animace rolování bez jQuery

selected=0;
$('.scroll').click(function(event){
    event.preventDefault();
    distance = $('.curtain').height(); // .curtain = třída obrazových pozadí o výšce jedné obrazovky
    selected = $(this).attr('href').substr(2);
    scrolldistance = distance*selected;
    $('html, body').animate({scrollTop: scrolldistance}, 500);
});

var selected=0;
var scrolls = document.querySelectorAll(".scroll");
for (var n=0; n<scrolls.length; n++) {
scrolls[n].addEventListener("click", function(event) {
    event.preventDefault();
    var distance = document.querySelector('.curtain').clientHeight;
    selected = this.getAttribute('href').substr(2);
    var scrolldistance = distance*selected;
    var start = new Date().getTime();
    var interval = setInterval(function() {
    var step = (new Date().getTime() - start) / 500;
    if (step == 1) clearInterval(interval);
    document.documentElement.scrollTop = scrolldistance*step;
    document.body.scrollTop = scrolldistance*step;
    }, 25);
});
}