async function* expression

Baseline Widely available

This feature is well established and works across many devices and browser versions. It’s been available across browsers since January 2020.

The async function* keywords can be used to define an async generator function inside an expression.

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

Try it

async function joinAll(generator) {
  let str = "";
  for await (const val of generator()) {
    str += val;
  }
  return str;
}
joinAll(async function* () {
  yield await Promise.resolve("a");
  yield await Promise.resolve("b");
  yield await Promise.resolve("c");
}).then((str) => console.log(str));
// Expected output: "abc"

Syntax

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

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

Note: An expression statement cannot begin with the keywords async function to avoid ambiguity with an async function* declaration. The async function keywords only begin an expression when they appear in a context that cannot accept statements.

Parameters

name Optional

The function name. Can be omitted, in which case the function is anonymous. The name is only local to the function body.

paramN Optional

The name of a formal parameter for the function. For the parameters' syntax, see the Functions reference.

statements Optional

The statements which comprise the body of the function.

Description

An async function* expression is very similar to, and has almost the same syntax as, an async function* declaration. The main difference between an async function* expression and an async function* declaration is the function name, which can be omitted in async function* expressions to create anonymous functions. An async function* expression can be used as an IIFE (Immediately Invoked Function Expression) which runs as soon as it is defined, allowing you to create an ad-hoc async iterable object. See also the chapter about functions for more information.

Examples

Using async function* expression

The following example defines an unnamed asynchronous generator function and assigns it to x. The function yields the square of its argument:

js
const x = async function* (y) {
  yield Promise.resolve(y * y);
};
x(6)
  .next()
  .then((res) => console.log(res.value)); // 36

Specifications

Specification

ECMAScript® 2027 Language Specification
# sec-async-generator-function-definitions

Browser compatibility

Enable JavaScript to view this browser compatibility table.

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This page was last modified on Jul 8, 2025 by MDN contributors.

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