Use generic rest parameters for combineLatest, zip, merge, concat, withLatestFrom

[ggradnig]

Use generic rest parameters for combineLatest, zip, merge, concat, withLatestFrom (more than 6 input Observables)

Is your feature request related to a problem? Please describe.
There are currently no type signatures for the mentioned operators/creation functions that allow more than 6 Observables while still keeping the correct types for the subscriber (anything above creates an impractical union type).

Describe the solution you'd like
Add a signature that makes use of generic rest parameters (documented here) and mapped types. Currently, the signatures look like this:

export function combineLatest<T, T2, T3, T4, T5, T6>(v1: ObservableInput<T>, 
    v2: ObservableInput<T2>, v3: ObservableInput<T3>, v4: ObservableInput<T4>, 
    v5: ObservableInput<T5>, v6: ObservableInput<T6>, scheduler?: SchedulerLike)
    : Observable<[T, T2, T3, T4, T5, T6]>;

This could for example be replaced with:

type CombinedObservable<T> = Observable<{
  [P in keyof T] : T[P] extends ObservableInput<infer U> ? U : never
}>

combineLatest<T extends (ObservableInput<any> | SchedulerLike)[]> (...obs: T) 
    : CombinedObservable<T>

These signatures would be more concise and enable more than 6 input Observables to be used with correct typings. The required TypeScript version is 3.0.

[cartant]

There's a lot that can be done to improve the types, once TypeScript version 3 becomes the minimum-supported version, but that won't be until v7.

[ggradnig]

Oh I see, didn't find the minimum supported TS version for RxJS 6 anywhere. Thanks for the info, will use own typings then :)

[cartant]

Using tuples and rest parameters - necessitating a minimum of TS 3.0 - would be a breaking change for many projects, so we'll need to wait for v7 to do this. The good news is that v7 is getting close to an alpha. Also, we now have a large portion of the API surface area covered with dtslint-based typings tests - so making changes to the typings ought to be less worrying. When we do get around to taking advantage of what TS 3.0 has to offer, any contributions you're able to make would definitely be appreciated.

[ggradnig]

Okay, thanks for the detailed reasoning and update on the development status. I'll stick around for v7 and see what I can contribute :-)