mergeMapTo-example.js

const { interval, Observable } = Rx;
const { map, take, switchMap, startWith, mergeMapTo } = RxOperators;

function outerObservable() {
  return new Observable(subscriber => {
    setTimeout(() => {
    	subscriber.next("A");
    }, 1000);
    
    setTimeout(() => {
    	subscriber.next("C");
    }, 1500);
    
    setTimeout(() => {
    	subscriber.next("B");
      subscriber.complete();
    }, 4000);
  }) 
}

// Emit the value 10 every second
function innerObservable() {
  return interval(1000).pipe(map(val => val + 1),take(3));
}

outerObservable().pipe(
  mergeMapTo(
    innerObservable(),
    (x, y) => x + y
  )
);