js animate scroll to the top of the page

// With easing equations:

// http://www.paulirish.com/2011/requestanimationframe-for-smart-animating/
window.requestAnimFrame = (function(){
  return  window.requestAnimationFrame       ||
          window.webkitRequestAnimationFrame ||
          window.mozRequestAnimationFrame    ||
          function( callback ){
            window.setTimeout(callback, 1000 / 60);
          };
})();

// main function
function scrollToY(scrollTargetY, speed, easing) {
    // scrollTargetY: the target scrollY property of the window
    // speed: time in pixels per second
    // easing: easing equation to use

    var scrollY = window.scrollY || document.documentElement.scrollTop,
        scrollTargetY = scrollTargetY || 0,
        speed = speed || 2000,
        easing = easing || 'easeOutSine',
        currentTime = 0;

    // min time .1, max time .8 seconds
    var time = Math.max(.1, Math.min(Math.abs(scrollY - scrollTargetY) / speed, .8));

    // easing equations from https://github.com/danro/easing-js/blob/master/easing.js
    var easingEquations = {
            easeOutSine: function (pos) {
                return Math.sin(pos * (Math.PI / 2));
            },
            easeInOutSine: function (pos) {
                return (-0.5 * (Math.cos(Math.PI * pos) - 1));
            },
            easeInOutQuint: function (pos) {
                if ((pos /= 0.5) < 1) {
                    return 0.5 * Math.pow(pos, 5);
                }
                return 0.5 * (Math.pow((pos - 2), 5) + 2);
            }
        };

    // add animation loop
    function tick() {
        currentTime += 1 / 60;

        var p = currentTime / time;
        var t = easingEquations[easing](p);

        if (p < 1) {
            requestAnimFrame(tick);

            window.scrollTo(0, scrollY + ((scrollTargetY - scrollY) * t));
        } else {
            console.log('scroll done');
            window.scrollTo(0, scrollTargetY);
        }
    }

    // call it once to get started
    tick();
}

// scroll it!
scrollToY(0, 1500, 'easeInOutQuint');

function scrollTo(element, to, duration) {
    if (duration <= 0) return;
    var difference = to - element.scrollTop;
    var perTick = difference / duration * 10;

    setTimeout(function() {
        element.scrollTop = element.scrollTop + perTick;
        if (element.scrollTop === to) return;
        scrollTo(element, to, duration - 10);
    }, 10);
}

// Experimental: https://developer.mozilla.org/en-US/docs/Web/API/Element/scrollIntoView#Browser_compatibility
document.body.scrollIntoView({behavior: 'smooth', block: 'start'});