Figwheel Tutorial


This document is intended to get you acquainted with the features and workflow of Figwheel.

This tutorial is based on the Clojure CLI tools.

Install the Clojure CLI tools

First we will want to install the clj and clojure command line tools.

If you are on Mac OSX and you can quickly install the Clojure tools via homebrew.

In the terminal at the shell prompt enter:

$ brew install clojure

If you’ve already installed Clojure, now is a great time to ensure that you have the latest version installed with:

$ brew upgrade clojure

You can check that everything has been installed correctly by running a Clojure REPL. In your terminal application at the shell prompt enter the following:

$ clj

You should see a user=> prompt where you can enter Clojure code.

Try entering (+ 1 2 3) you should get a response of 6 along with the next prompt asking you for more code. Type Control-C to get out of the Clojure REPL.

If you were able to start a REPL, you have successfully installed Clojure!

We will use the acronym REPL frequently. It stands for Read Eval Print Loop

Make a new directory to work in

Before we start working with ClojureScript, we’ll make a new directory to work in.

workspace$ mkdir hello-cljs

Specifying that you want to use Figwheel

Figwheel is a Clojure library, or rather it is a Jar of Clojure code that you will use. If you are familiar Ruby’s bundler and Gemfile, Python’s pip and requirements.txt, or Javascript’s npm/yarn and package.json then the concept of specifying a projects dependencies should be familiar to you.

When using the Clojure CLI Tools, the way that you specify that you want to have certain libraries available to you is in a deps.edn file.

So in our new hello-cljs directory, we’ll create a deps.edn file with the following contents:

{:deps {com.bhauman/figwheel-main {:mvn/version "0.2.18"}}}

Starting a ClojureScript REPL

Now let’s use Figwheel to start a ClojureScript REPL. Make sure you are still in the hello-cljs directory and enter:

clj -M -m figwheel.main

This command should fetch all the dependencies we need, boot up a ClojureScript REPL and finally pop open a Browser window with a page like that looks like this:

Repl host page in browser

If you see the green Connected next to the CLJS logo, it means that this page has successfully connected to the REPL that you just launched. This webpage is the host JavaScript environment for the REPL, and is where all the ClojureScript expressions that you type into the REPL will be evaluated.

Speaking of the REPL, if you head back to the terminal window where you launched figwheel.main from, you should now see something like this:

figwheel repl prompt in terminal

If you see this, you have successfully started a ClojureScript REPL and you can now type ClojureScript at the cljs.user=> prompt.

Let’s try some ClojureScript. Type the following expressions at the prompt as demonstrated in the example REPL session below:

cljs.user=> (println "Hello World!")
Hello World!
cljs.user=> (range 5)
(0 1 2 3 4)
cljs.user=> (map inc (range 5))
(1 2 3 4 5)
cljs.user=> (filter odd? (map inc (range 5)))
(1 3 5)
cljs.user=> (js/alert "ClojureScript!")

That last expression should cause a JavaScript Alert to pop up in the browser on our REPL host page.

Amping up the REPL

The REPL we just launched has a simple terminal readline support, meaning that it can handle editing a single line, and provide history by hitting the up arrow. This will do fine for trying simple expressions, but we often want to have a more fully featured terminal line reader that can:

  • syntax highlight Clojure code as you type it
  • facilitate multi-line editing of expressions
  • autocomplete the current function name that you are typing
  • display function signatures as you type
  • display the documentation for the function where your cursor is
  • display the source code of the function where your cursor is
  • allows you to query for functions that are similar to the word under your cursor

My library Rebel Readline provides these features for Clojure REPLs. Let’s install it now as it will be very helpful while exploring how to work with Figwheel and ClojureScript.

To use Rebel Readline let’s add com.bhauman/rebel-readline-cljs 0.1.4 as another dependency in the deps.edn file:

{:deps {com.bhauman/figwheel-main {:mvn/version "0.2.18"}
        com.bhauman/rebel-readline-cljs {:mvn/version "0.1.4"}}}

Now when you launch figwheel.main, it will detect the presence of com.bhauman/rebel-readline-cljs and use it when starting the ClojureScript REPL.

