JavaScript/CSS codes are displayed in excerpt

If I place some media other than ordinary image, such as gallery or image magnifier (from Nextend) at the beginning of a blog post, the source codes aren’t hidden from the excerpt.
I think the difference is an ordinary image is made from HTML, while some special media are made from CSS, JavaScript, jQuery, etc.
Here is the screenshot: http://widyagama.ac.id/informatika/wp-content/uploads/2013/05/codes-in-excerpt.png

How to hide these codes from the excerpt?