Baseline Widely available
An AsyncIterator object is an object that conforms to the async iterator protocol by providing a next() method that returns a promise fulfilling to an iterator result object. The AsyncIterator.prototype object is a hidden global object that all built-in async iterators inherit from. It provides an [Symbol.asyncIterator]() method that returns the async iterator object itself, making the async iterator also async iterable.
Note that AsyncIterator is not a global object, although it will be in the future with the async iterator helpers proposal. The AsyncIterator.prototype object shared by all built-in async iterators can be obtained with the following code:
const AsyncIteratorPrototype = Object.getPrototypeOf(
Object.getPrototypeOf(Object.getPrototypeOf((async function* () {})())),
);
Currently, the only built-in JavaScript async iterator is the AsyncGenerator object returned by async generator functions. There are some other built-in async iterators in web API, such as the one of a ReadableStream.
Each of these async iterators have a distinct prototype object, which defines the next() method used by the particular async iterator. All of these prototype objects inherit from AsyncIterator.prototype, which provides an [Symbol.asyncIterator]() method that returns the async iterator object itself, making the async iterator also async iterable.
Note: AsyncIterator.prototype does not implement [Symbol.iterator](), so async iterators are not sync iterable by default.
AsyncIterator.prototype[Symbol.asyncIterator]()
Returns the async iterator object itself. This allows async iterator objects to also be async iterable.
All built-in async iterators are also async iterable, so you can use them in a for await...of loop:
const asyncIterator = (async function* () {
yield 1;
yield 2;
yield 3;
})();
(async () => {
for await (const value of asyncIterator) {
console.log(value);
}
})();
// Logs: 1, 2, 3
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