To see it in action launch a REPL with figwheel.main again, from the hello-cljs directory:

$ clojure -M -m figwheel.main

Its better to use the clojure command when using rebel-readline because the clj command provides it’s own terminal line reader

After entering the above command, a Browser will open and a REPL will start just like before. However, you will now see the following line a few lines before the cljs.user=> prompt.

[Rebel readline] Type :repl/help for online help info

This confirms that we are using Rebel Readline.

If you type :repl/help command at the prompt, as you type you immediately notice that :repl/help character are syntax highlighted. Upon hitting enter, you will see a useful reference for the REPL’s capabilities displayed.

rebel readline help

Rebel Readline feature walkthrough

Let’s quickly walk through how to use some of the Clojure Key Bindings listed in the help above.


Type (ra at the prompt and don’t hit ENTER but hit the TAB key.

You will see a list of choices that you can TAB through and hit ENTER on your selection. You can also keep typing to narrow the selection down.

Select rand-int, you will now have cljs.user=> (rand-int on the line.

Inspecting functions

Let’s find out how to use rand-int. Hit Control-X Control-D to bring up the documentation for rand-int, upon doing so you will see

rebel redline displaying rand-int doc

One of the more helpful parts of this documentation is the line that is displaying ([n]). It’s helpful but concise, and we should take a moment to parse it.

([n]) is a list of function signatures for the rand-int function. It indicates that rand-int only has one function signature [n], which in turn indicates that rand-int takes a single argument n.

As for the type of n we can infer from the function name and documentation that n is most likely an integer. But there is another indication. The Clojure/Script core libraries, and many others, use the following conventions when naming arguments.

  • f, g, h - function
  • n - integer, usually a size
  • index, i - integer index
  • x, y - numbers
  • xs - sequence
  • m - map
  • s - string
  • re - regular expression
  • coll - a collection
  • pred - a predicate closure
  • & more - variable number of arguments

The argument name is n so we now know when we call rand-int we should supply one integer argument.

If we want to understand rand-int further, we can directly examine its source code.

Now, with your cursor anywhere on the rand-int function hit Control-X Control-S. You will see the source code for rand-int displayed.

rand-int source in rebel

Now that we know that rand-int takes a single integer argument let’s call it.

Let’s see if rand-int works as we’d expect it to. Complete the REPL line by typing (rand-int 5) and hit enter and you should indeed get a random number from 0 to 4.

If you hit the up arrow you can get (rand-int 5) back at the REPL prompt and you can hit enter again to get a different random result.

Let’s inspect a function with a more complex argument signature.

Enter (range at the prompt (use TAB completion if you like). Now look at it’s documentation with Control-X Control-D you will notice that the argument signature is described differently.

([] [end] [start end] [start end step])

This is showing us that range can be called with between 0 and 3 arguments. You can call range with no arguments [] which will return an infinite sequence (not recommended at the REPL), you can call it with one argument [end] specifying where a range starting at 0 should end, you can also call it with the other argument arities [start end] and [start end step].

Let’s try this:

cljs.user=> (range 4)
(0 1 2 3)
cljs.user=> (range 4 10)
(4 5 6 7 8 9)
cljs.user=> (range 4 10 2)
(4 6 8)

We intentionally didn’t try to use (range) as it will cause the JavaScript environment to go into a tight loop and prevent further REPL use or interaction.

It is a good exercise to experience this, so if you’re up for it, try entering (range) at the prompt. It should freeze the REPL because the browser is now stuck in a tight loop trying to iterate through all the integers up the maximum integer possible.

We can recover from this.

Sometimes it’s easy to forget that the REPL is backed by a browser tab, which is a very simple thing to reset. If your REPL gets stuck in a tight loop, you can return to the REPL host page, take note of the URL (most likely http://localhost:9500/ in our case) and then explicitly close the tab to kill the tight loop (a page reload often doesn’t work in this case) and then open new tab at the same URL.

At this point the REPL eval should have timed out and returned to functioning normally.

TIP If you are not in a tight loop and need to reset the state of the REPL at any point you can simply reload the REPL host page and that will give you a fresh slate to start from.

