emit numbers in sequence every specified duration using interval and timer in RxJS

interval

// RxJS v6+
import { interval } from 'rxjs';

//emit value in sequence every 1 second
const source = interval(1000);
//output: 0,1,2,3,4,5....
const subscribe = source.subscribe(val => console.log(val));

timer

// RxJS v6+
import { timer } from 'rxjs';

/*
  timer takes a second argument, how often to emit subsequent values
  in this case we will emit first value after 1 second and subsequent
  values every 2 seconds after
*/
const source = timer(1000, 2000);
//output: 0,1,2,3,4,5......
const subscribe = source.subscribe(val => console.log(val));

// RxJS v6+
import { timer } from 'rxjs';

//emit 0 after 1 second then complete, since no second argument is supplied
const source = timer(1000);
//output: 0
const subscribe = source.subscribe(val => console.log(val));

References
https://stackoverflow.com/questions/44165893/how-do-i-make-an-observable-interval-start-immediately-without-a-delay
https://www.learnrxjs.io/operators/creation/interval.html
https://www.learnrxjs.io/operators/creation/timer.html
https://rxjs.dev/api/index/function/interval
https://rxjs.dev/api/index/function/timer
https://stackoverflow.com/questions/44165893/how-do-i-make-an-observable-interval-start-immediately-without-a-delay