async function* - JavaScript

The async function* declaration creates a binding of a new async generator function to a given name.

You can also define async generator functions using the async function* expression.

Try it

async function* foo() {
  yield await Promise.resolve("a");
  yield await Promise.resolve("b");
  yield await Promise.resolve("c");
}

let str = "";

async function generate() {
  for await (const val of foo()) {
    str += val;
  }
  console.log(str);
}

generate();
/ Expected output: "abc"

Syntax

async function* name(param0) {
  statements
}
async function* name(param0, param1) {
  statements
}
async function* name(param0, param1, /* …, */ paramN) {
  statements
}

Note: Async generator functions do not have arrow function counterparts.

Note: function and * are separate tokens, so they can be separated by automatically inserted, causing async to become an identifier and the rest to become a function* declaration.

Parameters

name: The function name.
param Optional: The name of a formal parameter for the function. For the parameters' syntax, see the Functions reference.
statements Optional: The statements comprising the body of the function.

Description

An async function* declaration creates an Promise that resolves to the iterator result object.

An async generator function combines the features of yield keywords within the function body. This empowers you to handle asynchronous tasks ergonomically with await, while leveraging the lazy nature of generator functions.

When a promise is yielded from an async generator, the iterator result promise's eventual state will match that of the yielded promise. For example:

async function* foo() {
  yield Promise.reject(new Error("failed"));
}

foo()
  .next()
  .catch((e) => console.error(e));

Error: failed will be logged, because if the yielded promise rejects, the iterator result will reject as well. The value property of an async generator's resolved result will not be another promise.

async function* declarations behave similar to hoisted to the top of their scope and can be called anywhere in their scope, and they can be redeclared only in certain contexts.

Examples

Declaring an async generator function

Async generator functions always produce promises of results — even when each yield step is synchronous.

async function* myGenerator(step) {
  await new Promise((resolve) => setTimeout(resolve, 10));
  yield 0;
  yield step;
  yield step * 2;
}

const gen = myGenerator(2);
gen
  .next()
  .then((res) => {
    console.log(res); / { value: 0, done: false }
    return gen.next();
  })
  .then((res) => {
    console.log(res); / { value: 2, done: false }
    return gen.next();
  })
  .then((res) => {
    console.log(res); / { value: 4, done: false }
    return gen.next();
  })
  .then((res) => {
    console.log(res); / { value: undefined, done: true }
    return gen.next();
  });

Using an async generator function to read a series of files

In this example, we read a series of files and only access its content when requested, using Node's fs/promises module.

async function* readFiles(directory) {
  const files = await fs.readdir(directory);
  for (const file of files) {
    const stats = await fs.stat(file);
    if (stats.isFile()) {
      yield {
        name: file,
        content: await fs.readFile(file, "utf8"),
      };
    }
  }
}

const files = readFiles(".");
console.log((await files.next()).value);
/ Possible output: { name: 'file1.txt', content: '...' }
console.log((await files.next()).value);
/ Possible output: { name: 'file2.txt', content: '...' }

Specifications

Specification
ECMAScript® 2026 Language Specification # sec-async-generator-function-definitions