Okay, back from the brink? If not you can kill the REPL with Control-C Control-D and restart it.

Inline eval

There is one more Rebel Readline feature that I’d like to go over before moving on.

I’m assuming that you have a running REPL and that you are back at the cljs.user=> prompt.

At the prompt, enter the expression (+ 1 2 3 4), and after that when your cursor is just after the last ), hit Control-X Control-E.

You should see that the expression was evaluated and the result #_=> 10 displayed just under the line where your cursor is. Rebel Readline allows you to evaluate any expression or sub expression before hitting ENTER.

Let’s try this again, hit ENTER to get back to an empty prompt and type the expression (+ (+ 1 2 3) (+ 4 5 6)) and now place the cursor after the last paren of the sub-expression (+ 1 2 3). If you hit Control-X Control-E at this point you will see that you get the value 6. Experiment with evaluating in other parts of the expression on this line.

What’s the value of this feature? It allows you to work on larger multi-line expressions while verifying that the sub expressions are doing what you expect them to do.

Having easy to parse expressions is part of the magic of LISP. It allows tools to understand delimited expressions without having to implement a complete parser for the language.

Multi-line editing

By now, you may have noticed that you can create multi-line expressions at the REPL prompt in a fairly straight forward way.

Try entering the following expression at the prompt and make sure you format it so that it spans multiple lines.

cljs.user=> (+ 1
       #_=>    2
       #_=>    3)

You should notice that you were able to hit ENTER to create newlines in your expression while the cursor was inside of an open expression and that once you closed the expression (by adding the last paren), when you hit ENTER it was submitted for evaluation.

This concludes our tour of the Rebel Readline REPL features.

You can now exit the Rebel Readline REPL by hitting Control-C Control-D.

Break Time

Once you have made it this far you have learned how to add dependencies to deps.edn and how to start a figwheel.main REPL with Clojure’s CLI tools. You have also, learned how to include Rebel Readline and how to use it to introspect your environment.

This is more than enough to justify a break, may I suggest a nice walk or perhaps a chat with a co-worker nearby?

Working at the REPL

The ClojureScript REPL is a fantastic tool. It will be very helpful to understand a basic REPL driven workflow, as it is an important skill that is complementary to the more automated hot-reloaded workflow.

We are going to start up a generic figwheel.main REPL and then start to compose, require and reload ClojureScript source files.

Again, we will begin in our hello-cljs directory and start a REPL with figwheel.main.

$ clojure -M -m figwheel.main

Once the REPL has started we will turn our attention to creating a file that contains some ClojureScript source code.

We’ll have to create a file where it can be found, which means it will have to be named properly and reside on the classpath.

By default, the Clojure tool adds the local src directory to the classpath.

Create a file src/hello/cruel_world.cljs with the following contents:

(ns hello.cruel-world)

(defn what-kind? []
(js/console.log (what-kind?))

The file layout of your project should now be:

├── deps.edn
└── src
    └── hello
        └── cruel_world.cljs

Take note that in the ClojureScript file we are declaring a namespace hello.cruel-world and that the path to our file mirrors this namespace and it is rooted in the src directory, which is on the classpath. This is what will allow the ClojureScript compiler to find and compile our code.

TIP: an extremely common mistake is to forget to replace hyphens - with underscores _ when creating a namespace file path for a source file.

Now that we have created the file with the hello.cruel-world namespace let’s require it into our running REPL.

cljs.user=> (require 'hello.cruel-world)

That nil response is exactly what we want. If there was a problem finding or compiling the file the REPL will let you know.

Now we can call our what-kind? function defined in hello.cruel-world:

cljs.user=> (hello.cruel-world/what-kind?)

As expected, when we call (hello.cruel-world/what-kind?) it returns the string "Cruel".

But it seems like a bit much to type out the entire namespace for each call, so let’s require hello.cruel-world again and create an alias for it. Remember that you can hit the up arrow to get back to the original require statement and edit it.

cljs.user=> (require '[hello.cruel-world :as world])
cljs.user=> (world/what-kind?)

As you can see we created an alias world for our namespace and we can now invoke functions from hello.cruel-world with the alias world.

