[RxJS] mergeMap

• Maps values to a new observable on emission from source, subscribing to and emitting results of inner observables
• By default mergeMap does not limit the number of active inner observables
• Useful for HTTP requests you do not want cancelled, such as POSTs
• Inner observables whose lifetime you mill manage
• [X] Remember to clean up inner observable

import { fromEvent, interval } from 'rxjs';
import { ajax } from 'rxjs/ajax';
import {
  mergeMap,
  catchError,
  takeUntil,
  map
} from 'rxjs/operators';

/*
 * BEGIN FIRST SECTION
 */
const interval$ = interval(1000);
const click$ = fromEvent(document, 'click');

/*
   * mergeMap invokes the function you provide,
   * subscribing to each returned observable internally.
   * Any values emitted by these inner observables
   * are then emitted by mergeMap. By default there
   * is no limit to the number of active inner
   * subscriptions that can be active at a time
   * with mergeMap, so if you continually click on
   * the page more and more timers will be activated.
   * This can be dangerous if you have long 
   * running inner observables and forget to clean
   * them up.
   */
click$.pipe(
   mergeMap(() => interval$)
 ).subscribe(console.log);

const mousedown$ = fromEvent(document, 'mousedown');
const mouseup$ = fromEvent(document, 'mouseup');

/*
   * In this case, we are mapping to a new interval
   * observable on mousedown, but we are limiting it's
   * lifetime by using the takeUntil operator with
   * the mouseup$ stream.
   */

mousedown$.pipe(
  mergeMap(() => interval$.pipe(
    takeUntil(mouseup$)
  ))
).subscribe(console.log);

const coordinates$ = click$.pipe(
  map((event: any) => ({
    x: event.clientX,
    y: event.clientY
  }))
);

/*
   * mergeMap is good for 'fire and forget' save request
   * you do not want to be cancelled. For instance, in this
   * example we are emulating a save of coordinates
   * anytime the user clicks on the page.
   */
const coordinatesWithSave$ = coordinates$.pipe(
  mergeMap(coords => ajax.post(
    'https://www.mocky.io/v2/5185415ba171ea3a00704eed'
  ))
);

coordinatesWithSave$.subscribe(console.log);