next() in the ES2015 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 ES2015 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 ES2015 generator object.
Generator.prototype.throw()
Throws an error to a generator. This corresponds to the throw() method in the ES2015 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 2015 (6th Edition, ECMA-262)
The definition of 'Generator objects' in that specification.
Standard Initial definition.
ECMAScript Latest Draft (ECMA-262)
The definition of 'Generator objects' in that specification.
Draft  

Browser compatibility

Update compatibility data on GitHub
DesktopMobileServer
ChromeEdgeFirefoxInternet ExplorerOperaSafariAndroid webviewChrome for AndroidEdge MobileFirefox for AndroidOpera for AndroidSafari on iOSSamsung InternetNode.js
Basic supportChrome Full support 39Edge Full support 13Firefox Full support 26IE No support NoOpera Full support YesSafari Full support 10WebView Android Full support YesChrome Android Full support YesEdge Mobile Full support 13Firefox Android Full support 26Opera Android Full support YesSafari iOS Full support 10Samsung Internet Android Full support Yesnodejs Full support 4.0.0
Full support 4.0.0
Full support 0.12
Disabled
Disabled From version 0.12: this feature is behind the --harmony runtime flag.
nextChrome Full support 39Edge Full support 13Firefox Full support 26IE No support NoOpera Full support YesSafari Full support 10WebView Android Full support YesChrome Android Full support YesEdge Mobile Full support 13Firefox Android Full support 26Opera Android Full support YesSafari iOS Full support 10Samsung Internet Android Full support Yesnodejs Full support Yes
returnChrome Full support 50Edge Full support 13Firefox Full support 38IE No support NoOpera Full support 37Safari Full support 10WebView Android ? Chrome Android Full support 50Edge Mobile Full support 13Firefox Android Full support 38Opera Android Full support YesSafari iOS Full support 10Samsung Internet Android Full support 5.0nodejs Full support 6.0.0
throwChrome Full support 39Edge Full support 13Firefox Full support 26IE No support NoOpera Full support YesSafari Full support 10WebView Android Full support YesChrome Android Full support YesEdge Mobile Full support 13Firefox Android Full support 26Opera Android Full support YesSafari iOS Full support 10Samsung Internet Android Full support Yesnodejs Full support 4.0.0
Full support 4.0.0
Full support 0.12
Disabled
Disabled From version 0.12: this feature is behind the --harmony runtime flag.

Legend

Full support  
Full support
No support  
No support
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User must explicitly enable this feature.

See also

Legacy generators

ES2015 generators