autoscroll-bookmark.js

javascript:(function(window, document) {
  var isScrolling, scrollTimeout;
  var defaultScrollSpeed = 50;
  var scrollSpeed = 10;
  var adjustIncrement = 2;
  var jumpIncrement = 50;
  var stopPoint = -1;
  var scrolled = 0;
  var scrollDelay = 0;
  var reverse = false;

  function start(){
    var prompt_text = "Enter distance to scroll (in pixels, or fraction of the page '1/5'), the time it takes to scroll 1px (in milliseconds), and the delay before the scroll happens (in milliseconds). Add '-r' to the end to scroll up instead of down.\nEnter 0 for distance to go to the bottom of the page. Default speed is 10, default delay is 0.\n\nExamples:\n'800 10 1000'\n'1/5 10'\n'800 5 0 -r";
    var result = prompt(prompt_text);
    if(result){
      if(/(1\/)?\d{1,}( \d{1,})*( -r)?/.test(result)){
        var parts = result.split(' ');
        stopPoint = parts[0];
        if(stopPoint.includes('/')){
          fraction = parseInt(stopPoint.split('/')[1]);
          stopPoint = (document.documentElement.scrollHeight - window.innerHeight) / fraction;
        } else {
          stopPoint = parseInt(stopPoint);
        }
        speed = parseInt(parts[1]);
        delay = parseInt(parts[2]);
        if(speed){
          scrollSpeed = speed;
        }
        if(delay){
          scrollDelay = delay;
        }
        if(parts.includes("-r")){
          reverse = true;
        }
        setTimeout(startScroll, scrollDelay);
      } else {
        start();
      }
    }
  }

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

  function autoScroll() {
    if(scrollTimeout) {
      clearTimeout(scrollTimeout);
    }
    if(stopPoint == 0 || (stopPoint > 0 && scrolled < stopPoint)){
      if(reverse){
        window.scrollBy(0,-1);
      } else {
        window.scrollBy(0,1);
      }
      scrolled += 1;
      scrollTimeout = setTimeout(autoScroll, scrollSpeed);
      isScrolling = true;
    } else {
      isScrolling = false;
    }
  }

  var keysPressed = [];

  function keyPressed(event) {
    keysPressed[event.keyCode] = true;
    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) {
      if (isScrolling){
        turnOff();
      } else {
        autoScroll();
      }
    }
    if (downArrow) {
      event.preventDefault();
      window.scrollBy(0, jumpIncrement);
      scrolled += jumpIncrement;
      autoScroll();
    }
    if (upArrow) {
      event.preventDefault();
      window.scrollBy(0, -jumpIncrement);
      scrolled -= 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);
    isScrolling = false;
  }

  start();

})(window, document);