Iterator result of Iterable or AsyncIterable inferred to any

[ikokostya]

TypeScript Version: 3.7.0-dev.20191010

Search Terms:

Iterable, AsyncIterable, Iterator, IteratorResult

Code

Synchronous iterators:

declare let syncIterable: Iterable<number>;
let res = syncIterable[Symbol.iterator]().next();
let value = res.value;

Asynchronous iterator:

declare let asyncIterable: AsyncIterable<number>;

(async () => {
    let res = await asyncIterable[Symbol.asyncIterator]().next();
    let value = res.value;
});

Expected behavior:

In both cases value has number type.

Actual behavior:

Variable value has any type.

---

Investigation

Type IteratorResult has two type parameters T and TReturn:

TypeScript/lib/lib.es2015.iterable.d.ts

Line 41 in 6162001

type IteratorResult<T, TReturn = any> = IteratorYieldResult<T> | IteratorReturnResult<TReturn>;

Because by default TReturn is any (interface Iterable has only one type parameter), we got number | any type, which is narrowed to any.

There are two problems:

• There is no way to specify TReturn type parameter for IterableIterator and AsyncIterableIterator types.
• Current behavior is unsafe.

Playground Link:

https://www.typescriptlang.org/play/index.html?ssl=3&ssc=23&pln=1&pc=1#code/CYUwxgNghgTiAEEQBd4GcCeA7MBJZIMUARkgFzz6ElIA8WArgLbGEB8A3AFBKpxrwAvOmx4CRUiADaAZQwsA9hAB0AS3FRkCmAF0AFAEplWEAA9kh7r3gA3KBAYJh-ZXYcgOQA

Related Issues:

#2983

[ikokostya]

With explicit check done property of IteartorResult:

declare let syncIterable: Iterable<number>;
let res = syncIterable[Symbol.iterator]().next();
if (!res.done) {
    res.value // number
} else {
    res.value // any
}

[bterlson]

In addition to also wanting the fix suggested above (plumbing TReturn and TNext from AsyncIterableIterator to IteratorResult), I wonder if any is the best type? When I see an any I don't suspect I need to narrow, I suspect I've done something wrong. undefined could be a better default, though I haven't considered this at length.

[rbuckton]

This is a duplicate of 33353. Also, your expected behavior is incorrect. Without checking done, you cannot guarantee that value is a number:

const numbers: number[] = [];
const value = numers[Symbol.iterator]().next().value;
console.log(value); // undefined

While the correct result should be number | undefined, but may be a significant breaking change.

[ikokostya]

Duplicate of #33353