Type definition for what `for await` takes
lukiano opened this issue · 3 comments
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.
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.
I prefer ForAwaitable
for these reasons:
- It's more concise.
AsyncOrSyncIterable
is logically and semantically reduced toIterable
, but this is an incorrect deduction. Therefore, the name is counterintuitive.ForAwaitable
doesn't mean one must use it withfor await
, but rather that it is supported to do so. This exactly preserves the intent of the alias.ForAwaitable
trends withAwaitable
which is currently under discussion #31394
+1, I have this precise type definition locally in my code.