JSFiddle - React, Tailwind, and code Playground

HTML

<button id="btn">Start</button>
<div id="countdown"></div>

JavaScript

var seconds = 60;
var countdownTimer = null;

var btn = document.getElementById('btn');
btn.addEventListener('click', function() {
    setInterval('secondPassed()', 1000);
});

function secondPassed() {
    var minutes = Math.round((seconds - 30) / 60);
    var remainingSeconds = seconds % 60;
    if (remainingSeconds < 10) {
        remainingSeconds = "0" + remainingSeconds;
    }
    document.getElementById('countdown').innerHTML = minutes + ":" + remainingSeconds;
    if (seconds == 0) {
        clearInterval(countdownTimer);
        document.getElementById('countdown').innerHTML = "Times Up!!!";
    } else {
        seconds--;
    }
}