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"
js
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
, causing async to become an identifier and the rest to become a function* declaration.
The function name.
The name of a formal parameter for the function. For the parameters' syntax, see the
.
The statements comprising the body of the function.
An async function* declaration creates an
object. Each time when an async generator function is called, it returns a new
object, which conforms to the
. Every call to next() returns a
that resolves to the iterator result object.
An async generator function combines the features of
and
. You can use both the
and
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:
js
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
declarations — they are
to the top of their scope and can be called anywhere in their scope, and they can be redeclared only in certain contexts.
Declaring an async generator function
Async generator functions always produce promises of results — even when each yield step is synchronous.
js
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
module.
js
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: '...' }
Specification
ECMAScript® 2027 Language Specification# sec-async-generator-function-definitions
guide
guide