change reproducing speed using key

youtube_speed_key.user.js

// ==UserScript==
// @name         youtube speed key
// @namespace    http://tampermonkey.net/
// @version      0.2
// @match        https://www.youtube.com/watch?*
// ==/UserScript==

(function() {
  var down_key = 90, // z
      riset_key= 88, // x
      up_key= 67;    // c
  var criteria_speed = 0.25;

  var vids = document.getElementsByTagName('video')[0];

	// bind key-handler
	document.addEventListener('keydown', function(e){
    if ( (e.keyCode == down_key || e.keyCode == riset_key || e.keyCode == up_key) && !(e.altKey || e.ctrlKey || e.metaKey) ) {
        event.preventDefault();

      var current_rate = vids.playbackRate;

      switch (e.keyCode) {
        case down_key: // z
          if (0 < current_rate) {
            vids.playbackRate -= criteria_speed;
          }
          break;
        case riset_key: // x
          vids.playbackRate = 1;
          break;
        case up_key: // c
          if (current_rate < 3) {
            vids.playbackRate += criteria_speed;
          }
          break;
      }
    }
	} ,true);
})();