TheGenerator object is returned by agenerator function and it conforms to both theiterable protocol and theiterator protocol.

Generator is a subclass of the hiddenIterator class.

Constructor

There's no JavaScript entity that corresponds to theGenerator constructor. Instances ofGenerator must be returned fromgenerator functions:

function* generator() {  yield 1;  yield 2;  yield 3;}const gen = generator(); // "Generator { }"console.log(gen.next().value); // 1console.log(gen.next().value); // 2console.log(gen.next().value); // 3

There's only a hidden object which is the prototype object shared by all objects created by generator functions. This object is often stylized asGenerator.prototype to make it look like a class, but it should be more appropriately calledGeneratorFunction.prototype.prototype, becauseGeneratorFunction is an actual JavaScript entity. To understand the prototype chain ofGenerator instances, seeGeneratorFunction.prototype.prototype.

Instance properties

These properties are defined onGenerator.prototype and shared by allGenerator instances.

Generator.prototype.constructor

The constructor function that created the instance object. ForGenerator instances, the initial value isGeneratorFunction.prototype.

Note:Generator objects do not store a reference to the generator function that created them.

Generator.prototype[Symbol.toStringTag]

The initial value of the[Symbol.toStringTag] property is the string"Generator". This property is used inObject.prototype.toString().

Instance methods

Also inherits instance methods from its parentIterator.

Generator.prototype.next()

Returns a value yielded by theyield expression.

Generator.prototype.return()

Acts as if areturn statement is inserted in the generator's body at the current suspended position, which finishes the generator and allows the generator to perform any cleanup tasks when combined with atry...finally block.

Generator.prototype.throw()

Acts as if athrow statement is inserted in the generator's body at the current suspended position, which informs the generator of an error condition and allows it to handle the error, or perform cleanup and close itself.

Examples

An infinite iterator

With a generator function, values are not evaluated until they are needed. Therefore a generator allows us to define a potentially infinite data structure.

function* infinite() {  let index = 0;  while (true) {    yield index++;  }}const generator = infinite(); // "Generator { }"console.log(generator.next().value); // 0console.log(generator.next().value); // 1console.log(generator.next().value); // 2// …

Specifications

Browser compatibility

See also

• function*
• function* expression
• GeneratorFunction
• Iteration protocols