Async Iterators and for await...of

An async iterable yields values over time. for await...of awaits each next step and works naturally with streams or paginated reads. The producer can pause between items without forcing the caller to collect everything first.

Reading the code: Calling numbers creates an async generator. Each for await iteration requests and awaits its next result, then prints 2 and 4 in order. The second yield includes a resolved Promise.

Keep the boundary clear: Do not confuse `for await` with parallel mapping. It normally consumes elements sequentially, which can provide useful backpressure.

Save the code as async-iterators-await.ts. The result below was captured after compiling this exact example with strict TypeScript checking and executing the emitted JavaScript in Node.js.

TypeScript code

async function* numbers(): AsyncGenerator<number> {
  yield 2;
  yield await Promise.resolve(4);
}
async function main(): Promise<void> {
  for await (const value of numbers()) console.log(value);
}
main().catch(console.error);

Verified output

2
4

Build: npx tsc --strict --target es2022 --module commonjs --lib es2022,dom,esnext.disposable async-iterators-await.ts; run: node async-iterators-await.js. The exact filename may be changed freely. When a multi-file project exists, prefer its tsconfig.json and build scripts.

Calling numbers creates an async generator. Each for await iteration requests and awaits its next result, then prints 2 and 4 in order. The second yield includes a resolved Promise.

The output shown above comes from the initialized values in this program. Replacing a type annotation alone does not transform a JavaScript value.

Do not confuse for await with parallel mapping. It normally consumes elements sequentially, which can provide useful backpressure.

Check both the TypeScript diagnostic and the emitted JavaScript behavior. A program can compile correctly and still need runtime validation or error handling.

Write an async generator yielding the first three square numbers, then sum them in a for await loop.

Before running your change, write down the type you expect TypeScript to infer and the output you expect JavaScript to print. Then check both predictions.

An async iterable yields values over time.

Try the practice change and re-run the exact program to test your understanding.