Now let’s change the file and reload it.

Alter the what-kind? function in the src/hello/cruel_world.cljs file so that it returns "Brilliantly Cruel". When you are done the file should look like this:

(ns hello.cruel-world)

(defn what-kind? []
  "Brilliantly Cruel")
(js/console.log (what-kind?))

Now we can head back to the REPL prompt and reload the hello.cruel-world namespace like so:

cljs.user=> (require '[hello.cruel-world :as world] :reload)

And then verify that our changes were loaded:

cljs.user=> (world/what-kind?)
"Brilliantly Cruel"

There is one more quick trick that you will find helpful while working at the REPL. You can change your current namespace (cljs.user) to another loaded namespace.

For instance, we can change into the hello.cruel-world namespace like so:

cljs.user=> (in-ns 'hello.cruel-world)

You will see the prompt change and now that your current namespace is hello.cruel-world you can call functions in that namespace without needing to provide a namespace.

For example:

hello.cruel-world=> (what-kind?)
"Brilliantly Cruel"

Remember that you are working in a Browser environment so you can interact with the browser DOM as well.

hello.cruel-world=> (js/document.getElementById "app")
#object[HTMLDivElement [object HTMLDivElement]]

There is an HTML element with an id of "app" available on the REPL host page. This element contains all of the markup and style for the helper application. Let’s override the helper app content, with the string returned by our what-kind? function.

Make sure the REPL host page is visible while you type the following.

hello.cruel-world=> (def app-element (js/document.getElementById "app"))
hello.cruel-world=> (set! (.-innerHTML app-element) (what-kind?))
"Brilliantly Cruel"

After that, on the REPL host page, you should see the helper app disappear and be replaced by the words Brilliantly Cruel.

Well that was just a brief tour of a REPL driven workflow. This was a simple example but it starts to demonstrate that you could possibly get quite far into an application just using this simple setup.

TIP: There is a fantasic guide on Programming at the REPL on the official Clojure website. Much of the guide is also directly applicable to ClojureScript, when you are ready to learn more it is an excellent resource.

TIP: Much of what you learned above applies equally well to the Clojure language. So if you would like to try your hand at Clojure, as well, you can get a working Rebel Readline Clojure REPL by typing clojure -M -m rebel-readline.main in the hello-cljs directory. If you want to create and load Clojure files, everything is the same as above except Clojure files end with .clj

Starting the REPL already initialized with your code

While it is perfectly workable to require all the code you need at the REPL, most of the time you will want to initialize the REPL with something specific already loaded.

You can accomplish this by using the following command:

$ clojure -M -m figwheel.main --compile hello.cruel-world --repl

This will compile and load the hello.cruel-world namespace into the REPL environment. If hello.cruel-world required other namespaces they would get loaded as well.

You will notice some differences in the REPL startup output this time:

compile terminal output

The output looks similar to launching figwheel.main without any arguments, but there are some important differences.

The first thing of note is this warning:

target classpath warning

This is expected. When we started the figwheel.main REPL without any arguments, the compiled ClojureScript files are output to the target directory. This directory is automatically added to the classpath so that the compiled assets can be found and served by the built-in webserver.

Running figwheel.main without arguments indicates that you are likely experimenting with some code, not working on a local project.

When one starts compiling namespaces it indicates that we are commiting to a project, and thus we will want our compiled artifacts to be local to the project (for later use). It is also time to start being explicit about what is on our classpath. In order to provide a smooth initial experience, figwheel.main will try to be helpful and append the classpath with paths that should likely be there. When it does add a classpath, Figwheel will print a warning because it is best that you manage the classpath explicitly so that things work properly when you are not using figwheel.main.

We can fix this classpath warning by adding both "src" and "target" to the :paths key in our deps.edn file:

{:deps {com.bhauman/figwheel-main {:mvn/version "0.2.18"}
        com.bhauman/rebel-readline-cljs {:mvn/version "0.1.4"}}
 :paths ["src" "target"]}

We have to include "src" as well as "target" because when you add a :paths key to deps.edn the src path is no longer implicitly added.

Now we will return to run our REPL again from the hello-cljs directory.

