Async #7 — Async Iteration

Async iteration combines Promises with the iterator protocol, letting you loop over data that arrives asynchronously — paginated API responses, streaming chunks, event sequences. This article covers for await...of, async generators, and Symbol.asyncIterator.

Prerequisites: Async #3 — Promises, Async #6 — Generators

1. The Problem — Iterating Over Async Data

Regular iteration (for...of) expects synchronous values. When each value requires an async operation, you need something different:

2. Async Iterables — Symbol.asyncIterator

An async iterable uses Symbol.asyncIterator instead of Symbol.iterator, and its next() returns a Promise of { value, done }:

3. Async Generators — async function*

Async generators combine generators with async: yield produces values, await pauses for promises:

4. Building an Async Iterator Manually

5. Converting an Async Iterable to an Array

There's no spread for async iterables ([...asyncIterable] doesn't work). Build a helper:

6. Async map, filter, reduce Over Async Iterables

Key Takeaways

• Symbol.asyncIterator enables for await...of — next() returns a Promise<{ value, done }>.
• async function* creates an async generator — yield produces values and await pauses.
• yield* works with both sync and async iterables inside async generators.
• No spread for async iterables — use for await...of + array push, or a helper.
• Async iteration is the natural way to consume: paginated APIs, streaming responses, event sequences, queues.

Next: Async #8 — Concurrency Control & Retry Patterns — execute tasks sequentially, with a concurrency pool, and retry with backoff.