how to add a countdown timer and set condition in javascript code example

Example 1: add countdown timer to javascript quiz

var count = 15;
var interval = setInterval(function(){
  document.getElementById('count').innerHTML=count;
  count--;
  if (count === 0){
    clearInterval(interval);
    document.getElementById('count').innerHTML='Done';
    // or...
    alert("You're out of time!");
  }
}, 1000);

Example 2: countdown in js

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 fiveMinutes = 60 * 5,
        display = document.querySelector('#time');
    startTimer(fiveMinutes, display);
};