Note: After saving, you may have to bypass your browser's cache to see the changes.

• Firefox / Safari: Hold Shift while clicking Reload, or press either Ctrl-F5 or Ctrl-R (⌘-R on a Mac)
• Google Chrome: Press Ctrl-Shift-R (⌘-Shift-R on a Mac)
• Internet Explorer / Edge: Hold Ctrl while clicking Refresh, or press Ctrl-F5
• Opera: Go to Menu → Settings (Opera → Preferences on a Mac) and then to Privacy & security → Clear browsing data → Cached images and files.

var customizeToolbar = function() {
        /* Your code goes here */
};
 
/* Check if view is in edit mode and that the required modules are available. Then, customize the toolbar . . . */
if ( $.inArray( mw.config.get( 'wgAction' ), ['edit', 'submit'] ) !== -1 ) {
        mw.loader.using( 'user.options', function () {
                if ( mw.user.options.get('usebetatoolbar') ) {
                        mw.loader.using( 'ext.wikiEditor.toolbar', function () {
                                $(document).ready( customizeToolbar );
                        } );
                }
        } );
}