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The up to date version of this page can be found athttps://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 adefmethod which satisfiescljs.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)(defmethodjs-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 ofbabel-standalone using Nashorn. Thejs-transforms method gets and returns an object which satisfies theIJavaScript 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 tojs-transforms, is a map with the compiler options.The JavaScript preprocessing happens beforemodule conversion, however, both steps are independent of each other and can be used in disjunction.