RxJS operator to map emitted value to `true` and to emit `false` after some period of inactivity (On/AutoOff behavior)

index.ts

import { merge, Observable, Subject } from 'rxjs';
import { debounceTime, map } from 'rxjs/operators';

// Emit `true` on any source emission, then emit `false` after period of inactivity
const turnOffAfter = (timeout: number) => (source: Observable<any>) => merge(
  source.pipe(map(_ => true)),                         // map original value to `true`
  source.pipe(debounceTime(timeout), map(_ => false)), // emit `false` after timeout
);

// Example usage
const trackStart: Subject<void> = new Subject<void>();
const onOff$: Observable<boolean> = trackStart.asObservable()
  .pipe(
    turnOffAfter(3_000),
  );