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The up to date version of this page can be found at https://clojurescript.org/reference/javascript-library-preprocessing

ClojureScript allows you to add a custom transformation step for JavaScript libraries. This means that you are able to include JavaScript libraries in your project which are written in a dialect or make use of a JavaScript syntax extension. In order to effect the desired code transformation a defmethod which satisfies cljs.closure/js-transforms must be provided to the :preprocess option of the foreign library. For example, you can transform a JavaScript library using JSX into regular React function calls as follows:

(require '[cljs.build.api :as b]
         '[clojure.java.io :as io])
(refer 'cljs.closure :only '[js-transforms])
(import 'javax.script.ScriptEngineManager)

(defmethod js-transforms :jsx [ijs opts]
  (let [engine (doto (.getEngineByName (ScriptEngineManager.) "nashorn")
                 (.eval (io/reader (io/file "babel.min.js")))
                 (.put "input" (:source ijs)))]
    (assoc ijs :source
      (.eval engine (str "Babel.transform(input, {presets: ['react']}).code")))))

(b/build "src"
  {:main 'my-project.core
   :output-to "out/my_project.js"
   :output-dir "out"
   :foreign-libs [{:file "libs/example.js"
   :provides ["my.example"]
   :preprocess :jsx}]})

The example above uses Babel to transform the JavaScript code by loading a minified version of babel-standalone using Nashorn. The js-transforms method gets and returns an object which satisfies the IJavaScript protocol. The object can be a plain map or a record with keys like :url, :provides, :requires and :source. The second argument, which is passed to js-transforms, is a map with the compiler options. The JavaScript preprocessing happens before module conversion, however, both steps are independent of each other and can be used in disjunction.