YTLoop.user.js

// ==UserScript==
// @name        Loop
// @namespace   dumptyd
// @description Adds a loop button on YouTube videos
// @include     https://www.youtube.com/watch?v=*
// @version     1.5.5
// @grant       none
// @require 	https://code.jquery.com/jquery-2.1.4.min.js
// ==/UserScript==

vscript=document.createElement('script');
vscript.src='https://code.jquery.com/jquery-2.1.4.min.js';
vscript.type='text/javascript';
vscript.id='fuck';
document.getElementsByTagName('head')[0].appendChild(vscript);

 
   timer = '';
   button='<button status="0"  style="color:#fff; border:1px solid #f00; position:absolute; right:112px; top:13px; width:50px; cursor:pointer; background-color:transparent; border-radius:20px ; font-weight:bold;" id="loopBtn">Loop</button>'
   $('.ytp-right-controls').prepend(button);
   url=window.location.href;
  
  
  
   $('#loopBtn').on('click', function() {
    
	if ($('#loopBtn').attr('status') == '0')
		loopEnable();
    else if ($('#loopBtn').attr('status') == '1')
		loopDisable();
	
    }) 
  
   function checkVideo()
  {
    var currentTime = $('.ytp-time-current').text();
    var duration = $('.ytp-time-duration').text();
	var currentUrl=window.location.href;
	if(url!=currentUrl)
		loopDisable();
    if (currentTime == duration)
		$('.ytp-play-button').trigger('click');
  }  
  
  function loopDisable()
  {
	 $('#loopBtn').css('background-color', 'transparent');
     clearInterval(timer);
     $('#loopBtn').attr('status' , '0'); 
  }
  
  function loopEnable()
  {
	$('#loopBtn').css('background-color','#f00');
	timer = setInterval(function () 
	{
		checkVideo()
	}, 3000);
	$('#loopBtn').attr('status','1');  
  }