Add button to tinyMCE editor

custom-button-class.php

<?php 
/**
 * Plugin Name: TinyMCE Custom Link Class
 * Plugin URI: http://wpbeginner.com
 * Version: 1.0
 * Author: WPBeginner
 * Author URI: http://www.wpbeginner.com
 * Resource: http://www.wpbeginner.com/wp-tutorials/how-to-create-a-wordpress-tinymce-plugin/
 * Description: A simple TinyMCE Plugin to add a custom link class in the Visual Editor
 * License: GPL2
 */
 class TinyMCE_Custom_Link_Class {
    /**
    * Constructor. Called when the plugin is initialised.
    */
    function __construct() {
	}
}
 
$tinymce_custom_link_class = new TinyMCE_Custom_Link_Class;
//if ( is_admin() ) {
  add_action( 'admin_init', 'setup_tinymce_plugin'  );
//}

function setup_tinymce_plugin() {
	add_filter( 'mce_external_plugins','add_tinymce_plugin' ) ;
	add_filter( 'mce_buttons', 'add_tinymce_toolbar_button' );
  
  /* *** Couldn't get it to work without commenting all of this.  =P
  // Check if the logged in WordPress User can edit Posts or Pages
	// If not, don't register our TinyMCE plugin 
	//if ( ! current_user_can( 'edit_posts' ) && ! current_user_can( 'edit_pages' ) ) {
	//			return;
	//}
 
  // Check if the logged in WordPress User has the Visual Editor enabled
  // If not, don't register our TinyMCE plugin
  //	if ( get_user_option( 'rich_editing' ) !== 'true' ) {
  //		return;
  //	}
}

/**
* Adds a TinyMCE plugin compatible JS file to the TinyMCE / Visual Editor instance
*
* @param array $plugin_array Array of registered TinyMCE Plugins
* @return array Modified array of registered TinyMCE Plugins
*/
function add_tinymce_plugin( $plugin_array ) {
	$plugin_array['custom_link_class'] = plugin_dir_url( __FILE__ ) . 'tinymce-custom-link-class.js';
	return $plugin_array;
}
/**
* Adds a button to the TinyMCE / Visual Editor which the user can click
* to insert a link with a custom CSS class.
*
* @param array $buttons Array of registered TinyMCE Buttons
* @return array Modified array of registered TinyMCE Buttons
*/
function add_tinymce_toolbar_button( $buttons ) {
	array_push( $buttons, '|', 'custom_link_class' );
	return $buttons;
}

custom-button-js.js

(function() {
    tinymce.PluginManager.add( 'custom_link_class', function( editor, url ) {
        // Add Button to Visual Editor Toolbar
        editor.addButton('custom_link_class', {
            title: 'Insert Button Link',
			image: url + '/icon.png',
            cmd: 'custom_link_class',
        }); 
		// Add Command when Button Clicked
		editor.addCommand('custom_link_class', function() {
		 
			// Insert selected text back into editor, wrapping it in an anchor tag
			editor.execCommand('mceInsertContent', false, 'inserted content');
		});   
	});
})();