How to make a timer in JavaScript?

To make a countdown timer in JavaScript, decrease the seconds in setInterval, then output to a div.

For example, if this is your div:

<div id="timer"></div>

Then you can make a 30-seconds countdown timer with this JavaScript:

window.addEventListener('DOMContentLoaded', () => {
  let seconds = 30;

  const intervalId = setInterval(() => {
    document.getElementById('timer').textContent = `00:${seconds < 10 ? 0 : ''}${seconds}`;

    if (--seconds < 0) {
      clearInterval(intervalId);
    }
  }, 1_000);
});