Advanced Compile

If you are going to deploy your compiled ClojureScript to the web you will want to use advanced compilation as it will shrink your compiled code significantly.

The Google Closure Compiler’s advanced compilation may be the best possible way to compress your compiled ClojureScript. It shrinks names globally, it in-lines code, it removes code that is not reachable and just does a fantastic job of compressing your code.

However, there is a downside. It needs to know the variable and function names that it shouldn’t compress. It can be very frustrating when you are finally getting ready to deploy your project and then discover your code has some subtle incompatibilities with advanced compilation.

The way this plays out is that your code will compile without failure or warnings and then it just won’t run. And you will see in the developers console that some randomly compressed function name like zB does not exist.

It is for this reason that you should get in the habit of compiling your project in advanced mode fairly often, at least once a day, to verify that things are working.

How to advanced compile

Given our hello-world.core example project we can advanced compile it very simply by executing the following shell command:

$ clj -m figwheel.main -O advanced -bo dev

The -bo is the short flag for --build-once and it takes a build name just like the --build flag. The -O flag overrides the :optimizations configuration option in the build options. It is important that the -O flag and its options is before the final -bo option.

The -O flag is a capital letter and overrides the :optimizations build option. Not to be confused with the -o flag which overrides the :output-to config option.

The above command should have advanced compiled your code one time. If you examine the target/public/cljs-out/dev-main.js file you will see the compressed output of your program.

Now something to notice here is that the :output-to path target/public/cljs-out/dev-main.js has not changed. This means that the HTML that is hosting this file can still be used to verify that it all works.

You can verify that your advanced compiled code is working two different ways.

The first is to just build it once and start the server like so:

$ clojure -m figwheel.main -O advanced -bo dev -s

or alternatively you can watch and compile the advanced build so that you can have it recompiled after you change it.

$ clojure -m figwheel.main -O advanced -b dev -s

The above method can be very helpful when diagnosing advanced compile problems.

Keep in mind the above command will not hot-load your changes and you will need to manually refresh the browser to see if your fixes have taken. Also, remember that advanced compilation is significantly slower than :optimizations :none compiles.


If and when you run into problems please make sure that you have read and understood these guides on the ClojureScript site: