Auto Scroll Bookmarklet (for Chrome)

chrome-autoscroll-bookmarklet.js

javascript:(function(window, document) {
  var isScrolling, scrollTimeout;
  var defaultScrollSpeed = 50;
  var scrollSpeed = 50;
  var adjustIncrement = 2;
  var jumpIncrement = 50;

  function start() {
    window.addEventListener('keydown', keyPressed);
    window.addEventListener('keyup', keyReleased, false);
    autoScroll();
  }

  function autoScroll() {
    if(scrollTimeout) {
      clearTimeout(scrollTimeout);
    }
    window.scrollBy(0,1);
    scrollTimeout = setTimeout(autoScroll, scrollSpeed);
    isScrolling = true;
  }

  var keysPressed = [];

  function keyPressed(event) {
    keysPressed[event.keyCode] = true;
    console.log(keysPressed);
    var commandKey = (keysPressed[91] || keysPressed[93]);
    var leftArrow = keysPressed[37];
    var rightArrow = keysPressed[39];
    var upArrow = keysPressed[38];
    var downArrow = keysPressed[40];
    var escKey = keysPressed[27];
    if (escKey) {
      turnOff();
    }
    if (downArrow) {
      event.preventDefault();
      window.scrollBy(0, jumpIncrement);
      autoScroll();
    }
    if (upArrow) {
      event.preventDefault();
      window.scrollBy(0, -jumpIncrement);
      autoScroll();
    }
    if (rightArrow) {
      event.preventDefault();
      scrollSpeed -= adjustIncrement;
      autoScroll();
    }
    if (leftArrow) {
      event.preventDefault();
      scrollSpeed += adjustIncrement;
      autoScroll();
    }
  }

  function keyReleased(event) {
    delete keysPressed[event.keyCode];
  }

  function turnOff() {
    clearTimeout(scrollTimeout);
    window.removeEventListener('keydown', keyPressed);
    window.removeEventListener('keyup', keyReleased, false);
    isScrolling = false;
  }

  start();

})(window, document);

Thanks for making this!
If anyone was wondering: it can work when you open PDFs but you have to manually paste this code into the console for it to work.

Here's a fork where the escape button will resume scrolling as well as stop it.
https://gist.github.com/RayBB/359bb5946ac3c45e355bcf238e49a66e