Improve type inference of pluck operator

[aboyton]

RxJS version:
5.4.3

Code to reproduce:

const obs = Observable.of([{ a: { b: 1 } }]);
const a = obs.pluck('a');

Expected behavior:
a be of type Observable<{b: 1}>.

Actual behavior:
a is of type Observable<{}>

Additional information:
While I realise that I can provide the type of the return value to pluck, doing so doesn't give me any type safety. Instead, this could be inferred, at least for an arbitrary level of nesting. Anyone who goes beyond this level of nesting wouldn't be able to use type inference, very similarly to the current state of things.

type Pluck<T, K extends keyof T> = T[K];

export function pluck<
  T,
  P1 extends keyof T
>(
  this: Observable<T>,
  property1: P1,
): Observable<Pluck<T, P1>>;

export function pluck<
  T,
  P1 extends keyof T,
  P2 extends keyof T[P1]
>(
  this: Observable<T>,
  property1: P1,
  property2: P2,
): Observable<Pluck<Pluck<T, P1>, P2>>;

export function pluck<
  T,
  P1 extends keyof T,
  P2 extends keyof T[P1],
  P3 extends keyof T[P1][P2]
>(
  this: Observable<T>,
  property1: P1,
  property2: P2,
  property3: P3,
): Observable<Pluck<Pluck<Pluck<T, P1>, P2>, P3>>;

export function pluck<
  T,
  P1 extends keyof T,
  P2 extends keyof T[P1],
  P3 extends keyof T[P1][P2],
  P4 extends keyof T[P1][P2][P3]
>(
  this: Observable<T>,
  property1: P1,
  property2: P2,
  property3: P3,
  property4: P4,
): Observable<Pluck<Pluck<Pluck<Pluck<T, P1>, P2>, P3>, P4>>;
export function pluck<T>(
  this: Observable<T>,
  ...properties: string[]
): Observable<any> {
  return higherOrder(...properties)(this);
}

[kwonoj]

Surprisingly, our 5 version is based on TS compiler doesn't support keyof. it's dupe of #2295, while we will revisit this probably in 6 with bumped up TSC version (due to some changes around runtime inherited error etcs we consider bump up as breaking) but we can't do this in 5.

[aboyton]

Sorry, I'm a bit confused by the tickets. #2295 and #2179 are both closed (thus I didn't find them when searching). Is there an open ticket that's tagged with RxJS 6.0 so we make sure that this isn't forgotten about?

I noticed #2759 which has fixes to the type of an operator (mergeAll()) that's planned for the 6.0 release is still open.

[kwonoj]

Is there an open ticket that's tagged with RxJS 6.0 so we make sure that this isn't forgotten about

I'm planning to reopen one of original issue after we stabilize 6 branch and create separate label. For now it's closed as not actionable. There's mixed issues between some are opened and some are not and it's simply we haven't gone through all issues.

[aboyton]

Is this ready to be reopened?

[lock]

This thread has been automatically locked since there has not been any recent activity after it was closed. Please open a new issue for related bugs.