// RxJS v6+
import { BehaviorSubject } from 'rxjs';
​
const subject = new BehaviorSubject(123);
​
// two new subscribers will get initial value => output: 123, 123
subject.subscribe(console.log);
subject.subscribe(console.log);
​
// two subscribers will get new value => output: 456, 456
subject.next(456);
​
// new subscriber will get latest value (456) => output: 456
subject.subscribe(console.log);
​
// all three subscribers will get new value => output: 789, 789, 789
subject.next(789);
​
// output: 123, 123, 456, 456, 456, 789, 789, 789

// RxJS v6+
import { BehaviorSubject, fromEvent, interval, merge } from 'rxjs';
import { map, tap, mergeMap } from 'rxjs/operators';
​
const setElementText = (elemId, text) =>
  (document.getElementById(elemId).innerText = text.toString());
const addHtmlElement = coords =>
  (document.body.innerHTML += `
  <div 
    id=${coords.id}
    style="
      position: absolute;
      height: 30px;
      width: 30px;
      text-align: center;
      top: ${coords.y}px;
      left: ${coords.x}px;
      background: silver;
      border-radius: 80%;"
    >
  </div>`);
​
const subject = new BehaviorSubject(0);
​
const click$ = fromEvent(document, 'click').pipe(
  map((e: MouseEvent) => ({
    x: e.clientX,
    y: e.clientY,
    id: Math.random()
  })),
  tap(addHtmlElement),
  mergeMap(coords => subject.pipe(tap(v => setElementText(coords.id, v))))
);
​
const interval$ = interval(1000).pipe(
  tap(v => subject.next(v)),
  tap(v => setElementText('intervalValue', v))
);
​
merge(click$, interval$).subscribe();