Why this code doesn't work ? how to fix it?

var editor = CKEDITOR.replace( 'contentTextarea' ,{

     on : {
        dialogDefinition : function( ev ) {
            var dialogName = ev.data.name;
            var dialogDefinition = ev.data.definition;

            if ( dialogName == 'link' ) {

                var infoTab = dialogDefinition.getContents( 'info' );

                dialogDefinition.removeContents( 'target' );

                // Set the default value for the URL field.
                var urlField = infoTab.get( 'url' );
                urlField['default'] = 'www.example.com';

            }
        }
    }

});