Production Build

You may have noticed that out contains a lot of JavaScript. Fortunately the ClojureScript compiler generates output optimized for the Google Closure Compiler. The Google Closure Compiler performs many optimizations, but the most significant for browser-based clients are minification and dead code elimination.

Let's make a new helper build script release.clj, it should look like the following:

(require 'cljs.build.api)

(cljs.build.api/build "src"
  {:output-to "out/main.js"
   :optimizations :advanced})

(System/exit 0)

Under :advanced optimizations :main is not needed as advanced compilation creates a single JavaScript artifact. We also add a (System/exit 0) as the Google Closure Compiler creates a thread pool that isn't shutdown - we know we're done we can just exit.

Let's remove the dev time REPL bits from src/hello_world/core.cljs:

(ns hello-world.core)

(enable-console-print!)

(println "Hello world!")

Let's create a release build:

java -cp cljs.jar:src clojure.main release.clj

This process will take significantly longer which is why we don't use this compilation mode for development.

Open index.html, you should still see "Hello world!" printed.

Examine out/main.js, the file size should be around 80K. If you zip this file you'll see that it's around 19K. This is significantly smaller than a jQuery dependency yet when using ClojureScript you have implicit dependencies on the entire ClojureScript standard library (10KLOC) and the Google Closure Library (300KLOC). You can thank dead code elimination.

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