rxjs and angular router events: Set boolean to inital value after first emit

this function below should return true if an angular router event was triggered.

private routeChanged$(): Observable<boolean> {
    return this.router.events.pipe(
        filter(event => event instanceof NavigationEnd),
        mapTo(true),
        startWith(false),
        untilDestroyed(this)
    );
}

it works fine but it should also reset again the inital value (false) after the event was fired so i basically want to set the boolean back to false after it emits true the first time (when the route changed) (sorry for my bad english, i hope you get my point)

Do you mean it should immediately emit false again after true?

In that case you could do:

private routeChanged$(): Observable<boolean> {
    return this.router.events.pipe(
        filter(event => event instanceof NavigationEnd),
        switchMapTo([true, false]),
        startWith(false),
        untilDestroyed(this)
    );
}

switchMapTo will "subscribe" to the array - which is interpreted as an Observable emitting two values, true and false.