JSFiddle - React, Tailwind, and code Playground

HTML

<script src="https://unpkg.com/rxjs/bundles/Rx.min.js"></script>

JavaScript

const methodA = () => Rx.Observable.timer(500).mapTo("doing some work A")
const methodB = () => Rx.Observable.timer(250).mapTo("doing some work B")
const methodC = () => Rx.Observable.of("should be the last...")

const main = (some_condition) =>
  Rx.Observable.if(
    () => some_condition,
    methodA().merge(methodB()),
    methodA()
  )
  .concat(methodC())
  .subscribe(console.log)

main(true)