Compile Configuration

The ClojureScript compiler has a dizzying number of configuration options. However, there are really only a few key ones you need to understand to work effectively.

Figwheel handles setting up several compilation options for you, but it’s very important to understand these options, and how Figwheel uses them to inject itself into a build. Having this knowledge will empower you to understand how your build is working and how to adjust the options so that they serve you better.

Here are the compiler options we’ll cover:

  • --print-config
  • :output-to
  • :output-dir
  • :target
  • :optimizations
  • :main
  • :asset-path
  • :preloads
  • :closure-defines

The --print-config option

You can use the figwheel.main CLI option --print-config (or -pc) to examine the compile options that Figwheel is sending to the ClojureScript compiler. The --print-config CLI flag will only print out the configuration, it will not run the command.

It’s important to note that order matters for figwheel.main CLI options. The -pc flag is considered an init option and as such it must always come before a main option (-c, -b, -r, -m are all main options). In the commands that we’ve been using so far in this documentation, this means it has to come before the -b option.

Keeping with our hello-world.core example let’s print out the configuration.

$ clojure -m figwheel.main -pc -b dev -r

The above command should print out something like the following:

[Figwheel] :pprint-config true - printing config:
---------------------- Figwheel options ----------------------
{:pprint-config true, :mode :repl, :watch-dirs ("src")}
---------------------- Compiler options ----------------------
{:main hello-world.core,
 :output-to "target/public/cljs-out/dev-main.js",
 :output-dir "target/public/cljs-out/dev",
 :asset-path "cljs-out/dev",
 :aot-cache false,
 ([figwheel.repl :refer-macros [conns focus]]
   [stop-builds start-builds build-once reset clean status]])}

As you can see there are two sections; one displaying the current config options for Figwheel and one displaying the final compile options to be sent to the ClojureScript compiler.

Looking at the compiler options you will see that while we only supplied {:main hello-world.core} in our dev.cljs.edn, Figwheel added a few more options. We will focus on explaining these options.

The :output-to option

The :output-to option is “the path to the JavaScript file that will be output”. This is simple enough. You provide the :output-to option a path to a file where the compiled ClojureScript should be output. This file will always represent your main compiled artifact.

However, the contents of the file and how we load this file will vary based on the :optimizations, :main, and :target options.

The most important thing to remember when working with the Figwheel server is that the :output-to option needs to point to a path that is basically the classpath + public.

The Figwheel default for this path is in the target/public directory because we want to separate files that are compiled (and thus temporary) from files that we edit and keep in version control like HTML and CSS.

The Figwheel default as mentioned before uses the build name and looks like


If you don’t use a build file then the :output-to path may look like


This can lead to problems if you are using different command line options to obtain different compilation results.

The :output-dir option

The :output-dir sets the output directory for temporary files used during compilation.

Regardless of the :optimizations level, the compiler will output a file for each ClojureScript “namespace” in your project and all of its dependencies, as well as the Google Closure libraries and foreign libraries that are utilized.

If you have a hello-world.core example you can examine the :output-dir after a compile and see the temporary files:

├── cljs
│   ├── core.cljs
│   ├── core.js
│   ├──
│   ├── pprint.cljs
│   ├── pprint.cljs.cache.json
│   ├── pprint.js
│   ├──
│   ├── stacktrace.cljc
│   ├── stacktrace.cljc.cache.json
│   ├── stacktrace.js
│   ├──
... a lot more

These files also serve as a cache for the compiler which enables incremental compiles. If your source file has a timestamp newer than the file in the output directory, then the compiler will compile the source file. Caches can get stale however and you will want to delete your output directory on a regular basis.

When no :output-dir is defined, Figwheel will provide a default :output-dir which will have the form:


For example the default :output-dir for our dev.cljs.edn build file will be target/public/cljs-out/dev.

You may notice that the :output-dir path is placed so that its contents are available via the classpath. The reason for this is because when we use the default :optimizations level :none many of these “temporary files” are directly loaded into our client environment. For example, in a web environment when we load our :output-to file, the Google Closure base code will calculate the files that need loading and then load them in order. Hence the browser needs to serve these files and this is why they need to be available on the classpath.

