scroll by half

gistfile1.txt

(function() {
  var currentScroll = window.scrollY;
  var currentlyScrolling = false;

  var runOnScroll =  function(evt) {
    if (currentlyScrolling) {
      // preventDefault doesn't seem to work
      window.scrollTo(window.X, currentScroll);
    } 
  };

  var scrollHalf = function() {
    currentScroll = currentScroll + window.innerHeight / 2;
    currentlyScrolling = false;
    window.scrollTo({
      behavior: 'smooth',
      left: window.scrollX,
      top: currentScroll,
    });
  };

  window.addEventListener("scroll", runOnScroll);
  document.onkeypress = function (e) {
    e = e || window.event;
    if (e.keyCode = 32) {
      currentScroll = window.scrollY;
      currentlyScrolling = true;
      setTimeout(scrollHalf, 50);
    }
  };
})();