scrollAnimated.js

function scrollAnimated(to, duration) {
  var start = $(window).scrollTop(),
    change = to - start,
    currentTime = 0,
    increment = 20;

  function easeInOut(currentTime, startValue, change, duration) {
    currentTime /= duration / 2;

    if (currentTime < 1) {
      return change / 2 * currentTime * currentTime + startValue;
    }

    currentTime--;

    return -change / 2 * (currentTime * (currentTime - 2) - 1) + startValue;
  }

  function animateScroll() {
    currentTime += increment;

    var val = easeInOut(currentTime, start, change, duration);

    $(window).scrollTop(val);

    if (currentTime < duration) {
      setTimeout(animateScroll, increment);
    }
  }

  animateScroll();
}