Rxjs: Difference between Observable.First vs Single vs Filter

Question

I am exploring RxJS library and really a fan of using Observable instead of Promise. However, can someone provide any detailed information about the difference between using

• Observable.First
• Observable.Single
• Apply Filter in such a way that it returns only single item

What is the need for Single specifically in this library?

Answer 1

If by filter you mean something like:

let emitted = false;
obs = obs.filter(x => {
  if(emitted) {
    return false;
  } else {
    emitted = true;
    return true;
  }
});

Filter (in this particular case, check the code above)

Will emit as soon as first item appears. Will ignore all subsequent items. Will complete when source observable completes.

in : -1-2-3--|---
out: -1------|---

First

Will emit as soon as first item appears. Will complete right after that.

in : -1-2-3--|---
out: -1|----------

Single

Will fail if source observable emits several events.

in : -1-2-3--|---
out: -1-X---------

Will emit when source observable completes (and single can be sure nothing more can be emitted). Will complete right after that.

in : -1------|---
out: --------1|--