Type definition for what `for await` takes

[lukiano]

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.

[Jamesernator]

Having used the type quite a lot I'd prefer to see it called something like AsyncOrSyncIterable<T> rather than ForAwaitable as it's not necessarily the case you'll consume it with a for-await-of loop rather than just accessing iterator methods directly.

[Vandivier]

I prefer ForAwaitable for these reasons:

1. It's more concise.
2. AsyncOrSyncIterable is logically and semantically reduced to Iterable, but this is an incorrect deduction. Therefore, the name is counterintuitive.
3. ForAwaitable doesn't mean one must use it with for await, but rather that it is supported to do so. This exactly preserves the intent of the alias.
4. ForAwaitable trends with Awaitable which is currently under discussion Add a new type Awaitable<T> = T | PromiseLike<T> #31394

[jimblackler]

+1, I have this precise type definition locally in my code.