Rxjs操作员switchMap

Question

I'm new with Rxjs and write some examples for better understanding. However, the example below confuses me, why the output is 0 and 1? Can someone explain it?

var sources = Rx.Observable.timer(1000,1000);
var higherOrder = sources.switchMap(val => Rx.Observable.of(val).delay(1000*val));
higherOrder.subscribe(x => console.log(x));

In additon:

var sources = Rx.Observable.timer(1000,1000);
var higherOrder = sources.switchMap(val => Rx.Observable.of(val));
higherOrder.subscribe(x => console.log(x));

the outputs are : 0 1 2 3 4 5 ...

Answer 1

switchMap will abandon (unsubscribe) the previous emitted observable once a new one arrives. So eventually your values are delayed long enough for your timer to emit a new value creating a new Observable for your switchMap to subscribe to abandoning the previous one before retrieving its value.