$ clojure -M -m figwheel.main --compile hello.cruel-world --repl

You should see the familiar output when starting a figwheel.main REPL minus the warning about target.

You should also see that your ClojureScript source code is still being compiled to the local target directory.

You will also notice this line:

watching line in output

This line indicates that Figwheel is now watching the ClojureScript files in the src directory for changes. When a change is detected Figwheel will compile and reload the changed code into the JavaScript environment, without us needing to use (require 'hello.cruel-world :reload).

Let’s try this out.

First let’s verify that hello.cruel-world is loaded.

cljs.user=> (hello.cruel-world/what-kind?)
"Brilliantly Cruel"

Looks good! We successfully started the repl with our namespace already available so we don’t have to explicitly require it.

Now let’s change the namespace. Go to the src/hello/cruel_world.cljs source file and change the what-kind? function so it returns "Cruel No More" and then save the file. It should look like this:

(ns hello.cruel-world)

(defn what-kind? []
  "Cruel No More")
(js/console.log (what-kind?))

When you save the file, you should see a green Successfully compiled message appear in the REPL. You can now check the REPL to see that your code has indeed been reloaded automatically. (Don’t forget that you can use TAB to use autocomplete to help you enter the following)

cljs.user=> (hello.cruel-world/what-kind?)
"Cruel No More"

This verifies that our code has been loaded. Now, we don’t have to explicitly reload a namespace as we work, we can simply save the file we are working on and it will be reloaded instantly.

Automatically reloading code on save makes a significant impact on one’s workflow.

Feedback is King

We’ll take a look at the ways in which Figwheel provides feedback while you are working.

When we have a workflow where our files are being watched and compiled as we work on them, we have an opportunity to detect syntax/compile errors earlier than if we waited to reload by hand.

Figwheel provides feedback for compile time errors and warnings in the REPL and in the browser.

REPL Feedback

To experience this, arrange your REPL terminal and editor windows so that they are side by side.

image of terminal next to editor

Now in the editor edit the the src/hello/cruel_world.cljs file again by adding some bad code on a line at the end of the file that looks like this defn hello. When you are done the file should look like this:

(ns hello.cruel-world)

(defn what-kind? []
  "Cruel No More")
(js/console.log (what-kind?))

defn hello

Now when you save the file you should see some warnings that look like the following feedback in the REPL.

screen shot of defn hello warnings

Getting feedback like this as you are coding is more timely than waiting until a file is compiled and loaded by hand and allows you to concentrate more on the problem with less interruptions from the process.

Now let’s cause a compile error by adding parenthesis around defn hello so that it looks like this:

(ns hello.cruel-world)

(defn what-kind? []
  "Cruel No More")

(js/console.log (what-kind?))

(defn hello)

Upon saving the file you will see a compile error in the REPL.

image of compile error

So, we’ve demonstrated a workflow where you can edit your code and quickly get feedback from the REPL that informs you of any compile errors.

Heads-up display feedback

You may have noticed this already but if you go back to the “Default Figwheel Dev Page” in the Browser you will also see the same error above displayed in Figwheel’s heads-up display.

image of error in heads up display

Displaying compile time errors and warnings in the browser is an important feature of Figwheel as we can only keep our attention in so many places as we work. It’s far too easy to miss compile time error messages if they are tucked away in a terminal somewhere while you are focused on the front-end you are working on.

Now, keeping the browser window visible, change the file and remove the parenthesis from around the (defn hello) and hit save. You will now see the compiler error replaced with a warning like this:

image of warning

And finally delete the remaining defn hello line and save the file once again.

You will notice that the warning goes away.

The Build

The previous examples demonstrated how to start productively working with ClojureScript with very little configuration. However, when we are working on a larger project we will normally need to customize our environment beyond what is possible with the deps.edn file and the figwheel.main command line options.

There is also a need to name our build configurations so that we can identify them and help Figwheel keep their compiled artifacts separate.

To meet these needs figwheel.main relies on a build file to specify ClojureScript compiler options, figwheel options, and a stable name for the configuration.

To help explain what a build file is and how it works, we’ll start with an example.

