canonical-link.user.js

// ==UserScript==
// @name           canonical-link
// @namespace      http://www.hatena.ne.jp/hitode909
// @include        *
// ==/UserScript==

(function() {
     if (window != window.parent) {
         return;
     }
     var getValue = function (query, attr) {
         var element = document.querySelector(query);
         if (element) {
             return element[attr];
         }
     };
     var url =
         getValue('link[rel="canonical"]', 'href') ||
         getValue('meta[property="og:url"]', 'content');
     if (url == null) {
         return;
     }
     url = url.replace(/^([^\/]+:\/\/[^/]+)$/, "$1/");
     if (url != location.href) {
         var button = document.createElement('a');
         document.querySelector('body').appendChild(button);
         button.appendChild(document.createTextNode('canonical'));
         Object.assign(button.style, {
           position: 'fixed',
           width: '100%',
           left: 0,
           bottom: 0,
           color: '#fff',
           background: '#000',
           fontSize: '24px',
           textAlign: 'center',
           textDecoration: 'none',
           margin: 0,
           padding: 0,
           fontWeight: 'bold',
           opacity: 0.5,
           zIndex: 9999,
         });
         button.href = url;
     }
})();