JavaScript and Prototyping

I found the following requirements in the description of the Interaction Designer job at Google:
Requirements:

• Solid academic background in human-computer interaction or related field (BS or MS in Computer Science or related field a big plus.
• Demonstrated experience in designing usable web-based interfaces.
• Expert HTML skills.
• Excellent knowledge of JavaScript for rapid prototyping purposes.
• Strong, clean visual design sense.
• Excellent leadership, communication and teamwork skills.

I was wondering why they're asking for JavaScript knowledge. It looks like they're using it for prototyping. Actually I knew little about Html and nothing about JavaScript before this. So I spent a couple of hours picking up Html and JavaScript. During that I wrote my first html page (http://mypage.iu.edu/~li25) with one JavaScript line in it.

However, still don't get it, how do they especially need JavaScript to prototype?