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.

Start by reviewing the proposed ForAwaitable aliases and the use case for accepting Iterable and AsyncIterable values. Check the linked async iteration specification, especially CreateAsyncFromSyncIterator and precedence between Symbol.asyncIterator and Symbol.iterator. Done means reaching an agreed type-alias design that accurately models for-await inputs without changing emitted JavaScript.