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Externs Got You Down?

14 March 2014

Update: This post was written nearly a year ago. A lot of things can change in a year. People start using React and third party JavaScript packaging stories for ClojureScript can appear out of nowhere. This post describes a quick hack when you are in a bind but I would not use it in production setting. Use a CLJSJS package instead.

While ClojureScript’s strategy of generating and leveraging code optimized for the Google Closure Compiler has turned out to be fantastic for managing production code size, sometimes you find yourself needing a great JavaScript library that simply does not play well with advanced compilation. Integrating these libraries is problematic because of the externs file requirement. For example this is true for React. Fortunately someone has taken the time to write the externs file for React, but most of the time you won’t be so lucky.

For some JavaScript libraries there is an acceptable workaround - you can use the JavaScript library itself as an externs file! This would normally spew many warnings from Closure Compiler, but recent versions of ClojureScript expose knobs to control how Closure reports warnings.

For example imagine that an externs file did not exist for React. The following lein-cljsbuild build specification is enough to produce a proper advanced compiled production file:

{:id "release"
 :source-paths ["src"]
 :compiler {
   :output-to "main.js"
   :optimizations :advanced
   :preamble ["react/react.min.js"]
   :externs ["react/react.js"]
   :closure-warnings {:externs-validation :off
                      :non-standard-jsdoc :off}}}

That’s it!