middleware.clj

(defn externs
  "Middleware to add externs files (i.e. files with the .ext.js extension) and
  Google Closure libs (i.e. files with the .lib.js extension) from the fileset
  to the CLJS compiler options."
  [{:keys [tmp-src tmp-out main files opts] :as ctx}]
  (let [ext-paths (map core/tmppath (:exts files))
        lib-ins   (map core/tmpfile (:libs files))
        renamelib #(.replaceAll % "\\.lib\\.js$" ".js")
        merge-cfg (partial merge-with (comp vec distinct into))
        lib-paths (->> (map (comp renamelib core/tmppath) (:libs files))
                       (map #(.getPath (io/file tmp-out %))))]
    (doseq [[in out] (map list lib-ins lib-paths)]
      (file/copy-with-lastmod (io/file in) (io/file out)))
    (update-in ctx [:opts] merge-cfg {:libs lib-paths :externs ext-paths})))