Example of TS 3.1 Mapped Tuple Type

combineLatest.ts

interface Observable<T> {
  subscribe(cb: (v: T) => void): void;
}

type StreamValue<S> = S extends Observable<infer T> ? T : never;

type Unwrap<S> = { [P in keyof S]: StreamValue<S[P]> };

declare function combineLatest<U extends Observable<any>[]>(...streams: U): Observable<Unwrap<U>>;

function test(s1$: Observable<string>, s2$: Observable<number>, s3$: Observable<string>) {
  combineLatest(s1$, s2$).subscribe(([s1, s2]) => console.log(s1, s2));
  combineLatest(s1$, s2$, s3$).subscribe(([s1, s2, s3]) => console.log(s1, s2, s3));
}