// RxJS v6+
import { interval, of } from 'rxjs';
import { expand, take } from 'rxjs/operators';
​
//emit 2
const source = of(2);
const example = source.pipe(
  //recursively call supplied function
  expand(val => {
    //2,3,4,5,6
    console.log(`Passed value: ${val}`);
    //3,4,5,6
    return of(1 + val);
  }),
  //call 5 times
  take(5)
);
/*
    "RESULT: 2"
    "Passed value: 2"
    "RESULT: 3"
    "Passed value: 3"
    "RESULT: 4"
    "Passed value: 4"
    "RESULT: 5"
    "Passed value: 5"
    "RESULT: 6"
    "Passed value: 6"
*/
//output: 2,3,4,5,6
const subscribe = example.subscribe(val => console.log(`RESULT: ${val}`));