timer.sec.js

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

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

        console.log(minutes + ":" + seconds);

        if (--timer < 0) {
            clearInterval(intervalId);
        }
    }, 1000);
}

startTimer(15)