Typescript: RXJS `distinctUntilChanged` not working with strict / first parameter undefined

Question

The following code doesn't work:

let observable: Observable<{ key: number }> = of({ key: 123 });
observable.pipe(
    distinctUntilChanged(undefined, v => v.key)
);

Although there is an overload:

export function distinctUntilChanged<T, K>(
  comparator?: (previous: K, current: K) => boolean,
  keySelector: (value: T) => K = identity as (value: T) => K
): MonoTypeOperatorFunction<T>

What's the reason for it?

Answer 1

The example provided, is not a TypeScript example, so undefined is not allowed in the below types as a first argument! instead just specify a default comparator and set the keySelector!

Type Definition:

import { MonoTypeOperatorFunction } from '../types';
export declare function distinctUntilChanged<T>(comparator?: (previous: T, current: T) => boolean): MonoTypeOperatorFunction<T>;
export declare function distinctUntilChanged<T, K>(comparator: (previous: K, current: K) => boolean, keySelector: (value: T) => K): MonoTypeOperatorFunction<T>;

Code:

import { of, distinctUntilChanged, Observable } from 'rxjs';
export interface TypeObj {
  updatedBy: string;
  data: Array<string>;
}
// A stream of updates to a given account
const accountUpdates$: Observable<TypeObj> = of(
  { updatedBy: 'blesh', data: [] },
  { updatedBy: 'blesh', data: [] },
  { updatedBy: 'ncjamieson', data: [] },
  { updatedBy: 'ncjamieson', data: [] },
  { updatedBy: 'blesh', data: [] }
);

// We only want the events where it changed hands
const changedHands$ = accountUpdates$.pipe(
  distinctUntilChanged<TypeObj, string>(
    (prev: string, curr: string): boolean => prev === curr,
    (update: TypeObj) => update?.updatedBy
  )
);

changedHands$.subscribe(console.log);

Stackblitz Demo