Scroller - Add a button to scroll smoothly down a page

scroller.js

// run in a javascript console to add an auto-scroller to the page

var scroller = (function() {
  var button = document.createElement('button'),
      scroller = {},
      scrollerY = 0,
      going = false,
      last = 0,
      requestAnimationFrame = window.requestAnimationFrame || window.mozRequestAnimationFrame ||
                              window.webkitRequestAnimationFrame || window.msRequestAnimationFrame;

  scroller.speed = 10;

  function step(timestamp) {
    var progress = timestamp - last;
    // adjust if user scrolls
    if (Math.abs(window.scrollY - scrollerY) > 4) scrollerY = window.scrollY;
    // check if scrolling 1px
    if (progress > (1000/scroller.speed)) {
      last = timestamp;
      window.scrollTo(0,scrollerY++)
    }
    // reset at bottom of page
    if (window.scrollY < scrollerY - 4) {
      window.scrollTo(0,scrollerY=0)
    }
    // continue
    if (going) {
      requestAnimationFrame(step);
    }
  }

  // start scrolling
  scroller.go = function() {
    going = true;
    scrollerY = window.scrollY;
    requestAnimationFrame(step);
    button.onclick = scroller.pause;
    button.innerHTML = "pause";
    style(button, {
      background: '#aaa',
    });
  };

  // pause scrolling
  scroller.pause = function() {
    going = false;
    button.onclick = scroller.go;
    button.innerHTML = "play";
    style(button, {
      background: '#3b2',
    });
  };

  // create button
  button.innerHTML = "play";
  style(button, {
    position: 'fixed',
    bottom: '4px',
    right: '4px',
    zIndex: '100',
    fontSize: '11px',
    padding: '2px 4px',
    background: '#3b2',
    color: '#fff',
    lineHeight: '13px',
    fontWeight: 'bold',
    border: '1px solid #ccc',
    cursor: 'pointer'
  });

  function style(el, obj) {
    for (k in obj) {
      el.style[k] = obj[k];
    }
  };

  document.body.appendChild(button);

  button.onclick = scroller.go;

  return scroller;
})();