How can I hide or encrypt JavaScript code?

One of the best compressors (not specifically an obfuscator) is the YUI Compressor.

The only safe way to protect your code is not giving it away. With client deployment, there is no avoiding the client having access to the code.

So the short answer is: You can't do it

The longer answer is considering flash or Silverlight. Although I believe silverlight will gladly give away it's secrets with reflector running on the client.

I'm not sure if something simular exists with the flash platform.