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A development mode for ClojureScript. It allows you to develop Clojure web applications in combination with ClojureScript seamlessly. You can develop your normal Clojure web application (with Ring and Compojure for example) and whenever you change a ClojureScript source file it is automatically recompiled and you can test the changes in your web browser.


Follow the Quick Start Guide of ClojureScript (the bootstrap step is required)

After bootstrapping ClojureScript get cljs-devmode:

git clone

In the cljs-devmode folder invoke:

lein cljs-devmode-bootstrap

If you haven't set the $CLOJURESCRIPT_HOME environment variable yet, then pass the full path to your ClojureScript installation as the first command line argument.

The cljs-devmode-bootstrap leiningen plugin copies all necessary jar files into the lib folder of $CLOJURESCRIPT_HOME to make this development mode work.

Start the ClojureScript compiler and cljs-devmode from the REPL

To run the hello sample of ClojureScript in development mode do the following:

  • Start the ./script/repl in the $CLOJURESCRIPT_HOME folder.
  • Then invoke this lines on the REPL:
    (require '[cljs.closure :as cljsc]) 
    (defn compile-fn []
          (cljsc/build "samples/hello/src" 
                       {:output-dir "samples/hello/out"
                        :output-to "samples/hello/hello.js"})) 
    (use 'cljs-devmode.core)
    (start-devmode "samples/hello" compile-fn)
  • This brings up a Jetty server on port 9090, so open http://localhost:9090/hello-dev.html to see the hello sample of ClojureScript. On every request cljs-devmode checks, if any .cljs file inside samples/hello has changed. In the case of a change the above defined compile-fn is invoked.
  • So change the greeting message inside samples/hello/src/hello/core.cljs.
  • When you refresh the http://localhost:9090/hello-dev.html you will see the new greeting message.

cljs-devmode supports to automate the above steps

Take a look at the cljs-devmode-example. In the project folder of cljs-devmode-example you can invoke:

lein cljs-devmode

This generates a file in the project folder. The generated shell script starts the Clojure REPL in the $CLOJURESCRIPT_HOME folder. Furthermore 'lein cljs-devmode' has also generated a Clojure script, which is a string inside the shell script and is passed to the Clojure REPL process for evaluation. This Clojure script does the same as the script in the previous section. But the paths are adapted to the current project folder (here the cljs-devmode-example project). So if you execute the script:

chmod 755 

Then the ClojureScript compiler and the cljs-devmode are started automatically.

The cljs-devmode-example also demonstrates how to use the wrap-cljs-forward Ring middleware. This middleware takes care that all request to /cljs/* (for example) are forwarded to localhost:9090, where the cljs-devmode is running. So now you can develop your normal Clojure web application and let ClojureScript do the JavaScript part. Whenever you modify a .cljs file the whole ClojureScript project is automatically recompiled and you can test the changes immediately in your web browser.

'lein cljs-devmode' has some defaults / conventions for the folder structure of the project. But all defaults can be changed through a :cljs-devmode entry in the project.clj. These would be the default settings:

(defproject cljs-devmode-example
    :cljs-devmode {:dir "PROJECT_HOME/cljs"
                   :src-dir "PROJECT_HOME/cljs/src"
                   :output-dir "PROJECT_HOME/cljs/out")
                   :output-to "PROJECT_HOME/cljs/cljs-devmode-example.js")}

You do not have to specifiy the :cljs-devmode entry inside your project.clj as long as the folder structure of your project follows the conventions. Take a look at the folder structure of the cljs-devmode-example. The whole ClojureScript project is in the cljs/ subfolder. The ClojureScript project itself adheres to the folder structure of the ClojureScript samples. The :cljs-devmode entry can also be leveraged to pass additional parameters to the ClojureScript compiler. As you can see in the above example the :output-dir and :output-to entries are settings for the ClojureScript compiler, every additional map entry will also be passed to the ClojureScript compiler.


This is a really primitive prototype for a ClojureScript development mode. I only wrote it to get started with ClojureScript in one of my Clojure web applications. There are many things which can be improved. For example the generation of the .sh shell script is a little bit nasty ;-) You can also let the JVM process of leiningen start the ClojureScript compiler and the devmode via:

lein cljs-devmode start

But then you will not see the output (stdout and stderr) of the ClojureScript compiler and therefore you cannot read the helpful error messages of the ClojureScript compiler. Nevertheless there is no automated way to do a switch to "production mode" yet (e.g. invoke the Google Closure Compiler in advanced mode and move the resulting js file to the public/cljs folder).


Copyright (C) 2011 Maximilian Weber

Distributed under the Eclipse Public License, the same as Clojure.