Symbol.asyncIterator

The well-known symbol @@asyncIterator.

The Symbol.asyncIterator symbol is a builtin symbol that is used to access an object's @@asyncIterator method. In order for an object to be async iterable, it must have a Symbol.asyncIterator key.

You can define your own async iterable by setting the [Symbol.asyncIterator] property on an object.

const myAsyncIterable = {
  async *[Symbol.asyncIterator]() {
    yield "hello";
    yield "async";
    yield "iteration!";
  },
};

(async () => {
  for await (const x of myAsyncIterable) {
    console.log(x);
  }
})();
// Logs:
// "hello"
// "async"
// "iteration!"

When creating an API, remember that async iterables are designed to represent something iterable — like a stream of data or a list —, not to completely replace callbacks and events in most situations.

ReadableStream is the only built-in JavaScript object that has the Symbol.asyncIterator method set by default at the time of writing.

• Iteration protocols
• for await...of