var SCROLL_SPEED = 400;

var ready = function(callback) {
  if (document.readyState != "loading") callback();
  else document.addEventListener("DOMContentLoaded", callback);
}

ready(function() {
  jQuery(window).scroll(function() {
    if (jQuery(this).scrollTop() > 50) {
      jQuery('#back-to-top').fadeIn();
    } else {
      jQuery('#back-to-top').fadeOut();
    }
  });
  jQuery('#back-to-top').click(function() {
    jQuery('body,html').animate({
      scrollTop: 0
    }, SCROLL_SPEED);
    return false;
  });
});