Type definition for what `for await` takes

I haven't found this request on the current issues so I'm opening one.

Search Terms

"for await" asynchronous generators

Suggestion

If for await can take both an Iterable and an AsyncIterable then I'd like a type alias

type ForAwaitable<T> = Iterable<T> | AsyncIterable<T>;

// or maybe ForAwaitable<T, U = T> = Iterable<T> | AsyncIterable<U>;

Better names are welcome.

Use Cases

type ForAwaitable<T> = Iterable<T> | AsyncIterable<T>;

async function foo(chunks: ForAwaitable<Buffer>): Promise<void> {
  for await (const chunk of chunks) {
    // ...
  }
}

await foo(someReadableStream);
await foo(inMemoryBufferArray);

Looking at https://tc39.es/proposal-async-iteration/#sec-createasyncfromsynciterator and after a test, if an object has both fields Symbol.asyncIterator and Symbol.iterator then the former takes precedence, but that use case should rarely happen.

Checklist

My suggestion meets these guidelines:

• This wouldn't be a breaking change in existing TypeScript/JavaScript code
• This wouldn't change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, etc.)
• This feature would agree with the rest of TypeScript's Design Goals.

Commencez par examiner les alias ForAwaitable proposés et le cas d’utilisation consistant à accepter des valeurs Iterable et AsyncIterable. Consultez la spécification liée de l’itération asynchrone, en particulier CreateAsyncFromSyncIterator et la priorité entre Symbol.asyncIterator et Symbol.iterator. Le travail sera considéré comme terminé lorsqu’une conception d’alias de types faisant consensus aura été définie et modélisera correctement les entrées de for-await sans modifier le JavaScript généré.