Function to scroll to specified X coord with a map of easing functions to choose from.

scrollToX.js

function scrollTo(X, duration, easingFunction, callback) {
  callback = callback || function () {};

  var start = Date.now(),
      elem = document.scrollLeft ? document : document.body,
      from = elem.scrollLeft;

  if (from === X) {
    callback();
    return; /* Prevent scrolling to the X point if already there */
  }

  function min(a,b) {
    return a < b ? a : b;
  }

  function scroll(timestamp) {
    var currentTime = Date.now(),
        time = min(1, ((currentTime - start) / duration)),
        easedT = easingFunction(time);

    elem.scrollLeft = (easedT * (X - from)) + from;

    if (time < 1) requestAnimationFrame(scroll);
    else
      if (callback) callback();
  }

  requestAnimationFrame(scroll)
}

var easing = {
  // no easing, no acceleration
  linear: function (t) { return t },
  // accelerating from zero velocity
  easeInQuad: function (t) { return t*t },
  // decelerating to zero velocity
  easeOutQuad: function (t) { return t*(2-t) },
  // acceleration until halfway, then deceleration
  easeInOutQuad: function (t) { return t<.5 ? 2*t*t : -1+(4-2*t)*t },
  // accelerating from zero velocity
  easeInCubic: function (t) { return t*t*t },
  // decelerating to zero velocity
  easeOutCubic: function (t) { return (--t)*t*t+1 },
  // acceleration until halfway, then deceleration
  easeInOutCubic: function (t) { return t<.5 ? 4*t*t*t : (t-1)*(2*t-2)*(2*t-2)+1 },
  // accelerating from zero velocity
  easeInQuart: function (t) { return t*t*t*t },
  // decelerating to zero velocity
  easeOutQuart: function (t) { return 1-(--t)*t*t*t },
  // acceleration until halfway, then deceleration
  easeInOutQuart: function (t) { return t<.5 ? 8*t*t*t*t : 1-8*(--t)*t*t*t },
  // accelerating from zero velocity
  easeInQuint: function (t) { return t*t*t*t*t },
  // decelerating to zero velocity
  easeOutQuint: function (t) { return 1+(--t)*t*t*t*t },
  // acceleration until halfway, then deceleration
  easeInOutQuint: function (t) { return t<.5 ? 16*t*t*t*t*t : 1+16*(--t)*t*t*t*t }
};