JavaScript - Countdown minutes and seconds

by katalin_2003

HTML

<span id="countdown" class="timer"></span>

CSS

.timer {
    font-family: sans-serif;
    font-size: 5em;
}

JavaScript

var seconds, upgradeTime;
seconds = upgradeTime = 300;

function timer() {
    var days = Math.floor(seconds / 24 / 60 / 60);
    var hoursLeft = Math.floor((seconds) - (days * 86400));
    var hours = Math.floor(hoursLeft / 3600);
    var minutesLeft = Math.floor((hoursLeft) - (hours * 3600));
    var minutes = Math.floor(minutesLeft / 60);
    var remainingSeconds = seconds % 60;
    // add a 0 in front of single digit seconds
    if (remainingSeconds < 10) {
        remainingSeconds = "0" + remainingSeconds;
    }
    // add a 0 in front of single digit minutes
    if (minutes < 10) {
        minutes = "0" + minutes;
    }

    document.getElementById('countdown').innerHTML = /*days + ":" + hours + ":" + */
    minutes + ":" + remainingSeconds;

    if (seconds === 0) {
        clearInterval(countdownTimer);
        document.getElementById('countdown').innerHTML = "Completed :)";
        // do something when timer reaches 0 (reload page function etc..)
        // location.reload();
    } else {
        seconds--;
    }
}
var countdownTimer = setInterval(timer, 1000);