concatWith

Emits all of the values from the source observable, then, once it completes, subscribes to each observable source provided, one at a time, emitting all of their values, and not subscribing to the next one until it completes.

concat(a$, b$, c$) is the same as a$.pipe(concatWith(b$, c$)).

Example

Listen for one mouse click, then listen for all mouse moves.

import { fromEvent, map, take, concatWith } from 'rxjs';

const clicks$ = fromEvent(document, 'click');
const moves$ = fromEvent(document, 'mousemove');

clicks$.pipe(
  map(() => 'click'),
  take(1),
  concatWith(
    moves$.pipe(
      map(() => 'move')
    )
  )
)
.subscribe(x => console.log(x));

// 'click'
// 'move'
// 'move'
// 'move'
// ...