Simple timer object for calculating run-time with option to display the time in seconds.

Timer.js

/**
Example JS:
  const timer = new Timer();
  timer.start();
  timer.stop();

Example HTML:
  <span id="timerDisplay"><!-- not required --></span>
  <button onclick="timer.start();">start</button>
  <button onclick="timer.stop();">stop</button
*/
function Timer() {
  this.timerInterval = null;
  this.startTime = 0;
  this.timerDisplay = document.getElementById("timerDisplay");
  this.start = function() {
    clearInterval(this.timerInterval);
    this.startTime = (new Date()).getTime();
    console.log("START TIME: " + (new Date()).toLocaleTimeString());
    if (this.timerDisplay) {
      this.timerDisplay.innerHTML = 0;
      this.timerInterval = setInterval(() => {
        const seconds = ((new Date()).getTime() - thisRef.startTime) / 1000;
        if (thisRef.timerDisplay) {
          thisRef.timerDisplay.innerHTML = seconds + " seconds";// Math.round(seconds * 100) / 100;
        }
      }, 10);
    }
    const thisRef = this;
  }
  this.stop = function() {
    clearInterval(this.timerInterval);
    const timestamp = (new Date()).toLocaleTimeString();
    const seconds = (((new Date()).getTime() - this.startTime) / 1000);
    console.log("STOP TIME: " + timestamp + ", time=" + seconds + " seconds");
    if (this.timerDisplay) {
      this.timerDisplay.innerHTML = seconds + " seconds";
    }
    return seconds;
  }
}