TheAsyncGenerator object is returned by anasync generator function and it conforms to both theasync iterable protocol and the async iterator protocol.

Async generator methods always yieldPromise objects.

AsyncGenerator is a subclass of the hiddenAsyncIterator class.

Constructor

There's no JavaScript entity that corresponds to theAsyncGenerator constructor. Instances ofAsyncGenerator must be returned fromasync generator functions:

async function* createAsyncGenerator() {  yield Promise.resolve(1);  yield await Promise.resolve(2);  yield 3;}const asyncGen = createAsyncGenerator();asyncGen.next().then((res) => console.log(res.value)); // 1asyncGen.next().then((res) => console.log(res.value)); // 2asyncGen.next().then((res) => console.log(res.value)); // 3

There's only a hidden object which is the prototype object shared by all objects created by async generator functions. This object is often stylized asAsyncGenerator.prototype to make it look like a class, but it should be more appropriately calledAsyncGeneratorFunction.prototype.prototype, becauseAsyncGeneratorFunction is an actual JavaScript entity. To understand the prototype chain ofAsyncGenerator instances, seeAsyncGeneratorFunction.prototype.prototype.

Instance properties

These properties are defined onAsyncGenerator.prototype and shared by allAsyncGenerator instances.

AsyncGenerator.prototype.constructor

The constructor function that created the instance object. ForAsyncGenerator instances, the initial value isAsyncGeneratorFunction.prototype.

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

AsyncGenerator.prototype[Symbol.toStringTag]

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

Instance methods

Also inherits instance methods from its parentAsyncIterator.

AsyncGenerator.prototype.next()

Returns aPromise which will be resolved with the given value yielded by theyield expression.

AsyncGenerator.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.

AsyncGenerator.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

Async generator iteration

The following example iterates over an async generator, logging values 1–6 to the console at decreasing time intervals. Notice how each time a Promise is yielded, but it's automatically resolved within thefor await...of loop.

// An async task. Pretend it's doing something more useful// in practice.function delayedValue(time, value) {  return new Promise((resolve /*, reject */) => {    setTimeout(() => resolve(value), time);  });}async function* generate() {  yield delayedValue(2000, 1);  yield delayedValue(1000, 2);  yield delayedValue(500, 3);  yield delayedValue(250, 4);  yield delayedValue(125, 5);  yield delayedValue(50, 6);  console.log("All done!");}async function main() {  for await (const value of generate()) {    console.log("value", value);  }}main().catch((e) => console.error(e));

Specifications

Browser compatibility

See also

• function*
• async function*
• function* expression
• Generator Function
• Async Generator Function
• Iterators and generators guide