mergeMap + forkJoin with accumulation example

index.js

const { rxObserver } = require('api/v0.3');
const { forkJoin, timer, of } = require('rxjs');
const { map, mapTo, mergeMap } = require('rxjs/operators');


const parentTasksToSave = 1;
const childTasksToSave = [2, 3];

createWorkItem(parentTasksToSave).pipe(
  mergeMap(parentId => {
    if (parentId === null) {
      return of([ parentId ]);
    }
    
    const childTaskObservables = childTasksToSave.map(
      childId => createChildWorkItem(childId)
    );

    return forkJoin(childTaskObservables).pipe(
      map(ids => [parentId, ...ids])
    );
  })
)
.subscribe(rxObserver());

// MOCK that will create work item in 300ms
function createWorkItem(id){
  return timer(300).pipe(
    mapTo(id)
  );
}

// MOCK that will create child work item in 150ms
function createChildWorkItem(id) {
  return timer(150).pipe(
    mapTo(id)
  );
}