Should IteratorResult have optional value?

[christyharagan]

Looking at the ECMA spec for IteratorResult, it seems that value can be optional (when done is true)

In the TypeScript definition, this would translate to:

interface IteratorResult<T> {
    done: boolean;
    value?: T;
}

This does create a potentially annoying situation in which, when `strictNullChecksis true, thevalue``has to be cast to``T`` (unless the new control-flow based type guards are smart enough to recognise this).

As it is, however, it isn't possible to actually implement an Iterator with strictNullChecks because we can't return an undefined value for value.

What is the correct approach here?

[ivogabe]

That value was declared optional previously, but that indeed caused to much casts. See #8357. Ideally IteratorResult would be defined as:

type IteratorResult<TYield, TReturn>
  = { done: true, value: TYield }
  | { done: false, value: TReturn }

But that's not possible yet. Then you could define an iterator that does not return as IteratorResult<T, undefined>.

[mhegazy]

As noted by @ivogabe we need to have boolean literal types, to be able to model this one accurately.

[benmosher]

I'm fairly new to TypeScript, so maybe this is obvious to others, but I was able to work around with:

    return function next(): IteratorResult<T> {
      if (yielded < arr.length) {
        return { value: arr[yielded++], done: false }
      } else {
        return ({ done: true } as IteratorResult<T>)  // <= explicit cast here!
      }
    }

[mhegazy]

the definition is currently not correct, the correct one would be:

type IteratorResult<TYield, TReturn>
  = { done: false, value: TYield }
  | { done: done, value: TReturn | undefined }

[DanielRosenwasser]

Should this be closed in favor of #2983?

[mhegazy]

Yes.