Enhanced Code Splitting & Loading

10 July 2017
David Nolen

This is the first post in the Sneak Preview series.

As client applications increase in size it becomes desirable to optimize the time to load a logical screen. Network requests should be minimized while at the same the loaded code should be restricted to that which is absolutely necessary to produce a functioning screen. While tools like Webpack have popularized this optimization technique in the JavaScript mainstream, Google Closure Compiler and Library have supported this same optimization strategy in the form of Google Closure Modules for many years now.

Google Closure Modules also provide some unique advantages over tools like Webpack or Rollup which we will cover in the technical section of this post. In short, we optimally assign all sources to modules after which Google Closure Compiler employs dead code elimination (tree shaking) and cross module code motion to produce truly optimal splits.

While ClojureScript has provided basic integration with this facility for some time, the next release will feature greatly enhanced and comprehensive support for code splitting and asynchronous loading of these splits.


If you are familiar with Webpack terminology, in the following description please take note that module in this context refers to a code split or chunk.

By entry point we mean a source file which represents a logical entry point into your application (login, users, admin, etc.).

Enhanced Code Splitting

It is no longer necessary to hand-optimize module assignment of your sources. All sources will be optimally assigned to a module based upon the dependency graph of your application. If you have a module with many manual assignments you should now remove these. If you were using namespace wildcard matching, this is also no longer necessary. For details about how we assign an input to a particular module see the technical description below.

Concretely, the following is now an anti-pattern:

  {:vendor {:output-to "..."
            :entries '#{cljsjs.react reagent.* re-frame.*}}
   :main   {:output-to "..."
            :entries '#{myapp.core}
            :depends-on [:vendor]}}

Previously you would have to manually pin a source (re-frame in this case) and its dependencies to a module. Now all that is required is:

  {:vendor {:output-to "..."
            :entries '#{re-frame.core}
   :main   {:output-to "..."
            :entries '#{myapp.core}
            :depends-on [:vendor]}}

Another significant enhancement is that :modules now works under all optimization settings. By unifying :modules behavior under all compilation modes we eliminate a bit of incidental complexity around build configuration between development and production.


Asynchronous loading of module splits is now standardized with the introduction of the cljs.loader namespace. If any entry point in your application needs to invoke the load of another module due to some user action, you can now do so with cljs.loader.

cljs.loader provides a shared Google Closure ModuleManager singleton automatically initialized to your :modules graph regardless of the optimization level.

The following is a simple short example of cljs.loader functionality:

(ns views.user
 (:require [cljs.loader :as loader]
           [goog.dom :as gdom]
           [ :as events])
 (:import [ EventType]))

(events/listen (gdom/getElement "admin") EventType.CLICK
  (fn [e]
    (loader/load :admin
      (fn [e]
        ((resolve 'views.admin/init!))))))

Note that this example shows how to call across module boundaries without having the compiler complain about functionality not present in this code split. This is possible thanks to the recent inclusion of static resolve to the standard library.

For a complete walk through of the enhanced :modules functionality please refer to the new guide.

Technical Description

The following highlights some interesting technical details of the enhanced modules functionality.

Module Assignment

This section briefly describes the algorithm employed to automatically assign every source file to a module.

Assume a simplified module description like:

{:modules {:module-a {:entries '#{foo.core}}
           :module-b {:entries '#{bar.core}}}

This will be transformed into a module description that includes the implicit base module :cljs-base.

{:modules {:cljs-base {:entries []}
           :module-a  {:entries '#{foo.core}
                       :depends-on [:cljs-base]}
           :module-b  {:entries '#{bar.core}
                       :depends-on [:cljs-base]}}

We will then compute the depth of every module in the graph:

{:modules {:cljs-base {:entries [] :depth 0}
           :module-a  {:entries '#{foo.core} :depth 1
                       :depends-on [:cljs-base]}
           :module-b  {:entries '#{bar.core} :depth 1
                       :depends-on [:cljs-base]}}

We then use this to compute a mapping from all depended upon inputs to a set of possible module assignments. For example we find all dependencies of foo.core and we assume they will go into :module-a - even cljs.core, the standard library.

But of course :module-b will also assign cljs.core to itself. So cljs.core module assignment is [:module-a :module-b]. However we can only choose one. To choose we first find all the common parent modules. Once found, we pick the module with the greatest :depth value.

Finally, any orphans will be assigned to :cljs-base.

Readers familiar with Webpack will note that this approach treats splits and split loading as two independent concerns. Thus split definition does not require editing sources nor the introduction of additional plugins.

Cross Module Code Motion

Automatic module assignment pushes code upwards to produce code splits that match user expectations. However if we left it at that we would be missing a huge opportunity. Along with dead code elimination Google Closure Complier employs another useful optimization - cross module code motion. Individual program values (including functions and methods) which are free of side effects can be moved back down the module graph.

A functional programming language like Clojure is well suited for this kind of optimization and the ClojureScript compiler carefully generates code in many cases to take advantage of this capability.

In practice this means that if some function and its dependencies present in :cljs-base are only ever used in :module-a, they will all be moved back to :module-a.


While Google documented these capabilities in Closure: The Definitive Guide published in 2010, we believe they still represent the state of the art. Please give these enhancements a try in the next release!