First let’s reset things. Make sure you quit the REPL and then delete the target/public directory to get rid of our compiled assets.

After that, in the hello-cljs directory, place a new build file called cruel.cljs.edn with the following contents:

{:main hello.cruel-world}

The contents of the cruel.cljs.edn build file are specifying the ClojureScript compiler options for our cruel world project. These options will be passed to the ClojureScript compiler whenever source code needs compiling. There are quite a few compiler options but the above is all we’ll need as figwheel.main provides enough default compiler options to allow you to get started working.

The hello-cljs directory should now look like this:

├── cruel.cljs.edn
├── deps.edn
└── src
    └── hello
        └── cruel_world.cljs

It is easy to use a build file. From the hello-cljs directory, start figwheel.main again, using our new build file:

$ clojure -M -m figwheel.main --build cruel --repl

This will create a working environment that is practically identical to when we previously called clojure -M -m figwheel.main --compile hello.cruel-world --repl.

You will notice that if you edit the src/hello/cruel_world.cljs file you will see the same hot reloading and feedback behavior as before.

In the startup output for the REPL, you will notice a difference in the names of the compiled artifacts.

image of compiled artifacts output lines

Before the compiled output was getting compiled to target/public/cljs-out/main.js and now it’s getting compiled to target/public/cljs-out/cruel-main.js.

As your project grows it will more than likely have more than one build, and having a name to identify and separate your build’s compiled artifacts is very helpful.

Also, as you progress with your project there is normally a need to add more specific ClojureScript compiler options rather than rely the default ones that figwheel.main supplies. With a build file like cruel.cljs.edn you will have a logical place to add these options as needed.

You will also have a place to add figwheel.main configuration options.

As an exercise, we’ll add an option to modify figwheel.main’s behavior. Let’s say we’ll be mainly relying on the heads up display feedback and that we’d like to make figwheel print the compiler errors more concisely in the REPL.

We have seen that compile errors normally print out with some code context and a pointer to where the error was detected.

image of verbose error

We can make errors print out without the code context with the :log-syntax-error-style option.

We’ll configure this in our cruel.cljs.edn file. Go ahead and edit the build file to look like this:

^{:log-syntax-error-style :concise}
{:main hello.cruel-world}

The caret ^ character is very important and signifies that we are adding metadata to the map that follows it.

Learn more about Clojure Metadata here and see the official reference

Now when we start up the cruel build with figwheel.main again:

$ clojure -M -m figwheel.main --build cruel --repl

If we type an error in our code we’ll see a much more concise message like this:

concise message image

In summary, a build file is a useful way to specify compiler and Figwheel options and provide that configuration a useful name. I consider this the most useful way of working with figwheel.main.


Sometimes you will have several builds and you will want to share some or all of your figwheel.main configuration between them. You can do this by creating a figwheel-main.edn in your project directory.

For our current example, we can move our :log-syntax-error-style configuration out of our cruel.cljs.edn file and place it in a figwheel-main.edn file. Like this:

Example hello-cljs/figwheel-main.edn:

{:log-syntax-error-style :concise}

If you try this you will notice that the :log-syntax-error-style :concise configuration still works. This configuration will now take effect without out the need to specify it in any build files.

It is important to note that the metadata configuration that you add to your build files will override the configuration in figwheel-main.edn

Packaging up a single compiled artifact for production

The ClojureScript compiler has four :optimizations modes :none, :whitespace, :simple and :advanced. These determine the type of output that the complier produces. For example, the default mode that we have been using so far is :none. This mode produces many individual files when we compile our ClojureScript.

If you look at the target/public/cljs-out/cruel directory you will see these files.

