Timer (setInterval, OOP in js)

<div id="timer">00:00:00</div>
<button onClick="toggleTimer(event)">start</button>
<button onClick="timer.reset()">Reset</button>
<script>
  class Timer {
    constructor() {
      this.heurs = 0;
      this.minute = 0;
      this.seconde = 0;
      this.interval = null;
    }
    start() {
      if (!this.interval)
        this.interval = setInterval(() => {
          if (this.seconde >= 59) {
            ++this.minute;
            this.seconde = 0;
          } else ++this.seconde;
          if (this.minute >= 59) {
            ++this.heurs;
            this.minute = 0;
          }
          this.elem.innerText =
          (this.heurs < 10? "0" : "") + this.heurs + ":" +
          (this.minute < 10? "0" : "") + this.minute + ":" +
          (this.seconde < 10? "0" : "") + this.seconde;
        }, 1000);
    }
    stop() {
      if (this.interval) clearInterval(this.interval);
      this.interval = null;
    }
    reset() {
      this.heurs = 0;
      this.minute = 0;
      this.seconde = 0;
      this.elem.innerText =
          (this.heurs < 10? "0" : "") + this.heurs + ":" +
          (this.minute < 10? "0" : "") + this.minute + ":" +
          (this.seconde < 10? "0" : "") + this.seconde;
    }
    setDisplay(elem) {
      this.elem = elem;
    }
  }

  let timer = new Timer();
  timer.setDisplay(document.querySelector("#timer"));
  function toggleTimer(e) {
    if (!timer.interval) {
      e.target.innerText = "stop";
      timer.start();
    } else {
      e.target.innerText = "start";
      timer.stop();
    }
  }
</script>