async function*

The async function* declaration defines an async generator function, which returns an AsyncGenerator object.

You can also define async generator functions using the AsyncGeneratorFunction constructor or the async function* expression syntax.

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.

Parameters

name: The function name.
param Optional: The name of a formal parameter for the function.
statements Optional: The statements comprising the body of the function.

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

Unlike normal generator functions declared with function*, an async generator function return an AsyncGenerator object, which conforms to the async iterable protocol. Every call to next() returns a Promise that resolves to the iterator result object.

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(1);
}

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

1 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 are hoisted to the top of their scope and can be called anywhere in their scope.

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 Language Specification # sec-async-generator-function-definitions

Browser compatibility

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