A beginner guide to compile ClojureScript with shadow-cljs

shadow-cljs provides everything you need to compile your ClojureScript code with a focus on simplicity and ease of use.

thheller/shadow-cljs

Shadow CLJS User's Guide

To install shadow-cljs, use npm:

npm install -g shadow-cljs

shadow-cljs requires Java, make sure you have Java installed. Don’t worry, shadow-cljs handles Java for you. If you still want to learn more about ClojureScript, checkout this list.

Compile to browser

Say you have a project:

.
├── README.md
├── assets
│   └── index.html
├── package.json
├── shadow-cljs.edn
├── src
│   └── app
│       ├── lib.cljs
│       └── main.cljs
└── yarn.lock

assets/index.html(please copy it to target/index.html in order that it can be accessed) looks like:

<script src="main.js"></script>

src/app/main.cljs may look like:

(ns app.main)
(def value-a 1)
(defonce value-b 2)
(defn reload! []
  (println "Code updated.")
  (println "Trying values:" value-a value-b))
(defn main! []
  (println "App loaded!"))

The namespace for this project is just app. To compile this project, first create a shadow-cljs.edn file, with configs:

{:source-paths ["src"]
 :dependencies []
 :dev-http {8080 "target/"}
 :builds {:app {:output-dir "target/"
                :asset-path "."
                :target :browser
                :modules {:main {:init-fn app.main/main!}}
                :devtools {:after-load app.main/reload!}}}}

Here’s how it’s configured:

• :source-paths is where you put source code
• :dependencies is ClojureScript deps of this projects
• :builds is where you specify build configurations
• :app is a build id named by us, we will need it to run specific compilations from command line tools or from APIs
• :output-dir decides where the generated files are saved
• :asset-path is the base path of files of the hot updated code
• :target is short for “compilation target”, :browser is for browser apps
• :modules specifies the modules we want to generate, :main also means the bundle will be named main.js
• :init-fn specifies the main function of the whole program
• :devtools specifies configurations for development tools
• :dev-http tells shadow-cljs to serve the folder target/ on port 8080.

During development, we can compile with hot code swapping, watch how we use the build id :app :

shadow-cljs watch app

After initial compilation finished, shadow-cljs would generate code in target/main.js and serve target/ folder on http://localhost:8080. When you open the url, the page will connect to shadow-cljs’ server and listen for updates. As you modify code and save files, the app will be hot reloaded and then app.main/reload! will be called.

To compile and optimize code:

shadow-cljs release app

For the complete example, please clone this repo:

minimal-xyz/minimal-shadow-cljs-browser

Compile to Node.js

It’s quite similar steps to compile ClojureScript targeting Node.js , for the project:

.
├── README.md
├── package.json
├── shadow-cljs.edn
├── src
│   └── server
│       └── main.cljs
└── yarn.lock

src/server/main.cljs may look like:

(ns server.main)
(def value-a 1)
(defonce value-b 2)
(defn reload! []
  (println "Code updated.")
  (println "Trying values:" value-a value-b))
(defn main! []
  (println "App loaded!"))

We can use shadow-cljs.edn with configurations:

{:source-paths ["src"]
 :dependencies []
 :builds {:app {:target :node-script
                :output-to "target/main.js"
                :main server.main/main!
                :devtools {:after-load server.main/reload!}}}}

Now we use :node-script as the compilation :target . And there are two fields:

• :output-to specifies the output file
• :main specifies the main function

Like in previous demo, we can start compilation in development mode, and run it with Node.js :

shadow-cljs watch app
# in another terminal
node target/main.js

As Node.js runs, it will connect to shadow-cljs’ WebSocket server listening for updates. Some npm modules(wsand source-map-support) are required, please install them. It also does hot code swapping as well as calling reload! function.

To create a release bundle, use the sub-command:

shadow-cljs release app

You may also add :compiler-options {:optimizations :simple}} to disable mangling. To try the full example, clone this repo:

minimal-xyz/minimal-shadow-cljs-nodejs

More

There are also examples you can browse in shadow-cljs, like using Macros:

minimal-xyz/minimal-cljs-macro

Or like compile code with long term caching:

minimal-xyz/minimal-shadow-cljs-release

And some examples run in several environments of JavaScript:

thheller/shadow-cljs-examples

Explore more on shadow-cljs and send feedbacks on GitHub :)