how to map the observable if there is need to subscribe inside data before forming it in mapped result?

I am trying to do a subscribe of observable which formed through multiple times with map operator to build that final result.

So let say If I have outer observable functionA() where we had subscribed to it,but there is functionB() : observable where data is coming through two observable and second observable is dependent to first one result, then how can we get the return the final mapped observable . please check code what I need to describe:-

functionB() : observable<any>{
return this.datastream1().map((objectA)=>{
return [...objectA,...objectB]



So in this example while I am subscribing it then I am getting the observable in console instead of getting the spreaded data coming from both observables.

how can I make wait to resolve nested observable dependencies first while subscribing.In this scenario both observable should be resolved and then data should be returned back to subscribing function that is textfunctionA()

2 answers

  • answered 2019-09-15 19:20 mbojko

    A flatMap (or one of its siblings) will return a new stream, with one common .subscribe() at the end of the chain getting the business going. There probably is a neater method of including objectA in the resulting stream, but this should do the job:

    functionB() : observable<any> {
        return this.datastream1().pipe(
            flatMap(objectA => combineLatest(of(objectA), this.datastream2(objectA))

  • answered 2019-09-16 07:27 ChrisY

    I think you can use the concatMap operator, which internally does not subscribe to the next observable until the previous is completed.

    const datastream1 = () =>
      of([{ value: 1 }, { value: 2 }, { value: 3 }]).pipe(delay(3000));
    const datastream2 = dependentData => {
      const result = [
        { value: dependentData[0].value + 100 },
        { value: dependentData[1].value + 100 },
        { value: dependentData[2].value + 100 }
      return of(result);
    const example = datastream1().pipe(
      concatMap(val1 => datastream2(val1).pipe(map(val2 => [...val1, ...val2])))
    example.subscribe(val => {
      val.forEach(v => console.log("Value: " + v.value));

    In the example above we delay the emits of datastream1 by 3 seconds. datastream2 though is waiting for the completion of datastream1 before it starts emitting. You can try it here.