example of concatMap with a Promise

index.js

const { rxObserver } = require('api/v0.3');
const { timer } = require('rxjs');
const { concatMap } = require('rxjs/operators');


// our source$ will emit values at
// 0, 100, 200, 300...
const source$ = timer(0, 100);

const concatMap$ = source$.pipe(
    concatMap(i => makeAPromise(i * 50))
  );

// visualization
source$.subscribe(rxObserver('source$'));
concatMap$.subscribe(rxObserver('concatMap( promise )'));


// helpers
function makeAPromise(timeout){
  // return a promise that will be resolved
  // after a `timeout`

  return new Promise(resolve=>{
    setTimeout(()=>resolve(Date.now()), timeout);
  });
}