Generator
Baseline Widely available
This feature is well established and works across many devices and browser versions. It’s been available across browsers since September 2016.
The Generator
object is returned by a generator function and it conforms to both the iterable protocol and the iterator protocol.
Generator
is a subclass of the hidden Iterator
class.
Try it
Constructor
There's no JavaScript entity that corresponds to the Generator
constructor. Instances of Generator
must be returned from generator functions:
function* generator() {
yield 1;
yield 2;
yield 3;
}
const gen = generator(); // "Generator { }"
console.log(gen.next().value); // 1
console.log(gen.next().value); // 2
console.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 as Generator.prototype
to make it look like a class, but it should be more appropriately called GeneratorFunction.prototype.prototype
, because GeneratorFunction
is an actual JavaScript entity. To understand the prototype chain of Generator
instances, see GeneratorFunction.prototype.prototype
.
Instance properties
These properties are defined on Generator.prototype
and shared by all Generator
instances.
Generator.prototype.constructor
-
The constructor function that created the instance object. For
Generator
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 parent Iterator
.
Generator.prototype.next()
-
Returns a value yielded by the
yield
expression. Generator.prototype.return()
-
Acts as if a
return
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 a
throw
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); // 0
console.log(generator.next().value); // 1
console.log(generator.next().value); // 2
// …
Specifications
Specification |
---|
ECMAScript Language Specification # sec-generator-objects |
Browser compatibility
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