Simple Javascript countdown timer

javascript_timer.js

// index.html
<p id="timer">
  00:00:00
</p>

// script.js
let count = 0;
let intervalRef = null;

intervalRef = setInterval(_ => {
  count+=10;

  let ms = count % 1000;
  let s = Math.floor((count /  1000)) % 60;
  let m = Math.floor((count / 60000)) % 60;

  $('#timer').text(m + ":" + s + ":" + ms);
}, 10);