JSFiddle - React, Tailwind, and code Playground

HTML

<div>Timeout in <span id="time">00:00</span> minutes</div>

JavaScript

function startTimer(duration, display) {
    var timer = duration, minutes, seconds;
    setInterval(function () {
        minutes = parseInt(timer / 60, 10)
        seconds = parseInt(timer % 60, 10);

        minutes = minutes < 10 ? "0" + minutes : minutes;
        seconds = seconds < 10 ? "0" + seconds : seconds;

        display.textContent = minutes + ":" + seconds;

        if (--timer < 0) {
            timer = duration;
        }
    }, 1000);
}

window.onload = function () {
    var time = 5,
        display = document.querySelector('#time');
    var isDone = startTimer(time, display);

		if (isDone){
			alert("a");
    }
};