How can I scroll to the top using vanilla JavaScript?

I'm trying to emulate scrollTop

(jQuery) in vanilla JS, so clicking it scrolls the element. This works great - if you haven't already scrolled the element. Therefore, it does not scroll the way back. Should my formula include window.pageYOffset

?

var moves = function(scrollz) {
    var scrollPos = document.getElementById(scrollz).offsetTop - ((document.documentElement.clientHeight - document.getElementById(scrollz).offsetHeight) / 2);

    var timerID = setInterval(function() {
        window.scrollBy(0, speed);
        if (window.pageYOffset >= scrollPos) {
            clearInterval(timerID);
        }
    }, 13);
}