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wp-content/plugins/easy-table-of-contents/assets/js/toc-mce-button.js

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+(function() {
+       tinymce.PluginManager.add('toc_mce_button', function( editor, url ) {
+           editor.addButton('toc_mce_button', {
+                       text: 'TOC',
+                       icon: false,
+                       onclick: function() {
+                         // change the shortcode as per your requirement
+                          editor.insertContent('[ez-toc]');
+                      }
+             });
+       });
+})();