I want to give this a try! 😄 I've already started with a few types to see what could work, and I need some suggestions on how to proceed.

Ideally, we'd take the existing .d.ts from DefinitelyTyped and merge it into dom.generated.d.ts. However, it seems that the lib generator doesn't work like that: it only consumes WebIDL files or modifications from {added,overriding,removed}Types.json files. As @mhegazy already mentioned, it seems like we should either:

Rewrite the .d.ts as WebIDL, and add some tweaks in the JSON files for types that can't be expressed in WebIDL. This will not be an official WebIDL file, which might be a bit of a risk, although we'll have to tweak and maintain the types anyway.
Rewrite the .d.ts entirely as JSON. This will probably be quite verbose, but you wouldn't need to go back-and-forth between WebIDL and JSON to figure out where a type is defined.

Any suggestions on what approach is more desirable would be much appreciated.

The read method of ReadableStreamDefaultReader currently has the following type:

read(): Promise<IteratorResult<R>>;

However, IteratorResult is defined in es2015.iterable, which is not available when targeting ES5 or lower.

I think we can get away with defining a separate interface (e.g. ReadResult<R>) that is just a copy of IteratorResult<R>. Since TypeScript uses structured typing, the type checker should treat both as equivalent. Or is there a cleaner way to handle this?

I noticed that many WritableStream types are already defined (because they're implemented in Edge), but some of them are quite awkward. For example, the callbacks in UnderlyingSink are defined as callable interfaces. I think this is because they're generated from a WebIDL callback definition.

Is it okay if we change these to be function types? For example:

// old
interface WritableStreamDefaultControllerCallback {
    (controller: WritableStreamDefaultController): void;
}
// new
type WritableStreamDefaultControllerCallback = (controller: WritableStreamDefaultController) => void;

Or should we keep using callable interfaces, including for the new callbacks (e.g. UnderlyingSource.pull)?

Add Streams types #521

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