How can I encrypt JavaScript code so that it's not decryptable?

The Javascript code is executed in the browser, i.e. on the client side, which means it must be available not-encrypted on the client side.

The "best" you can do is probably to minify it, which will make it harder to understand it -- and a bit of obfuscation might do too -- even if someone really motivated will still be able to read it.

See for instance the YUI Compressor, which can both minify and obfuscate JS code.

If you truly want to encrypt it so that no one else can decrypt it, you could use one of any number of encryptions. For instance, to do it manually, you could use TrueCrypt; or to do it programatically, use the encryption libraries available in most languages.

If what you are really asking, though, is how to obfuscate the code so that no one can reverse-engineer it, the answer is: you can't, not in Javascript or any other language. The best you can do is make it more difficult on the reverse-engineers by making it hard to read; for that, there are any number of tools:

• javascriptobfuscator.com
• javascript obfuscator
• ObfuscateJS

There is no way to do that. You can obscure it and have some domain checking code in it. Or you could have it served with a server-side script that checks the referring domain.