// RxJS v6+
import { fromEvent, interval } from 'rxjs';
import { tap, scan, map, switchMap, takeWhile } from 'rxjs/operators';
import {
  dot,
  updatedDot,
  setTimerText,
  resetDotSize,
  moveDot
} from './dom-updater';
​
interface State {
  score: number;
  intrvl: number;
}
const makeInterval = (val: State) =>
  interval(val.intrvl).pipe(
    map(v => 5 - v),
    tap(setTimerText)
  );
const gameState: State = { score: 0, intrvl: 500 };
const nextState = (acc: State) => ({
  score: (acc.score += 1),
  intrvl: acc.score % 3 === 0 ? (acc.intrvl -= 50) : acc.intrvl
});
const isNotGameOver = intervalValue => intervalValue >= 0;
​
const game$ = fromEvent(dot, 'mouseover').pipe(
  tap(moveDot),
  scan < Event,
  State > (nextState, gameState),
  tap(state => updatedDot(state.score)),
  switchMap(makeInterval),
  tap(resetDotSize),
  takeWhile(isNotGameOver)
);
​
game$.subscribe(n => {}, e => {}, () => setTimerText('ouch!'));

const random = () => Math.random() * 300;
const elem = id => document.getElementById(id);
const setElementText = (elem, text) => (elem.innerText = text.toString());
const timer = elem('timer');
const setDotSize = size => {
  dot.style.height = `${size}px`;
  dot.style.width = `${size}px`;
};
​
export const dot = elem('dot');
export const updatedDot = score => {
  if (score % 3 === 0) {
    dot.style.backgroundColor =
      '#' + ((Math.random() * 0xffffff) << 0).toString(16);
  }
  setElementText(dot, score);
};
export const setTimerText = text => setElementText(timer, text);
export const moveDot = () => {
  setDotSize(5);
  dot.style.transform = `translate(${random()}px, ${random()}px)`;
};
export const resetDotSize = () => setDotSize(30);

<style>
  #dot {
    margin-top: 10px;
    height: 30px;
    width: 30px;
    background-color: lightgray;
    border-radius: 50%;
    transition: all 0.6s ease-in-out;
    text-align: center;
    color: white;
  }
​
  #timer {
    position: absolute;
    top: 150px;
    left: 150px;
    opacity: 0.1;
    font-size: 60px;
  }
</style>
​
<div id="timer"></div>
<div id="dot"></div>