Avoiding nesting Observables ( mergeMap on complete )

Question

Got this pseudo code for a scraping service that first explores the pagination urls then the subpage urls on the scraped "parent" website

const pagination = [["url-1", "url-2", "url-3", "url-4"], ["url-5"]];
const timeInterval = 2;

sendRequestsInIntervals(pagination, timeInterval )
  .pipe()
  .subscribe({
    next(x) {
      console.log(x);
    },
    error(err) {
      console.error(err);
    },
    complete() {
      console.log("done");
      sendRequestsInIntervals(resDataArray, timeInterval ).subscribe({
        next(x) {
          console.log(x);
        },
      });
    },
  });

I wanna avoid nesting as its an incorrect way to use observables

Is there a way to convert this to something like this:

sendRequestsInIntervals(pagination, timeInterval )
  .pipe(
    waitUntilCompletes()
    mergeMap((resDataArray) => {
      return sendRequestsInIntervals(resDataArray, timeInterval );
    })
  )
  .subscribe({
    next(x) {
      console.log(x);
    },
    error(err) {
      console.error(err);
    },
    complete() {
      console.log("done");
    },
  });

Added a pseudo function called waitUntilCompletes()

Is there such a thing in rxJS that it makes the observable in the mergeMap wait before runing until the previous observable is completed?

Answer 1

the last operator will ignore all emissions until the final one upon completion:

sendRequestsInIntervals(pagination, timeInterval )
  .pipe(
    last(),
    mergeMap((resDataArray) => { // resDataArray is only the final emitted value from sendRequestsInIntervals() 
      return sendRequestsInIntervals(resDataArray, timeInterval );
    })
  )
  .subscribe({
    next(x) {
      console.log(x);
    },
    error(err) {
      console.error(err);
    },
    complete() {
      console.log("done");
    },
  });

or you can use reduce to gather the emissions and emit once complete:

sendRequestsInIntervals(pagination, timeInterval )
  .pipe(
    // this will gather all outter emissions into an array and emit once outter completes
    reduce((acc, val) => acc.concat(val), []),
    mergeMap((resDataArray) => { // resDataArray is all values emitted from sendRequestsInIntervals() in an array
      return sendRequestsInIntervals(resDataArray, timeInterval );
    })
  )
  .subscribe({
    next(x) {
      console.log(x);
    },
    error(err) {
      console.error(err);
    },
    complete() {
      console.log("done");
    },
  });