drupal_add_js() to add a jquery plugin, but there is an error
TypeError: $ is undefined at console of firebug. So how should I add the js?
I think you'd find this article in the Drupal documentation relevant and helpful.
I'm not really sure but either before/after some version of jQuery, the
$ was a global alias for the jQuery object itself and could be used directly without the closure but was later removed as it conflicted with other libraries.