To create a countdown timer please check the below example :-

Example -

In this example let's suppose a user fill a form and at that time we want to limit the form filling time to 5 minutes and at the same time we want to show that user how many minutes/seconds are still left there. if the set time limit will exceed then user will informed with a time out alert message.

So let's start code . create a file name timer.js and put below code over there.

var timer = {
    minutes :0,
    seconds : 0,
    elm :null,
    samay : null,
    sep : ':',
    init : function(m,s,elm)
    {
        m = parseInt(m,10);
        s = parseInt(s,10);
        if(m < 0 || s <0 || isNaN(m) || isNaN(s)) { alert('Invalid Values'); return; }
        this.minutes = m;
        this.seconds = s;
        this.elm = document.getElementById(elm);
        timer.start();
    },
    start : function()
    {
        this.samay = setInterval((this.doCountDown),1000);
    },
    doCountDown : function()
    {
        if(timer.seconds == 0)
        {
            if(timer.minutes == 0)
            {
                clearInterval(timer.samay);
                timerComplete();
                return;
            }
            else
            {
                timer.seconds=60;
                timer.minutes--;
            }
        }
        timer.seconds--;
        timer.updateTimer(timer.minutes,timer.seconds);
    },
    updateTimer :  function(min,secs)
    {
        min = (min < 10 ? '0'+min : min);
        secs = (secs < 10 ? '0'+secs : secs);
        (this.elm).innerHTML = min+(this.sep)+secs;
    }
}
function timerComplete()
{
    alert('time out buddy!!!');
}

For starting the timer write below lines just after where you included the timer.js

window.onload = init;
function init()
{
    timer.init(0,55,'container');
}

On the html side please create an element having id named as container.

In the above code init function have 3 parameters : minutes, seconds and the id of element where the countdown will be show.

Above created timer will start a countdown for 55 seconds. and after 55 seconds, timerComplete() function will executed and alert a message "time out buddy!!!".