<![endif] Knowing this one ClojureScript gotcha will save you hours September 20, 2015 Summary: ClojureScript optimizes names by replacing them with shorter ones. Usually, that's a good thing. But it can get carried away. Externs are how you help it know what's unsafe to optimize. Here's the situation: you're writing ClojureScript code, compiling it with no optimizations (because it's faster for development). Everything is working great. You compile it with advanced compilation and test it, and things start breaking . Hopefully it's just on your local machine and not on production. What's happening? Surprisingly, this happens a lot. As a Clojure programmer, I'm not used to really having a difference between development and production. It's the same language and everything is available in both environments. But with ClojureScript, it helps to think of ...