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Hello Google Closure Modules

23 February 2015

By now you may have heard some buzz about webpack, a tool for managing web application assets. webpack can manage images and stylesheets, but only the facilities for managing JavaScript sources and more specifically the facilities for code-splitting are of interest to us in this post. We'll briefly look at webpack's support for splitting and compare it to a little known feature of the Google Closure Compiler: Google Closure Modules.

webpack describes its support for code-splitting here. It's clear without reading the entire page that the unit of modularity that webpack operates on is a code module that was written by an actual human being. Splitting happens on explicit code module dependencies expressed in the source itself. webpack presents various knobs to control how chunking of the split occurs because modules written by humans are actually a terrible unit of modularity when it comes to optimizing the production artifact. I've already talked about this from a different angle elsewhere.

Google Closure Modules maintains the simple Closure Compiler philosophy: don't let a human do anything a computer can do for you. Code splits are not defined in the source code and the modules you end up with may have nothing to do with the modules you actually wrote. This is a good thing. Closure Compiler may freely move code between the modules you wrote to get optimized modules that you would have never written by hand that contain precisely what is needed.

Code Motion

Google searching for code motion will probably lead you to the Wikipedia article on Loop-invariant code motion. The idea is simple, by moving code without changing the semantics of the program you can get an optimal result. In our case it isn't faster loops but smaller modules (and faster page loads).

ClojureScript now has full support for Google Closure Modules. Let's see how it works out in practice.

Imagine we have the following namespace in ClojureScript that we want load on some page of our application:

  (:require-macros [cljs.core.async.macros :refer [go]])
  (:require [cljs.core.async :refer [timeout chan >! <!]]))


(def c (chan))

  (<! (timeout 1000))
  (println "Hello world!"))

  (<! c)
  (println "Goodbye!"))

  (>! c :knock))

This namespace has a dependency on core.async, a large-ish ClojureScript library (~2000 lines).

Now lets imagine another namespace that we would like to load:



(println "Hello world from module bar!")

We'd like to split our application into three pieces, the shared bit, the bit for and the bit for So in our project.clj file we would define a :modules entry like so:

  :modules {:foo {:output-to "out/foo.js"
                  :entries #{}}
            :bar {:output-to "out/bar.js"
                  :entries #{}}}

Notice that we don't need to specify the shared module. The ClojureScript compiler will automatically move any namespace not explicitly placed into a module into the shared module.

This sounds like a disaster but Google Closure Compiler will employ cross module code motion to ensure these modules only get the code they need. In fact we would hope that anything from core.async that wasn't dead code eliminated got moved into

Running Closure advanced compilation with pretty-printing and human readable names shows this is in fact the case.

This is the entire file bar.js which does not depend on core.async:

    $cljs$core$array_seq$$(["Hello world from module bar!"], 0)

And this is a snippet of foo.js which does depend on core.async:

var $cljs$core$async$impl$ioc_helpers$t23998$$ ...
function $cljs$core$async$impl$protocols$active_QMARK_$$($h$$128$$) {
    if ($h$$128$$ ? ...) {

As predicted core.async got moved into foo.js!

The final gzipped sizes of the modules for the example above:

For large ClojureScript applications I think it's an understatement to say that this is a "game changer".

I'd like to thank Thomas Heller for his work incorporating Google Closure Modules into his shadow-build project. He provided a lot of inspiration and rationale that convinced me to land this functionality into ClojureScript itself.

For more details on actual usage checkout the ClojureScript wiki.

Happy hacking!