RxJs Observable to run a series of promises in series with no concurrency

rxjs.sync.js

const Rx = require('rxjs'); // Did this in node using `npm install rxjs`

// -- Our fake async function
const fakeAsync = (input) => {
  return new Promise((resolve, reject) => {
    console.log(`Start: ${input}`);
    setTimeout(() => {
      console.log(`Finish: ${input}`);
      resolve(`Processed ${input}`);
    }, Math.random()*1500+100);
  })
};

// -- Our observable Subject and Suscriber
var subject = new Rx.Subject();

var subscription = subject
  .concatMap((item) => fakeAsync(item))
  .subscribe(
    (item) => console.log('Done: %s', item),
    (error) => console.log('Error: %s', error),
    () => console.log('Completed')
  );

// --- Create 10 jobs over first 3 seconds
for (var fileIndex = 0; fileIndex < 10; fileIndex ++) {
  setTimeout((index) => {
    console.log(`Add: ${index}`);
    subject.next(index);
  }, 300 * fileIndex, fileIndex);
}

// --- Create 5 jobs a little after 5 seconds (while batch 1 is busy)
for (var fileIndex = 10; fileIndex < 15; fileIndex ++) {
  setTimeout((index) => {
    console.log(`Add: ${index}`);
    subject.next(index);
  }, 5000, fileIndex);
}

// --- Create 5 jobs a little after 15 seconds (after other batches)
for (var fileIndex = 20; fileIndex < 25; fileIndex ++) {
  setTimeout((index) => {
    console.log(`Add: ${index}`);
    subject.next(index);
  }, 15000, fileIndex);
}