This operator can take arrays or a Promise as input and turn them into a stream. To use it, simply call it like this:

// creation-operators/from.js

const promiseStream$ = Rx.Observable.from(
  new Promise(resolve => setTimeout(() => resolve("data"),3000))
);

const arrayStream$ = Rx.Observable.from([1, 2, 3, 4]);

promiseStream$.subscribe(data => console.log("data", data));
// emits data after 3 seconds

arrayStream$.subscribe(data => console.log(data));
// emits 1, 2, 3, 4

This saves us a lot of headache by not having to deal with different types of asynchronous calls.