So if you choose to customize the :output-dir path keep in mind that they will need to be found by the client environment if you are using :optimizations :none.

But … you should never put the :output-dir directly on the classpath. This means if target is in the classpath, you should never set :output-dir to the target directory. If you do this you are making your compiled artifacts resolvable by the CLJS compiler. This means that when the CLJS compiler looks for a required CLJS file it will possibly find it in the target directory and this will cause major problems.

The :optimizations option

The :optimizations compiler option designates the optimizations level that the Google Closure compiler should use. The available optimization levels are:

  • :none - the default level, no code optimizations
  • :whitespace - basically just removes whitespace and comments
  • :simple - removes whitespace and shortens local variable names
  • :advanced - more aggressive renaming, dead code removal, global inlining

The Figwheel --build option and the ClojureScript REPL both require :optimizations to be set to :none. :none is the default so it does not need to be specified in the configuration options map.

As we noted before the :none level does not produce a single self-contained compiled artifact, but rather creates an artifact that loads all of the separately compiled namespaces.

All the other levels (:whitespace, :simple, and :advanced) produce a single compiled artifact to the :output-to file. These are often used for producing your final deployable asset when you use these optimizations settings.

The :advanced level provides absolutely amazing compression and optimization but it is also the most finicky. This guide can be helpful.

The :main option

The :main option specifies an entry point (root namespace) for your compiled artifact. When :optimizations is :none the :main option will cause the compiler to generate a file that will in turn load all the files that are needed by the :main namespace. Actually, it does more than this. It adds all the :closure-defines that we have specified in our configuration, and requires all of the :preloads.

The :main option bootstraps your client environment and as such it behaves differently depending on the :target option.

Figwheel does not have a default for the :main option and requires that you provide a namespace value.

You can provide a symbol or a string, and it needs to be a namespace that is available in your source path directories that are on the classpath.

The example we have been using in this documentation has been:

:main hello-world.core

The :asset-path option

This only affects the build if you are using the :main option with :optimizations :none. The generated bootstrap script in the :output-to file needs to know the path to your temporary files in :output-dir. This path is relative to your webroot.

As you can see above in the compile options for our example hello-world.core project our :asset-path is this:

:asset-path "cljs-out/dev"

Basically you can think of the :asset-path as your :output-dir minus your webroot directory.

So in our case:

"target/public/cljs-out/dev" - "target/public/" => "cljs-out/dev"

If your application can’t seem to find the files it needs to load, it normally means you have a misconfigured :asset-path.

The :target option

There are three values for the :target option. Actually there are two values and the absence of a :target option. If you don’t specify a :target then it will be assumed that our client environment is the Browser.

The other valid targets are :nodejs and :webworker.

The :target option will change the output of the :main bootstrap script which gets output to the :ouput-to file. The script will handle the environmental needs of loading your ClojureScript code for that particular environment.

The :target option will also change the output code based on what the environment needs.

Figwheel does not add or change the :target option. It will respond to it and ensure that it starts a Node backed REPL if it needs to.

The :preloads option

When we load a ClojureScript application we start at the :main namespace to find all the files that need to be loaded. Sometimes we want to inject functionality (like REPL support) into our applications.

The :preloads option allows you to inject namespaces into your runtime environment.

Figwheel takes advantage of this to inject its code into your environment. You can see this in the hello-world.core example project above. Figwheel appended several :preloads to the configuration:


All of these namespaces add additional functionality to your application. Notice that this is how we add devtools and the figwheel.repl connection to your build.

You can add pre-loads as well to add behavior to your development or production builds.

The :closure-defines option

The :closure-defines option is especially powerful. It allows us to define namespaced constants with goog-define and configure their values in the :closure-defines map.

Let’s say we want to have DEBUG and LOCALE variables that we set at compile time.

(ns hello-world.core)

(goog-define DEBUG false)
(goog-define LOCALE "en")

Important: goog-define only works with String, Booolean, and Number values.

So we’ve defined these constants along with their default values. We can override these default values in our configuration with the :closure-defines option like so:

:closure-defines {hello-world.core/DEBUG true
                  hello-world.core/LOCALE "fr"}

Figwheel uses :closure-defines to supply the connection URL to the figwheel.repl connection code.

You can see in our example above that its value is: