async function* expression

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

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
}

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:

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

Specifications

Browser compatibility

See also

• Functions guide
• Functions
• async function*
• AsyncGeneratorFunction
• Iteration protocols
• yield
• yield*