MediaWiki:Gadget-BackToTop.js

$('body').prepend('<a href="#" class="back-to-top">Back to Top</a>');

var amountScrolled = 300;

$(window).scroll(function () {
  if ($(window).scrollTop() > amountScrolled) {
    $('a.back-to-top').fadeIn('slow');
  } else {
    $('a.back-to-top').fadeOut('slow');
  }
});

$('a.back-to-top, a.simple-back-to-top').click(function () {
  $('html, body').animate({
    scrollTop: 0
  }, 'fast');
  return false;
});