Double digit 30 seconds countdown

Question

I would like to have the countdown like

:10, :09, :08

and so on. Please help a beginner.

JS:

<script type="text/javascript">

    var timeleft = 30;
    var downloadTimer = setInterval(function(){
    timeleft--;
    document.getElementById("countdowntimer").textContent = ":" + timeleft;
    if(timeleft <= 0){
        clearInterval(downloadTimer);
        window.location.replace("next_slide.html");
        }
    },1000);

</script>

Answer 1

This is how you can proceed:

<script type="text/javascript">

    var timeleft = 30;
    var downloadTimer = setInterval(function(){
    timeleft--;
    document.getElementById("countdowntimer").textContent = ":" + ((timeleft >= 10) ? timeleft : ("0" + timeleft));
    if(timeleft <= 0){
        clearInterval(downloadTimer);
        window.location.replace("next_slide.html");
        }
    },1000);

</script>