Add self-closing shortcode button to TinyMCE in WP 4.6

We start by adding the custom TinyMCE Button:

function add_mce_button_custom_em() {
    // check user permissions
    if ( !current_user_can( 'edit_posts' ) && !current_user_can( 'edit_pages' ) ) {
        return;
    }
    // check if WYSIWYG is enabled
    if ( 'true' == get_user_option( 'rich_editing' ) ) {
        add_filter( 'mce_external_plugins', 'add_tinymce_plugin_custom_em' );
        add_filter( 'mce_buttons', 'register_mce_button_custom_em' );
    }
}
add_action('admin_head', 'add_mce_button_custom_em');

Then we declare and register the new button:

// Declare script for new button
function add_tinymce_plugin_custom_em( $plugin_array ) {
    $plugin_array['custom_em'] = get_template_directory_uri() .'/plug/custom-em/custom-em.js';
    return $plugin_array;
}

// Register new button in the editor
function register_mce_button_custom_em( $buttons ) {
    array_push( $buttons, 'custom_em' );
    return $buttons;
}

Finally, we decide which buttons (more on buttons can be found at Content Formatting) we want to display. Obviously, if you have UX in mind, you would only display a few of them, example:

// TinyMCE: TinyMCE choose which buttons you want to display
function myformatTinyMCE( $in ) {
    $in['toolbar1'] = 'styleselect,bold,custom_em,blockquote,hr,aligncenter,link,unlink,spellchecker,undo,removeformat';
    return $in;
}
add_filter( 'tiny_mce_before_init', 'myformatTinyMCE' );

As you can see in the add_tinymce_plugin_custom_em function, we are declaring a javascript file inside get_template_directory_uri() .'/plug/custom-em/custom-em.js'

So create the custom-em.js file, and then you have two ways to go about this.

Either you can go with the following shortcode format [shortcode foo="" bar=""] or [shortcode][/shortcode].

Let’s start with the [shortcode foo="" bar=""] format:

(function() {
    tinymce.create('tinymce.plugins.custom_em', {
        init : function(ed, url) {
            ed.addButton('custom_em', {
                title : 'Custom EM',
                image : url+'/images/btn_custom_em.png',
                onclick : function() {
                    ed.execCommand(
                        "mceInsertContent",
                        false,
                        "[shortcode foo=\"\" bar=\"\"]"
                    );
                }
            });
        }
    });
    tinymce.PluginManager.add('custom_em', tinymce.plugins.custom_em);
})(); 

As you can see, we use an image as the button icon. You can change that into text as outlined in the example below.

The following is what we use on our own platform, it wraps the selection into: <em class="whatever">hello world</em>:

(function() {
    tinymce.PluginManager.add('custom_em', function( editor, url ) {

        editor.on('init', function(e) {
            this.formatter.register('thetarget', {
                inline : 'em',
                classes : 'whatever'
            });
        });

        editor.addButton('custom_em', {
            text: 'Custom EM',
            icon: false,
            onclick : function() {
                var contents = editor.selection.getContent(),
                tags = jQuery(jQuery.parseHTML(contents)).find('em.whatever').andSelf();
                editor.formatter.toggle('thetarget');
            }
        });
    });
})(jQuery);

Please post results and perform edits. TinyMCE is a plague and requires headache but can be solves in a collaborative manner.