How can i format dynamic value with javascript

How can i format dynamic value with javascript? I have a dynamic value in seconds in repeat region, how can i format seconds in to time value?

My javascript function formats seconds into HHmmss;
String.prototype.toHHMMSS = function () {

    var sec_num = parseInt(this, 10); // don't forget the second param

    var hours = Math.floor(sec_num / 3600);

    var minutes = Math.floor((sec_num - (hours * 3600)) / 60);

    var seconds = sec_num - (hours * 3600) - (minutes * 60);

    if (hours < 10) { hours = "0" + hours; }

    if (minutes < 10) { minutes = "0" + minutes; }

    if (seconds < 10) { seconds = "0" + seconds; }

    return hours + ':' + minutes + ':' + seconds;

}