var autoScroll = function() {

  var speed = 0;
  var scrollDir = 'down';
  var viewport = window.innerHeight;
  var bottom = document.body.scrollHeight;

  var getSpeed = function() {
    speed = prompt("Please Enter a Scroll Speed");
    scrollMove();
    stopButton();
  };

  var stopButton = function() {
    scrolling = true;
    var stop = document.createElement('button');
    stop.innerText = 'Stop Scroll';
    stop.style.position = 'fixed';
    stop.style.top = 0;
    stop.style.left = 0;
    stop.onclick = function() {
      if (scrolling) {
        clearTimeout(scrolldelay);
        scrolling = false;
        stop.innerText = 'Start Scroll';
      }
      else {
        speed = prompt("Please Enter a Scroll Speed");
        scrollMove();
        scrolling = true;
        stop.innerText = 'Stop Scroll';
      }
    };
    document.body.appendChild(stop);
  };


  var scrollMove = function() {
    if (scrollDir === 'down') {
      window.scrollBy(0,speed);
      scrolldelay = setTimeout(scrollMove,10);
    }
    if (scrollDir ==='up') {
      window.scrollBy(0,-speed);
      scrolldelay = setTimeout(scrollMove,10);
    }
    var sp = document.body.scrollTop;
    if (sp + viewport === bottom) {
      scrollDir = 'up';
    }
    if (sp === 0) {
      scrollDir = 'down';
    }
  };
  getSpeed();
};

autoScroll();