JavaScript: keydown make url jump.

keydown_nav_url.js

<script type="text/javascript">
document.addEventListener("keydown", function ( event ) {
      if ( event.keyCode === 9 || ( event.keyCode >= 32 && event.keyCode <= 34 ) || (event.keyCode >= 37 && event.keyCode <= 40) ) {
          event.preventDefault();
      }
  }, false);
  
document.addEventListener("keyup", function ( event ) {
    if ( event.keyCode === 9 || ( event.keyCode >= 32 && event.keyCode <= 34 ) || (event.keyCode >= 37 && event.keyCode <= 40) ) {
        switch( event.keyCode ) {
            case 33: // pg up
            case 37: // left
            case 38: // up
              {% if page.previous %}
                url = '{{ site.url }}{{ page.previous.url }}';
              {% endif %}
                     break;
            case 34: // pg down
            case 39: // right
            case 40: // down
              {% if page.next %}
                url = '{{ site.url }}{{ page.next.url }}';
              {% endif %}
                     break;
        }
        if (url) {
            window.location = url;
        }
        event.preventDefault();
    }
}, false);
</script>