I’m trying to adda custom button to my editor following this guide

Can’t get it to work. I’ve added the following code to my funcitons.php

  add_action('admin_head', 'gavickpro_add_my_tc_button');

function gavickpro_add_my_tc_button() {
    global $typenow;
    // check user permissions
    if ( !current_user_can('edit_posts') && !current_user_can('edit_pages') ) {
    return;
    }
    // verify the post type
    if( ! in_array( $typenow, array( 'post', 'page' ) ) )
        return;
    // check if WYSIWYG is enabled
    if ( get_user_option('rich_editing') == 'true') {
        add_filter("mce_external_plugins", "gavickpro_add_tinymce_plugin");
        add_filter('mce_buttons', 'gavickpro_register_my_tc_button');
    }
}

function gavickpro_add_tinymce_plugin($plugin_array) {
    $plugin_array['gavickpro_tc_button'] = plugins_url( '/kevin/editor_plugin.js', __FILE__ ); // CHANGE THE BUTTON SCRIPT HERE
    return $plugin_array;
}

function gavickpro_register_my_tc_button($buttons) {
   array_push($buttons, "gavickpro_tc_button");
   return $buttons;
}

and the following js file in my plugins folder kevin/editor_plugin.js

(function() {
    tinymce.PluginManager.add('gavickpro_tc_button', function( editor, url ) {
        editor.addButton( 'gavickpro_tc_button', {
            title: 'My test button',
            icon: 'wp_code',
       

 onclick: function() {
            editor.insertContent('Hello World!');
        }
    });
});

})();