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Generator

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

Browser compatibilityUpdate compatibility data on GitHub

Desktop
Chrome Edge Firefox Internet Explorer Opera Safari
Basic support 39 13 26 No Yes 10
next 39 13 26 No Yes 10
return 50 13 38 No 37 10
throw 39 13 26 No Yes 10
Mobile
Android webview Chrome for Android Edge Mobile Firefox for Android Opera for Android iOS Safari Samsung Internet
Basic support Yes Yes 13 26 Yes 10 Yes
next Yes Yes 13 26 Yes 10 Yes
return ? 50 13 38 Yes 10 5.0
throw Yes Yes 13 26 Yes 10 Yes
Server
Node.js
Basic support 4.0.0
4.0.0
0.12
Disabled
Disabled From version 0.12: this feature is behind the --harmony runtime flag.
next Yes
return 6.0.0
throw 4.0.0
4.0.0
0.12
Disabled
Disabled From version 0.12: this feature is behind the --harmony runtime flag.

See also

Legacy generators

ES2015 generators

© 2005–2018 Mozilla Developer Network and individual contributors.
Licensed under the Creative Commons Attribution-ShareAlike License v2.5 or later.
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Generator