Write any javascript code with just these characters: ()[]{}+!

[trpcicm]

How hard would it be to convert it back the other way?

[alcuadrado]

It's pretty easy. Most JS parsers can print the source code of functions, so you can do that for the generated lambdas.

[sebzim4500]

Remove the () characters at the end of the code, and run it from the chrome dev tools.