└── public
    └── cljs-out
        ├── cruel
        │   ├── cljs
        │   │   ├── core.cljs
        │   │   ├── core.js
        │   │   ├──
        │   │   ├── pprint.cljs
        │   │   ├── pprint.cljs.cache.json
        │   │   ├── pprint.js
        │   │   ├──
        │   │   ├── stacktrace.cljc
        │   │   ├── stacktrace.cljc.cache.json
        │   │   ├── stacktrace.js
        │   │   ├──
        │   │   ├── test.cljs
        │   │   ├── test.cljs.cache.json
        │   │   ├── test.js
        │   │   └──
        │   ├── cljs_deps.js
        │   ├── cljsc_opts.edn
        │   ├── clojure
        │   │   ├── data.cljs
        │   │   ├── data.cljs.cache.json
        │   │   ├── data.js
        │   │   ├──
        │   │   ├── set.cljs
        │   │   ├── set.cljs.cache.json
        │   │   ├── set.js
        │   │   ├──
        │   │   ├── string.cljs
        │   │   ├── string.cljs.cache.json
        │   │   ├── string.js
        │   │   ├──
        │   │   ├── walk.cljs
        │   │   ├── walk.cljs.cache.json
        │   │   ├── walk.js
        │   │   └──
... and many more

When we want to deploy our final project we are normally going to want to produce a single JavaScript file, to make load times more efficient.

All of the other :optimizations modes produce a single file as output.

  • :whitespace only optimizes whitespace
  • :simple only makes safe simple optimizations in addition to optimizing whitespace
  • :advanced performs a state of the art JavaScript optimization

The :advanced optimization level is normally what you will want when you are ready to deploy code.

Let’s first output a :whitespace optimized file.

First lets delete the target/public directory.

$ rm -rf target/public

Because we just want to compile a file once and not start a watching process we are going to use the figwheel.main --build-once flag. We will also specify the --optimizations level as whitespace.

Here’s the long version of the command:

$ clojure -M -m figwheel.main --optimizations whitespace  --build-once cruel

and here’s the command using abbreviated flags:

$ clojure -M -m figwheel.main -O whitespace -bo cruel

TIP: you can use clj -M -m figwheel.main --help to learn all of the figwheel.main CLI flags and their abbreviations

If you execute the above command and then view target/public/cljs-out/cruel-main.js you will see that it is now a large file that bundles all the required code in it.

You may think that this is a lot of code considering the size of our source file. Unfortunately we are getting all the code that is needed by the core ClojureScript library even though we are not using it.

We can remedy this by using advanced compilation which will perform a static code analysis and perform DCE (Dead code elimination) and remove any code that is not used.

$ clojure -M -m figwheel.main -O advanced -bo cruel

If you now view the contents of target/public/cljs-out/cruel-main.js and see that it is now significantly smaller.


In all of the examples above, we have used the host HTML page provided by the figwheel.main helper application. You will very quickly get to the point where you want to supply your HTML page to host your application.

Let’s look at how you can provide your own index.html to host your application.

The webserver used by figwheel uses the classpath to find static files. Anything that is on the classpath in a public directory will be served as a static file.

Add a resources directory to the classpath by adding "resources" to the :paths key in you deps.edn file. When you finish the file should look like this:

{:deps {com.bhauman/figwheel-main {:mvn/version "0.2.18"}
        com.bhauman/rebel-readline-cljs {:mvn/version "0.1.4"}}
 :paths ["src" "target" "resources"]}

Now let’s make a resources/public directory that we will use as a web-root directory for our web assets.

$ mkdir -p resources/public

Now, we’ll add a CSS file, because let’s face it we’re gonna need it.

Create a CSS file resources/public/css/style.css with the following content:

/* style */
body {
	color: red;

Then create an resources/public/index.html file with the following in it:

<!DOCTYPE html>
    <!-- this refers to resources/public/css/style.css -->
    <link href="css/style.css" rel="stylesheet" type="text/css">
    <div id="app"></div>
	<!-- this refers to target/public/cljs-out/cruel-main.js -->
    <script src="cljs-out/cruel-main.js"></script>

Once this file is in place, when you start the cruel build with figwheel.main you will now see your index.html file rather than the default helper app host page.

Live Reload CSS

You can get figwheel.main to watch and reload the css file above by adding :css-dirs ["resources/public/css"] configuration to your cruel.cljs.edn file as follows:

^{:css-dirs ["resources/public/css"]}
{:main hello.cruel-world}


You made it! Thanks for taking the time to learn more about figwheel.main. I’ve hopefully provided you enough information to get started with your ClojureScript explorations.