Lately I've seen on HN a few attempts to add runtime type checking to JavaScript. I'm happy with that, as in my experience adding a bunch of asserts in strategic places can really speed up prototyping and development, you become more confident, and chances are that you can catch bugs earlier. In the same vein I wrote a library that adds runtime type checking based on type combinators [1], check it out, we can share some ideas.

[1] https://github.com/gcanti/tcomb