groupBy should support typeguards

[benlesh]

I'm not even sure this is possible, but it would be great if this "just worked":

function isNumber(input: any): input is number {
   return typeof input === 'number';
}

of('a', 1, 'b', 2).pipe(
   groupBy(isNumber),
   mergeMap(group => {
     if (group.key) {
         return group; // Observable<number>
     } else {
         return group; // Observable<string>
     }
   });
)

[cartant]

This sounds like splitBy - see #3807 - which I implemented here but I didn't open a PR for it - I guess I wasn't entirely happy with it. I can take another look.

[cartant]

Hmm, this is a little weird. It would seem that adding this signature should do it:

  export function groupBy<T, K extends T>(
    keySelector: (value: T) => value is K
  ): OperatorFunction<T, GroupedObservable<true, K> | GroupedObservable<false, T>>;

And the key property needs to be made const:

rxjs/src/internal/operators/groupBy.ts

Line 281 in f5c6332

constructor(public key: K,

However, the type is only narrowed within the if and not the else - which is weird:

of("a", 1, "b", 2).pipe(
  groupBy(isNumber),
  mergeMap(group => {
    if (group.key) {
      return group; // GroupedObservable<true, number>
    } else {
      return group; // GroupedObservable<true, number> | GroupedObservable<false, any>
    }
  })
);

It looks like the inference isn't taking into account that the type of key is true | false which is a boolean.