UNB/ CS/ David Bremner/ teaching/ cs2613/ books/ mdn/ Reference/ Statements/ async function*

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

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

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 whitespace or line terminators. However, there cannot be a line terminator between async and function, otherwise a semicolon is automatically inserted, causing async to become an identifier and the rest to become a function* declaration.

Parameters

Description

An async function* declaration creates an AsyncGeneratorFunction object. Each time when an async generator function is called, it returns a new AsyncGenerator object, which conforms to the async iterator protocol. Every call to next() returns a Promise that resolves to the iterator result object.

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.

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 behave similar to function declarations — they are 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

Browser compatibility

See also