Shadow offers many helpers to compile and develop your app, but we only need to know about three methods right now:

• compile

• watch

• release

compile and watch are similar in scope. Both of them convert the ClojureScript code to JavaScript code based on the build target. The only difference is that watch re-compiles each time a file on the source path changes.

The JavaScript code produced by both compile and watch is inspectable, ie it's not uglified or minified.

The release method bundles the code for production. It applies advanced optimizations, minification, and uglification.

ClojureScript relies on Google's Closure Library (GCP) to transform CLJS to JavaScript. GCP is Google's in-house alternative for Webpack and was open-sourced over ten years ago. All major Google apps including Gmail rely on GCP.

Start watching#

We are ready to start our first build. We have set up our configuration, installed Shadow, and created a namespace. Our main function prints "Hello World".

To start Shadow watch, we need to issue the watch command with a build target:

yarn shadow-cljs watch :script

# or npx shadow-cljs watch :script

The :script is the name of our build target, defined in Shadow config.

When you run the watch command for the first time, Shadow will install the required dependencies.

Your output will be similar to:

$ yarn shadow-cljs watch :script

yarn run v1.21.1

shadow-cljs - updating dependencies

Retrieving thheller/shadow-cljs/2.10.15/shadow-cljs-2.10.15.pom from https://repo.clojars.org/

Retrieving cider/piggieback/0.5.0/piggieback-0.5.0.pom from https://repo.clojars.org/

...

Retrieving thheller/shadow-cljsjs/0.0.21/shadow-cljsjs-0.0.21.jar from https://repo.clojars.org/

shadow-cljs - dependencies updated

If everything goes well, you should see some log statements saying that the build completed and the nREPL server is running:

shadow-cljs - server version: 2.10.15 running at http://localhost:9630

shadow-cljs - nREPL server started on port 9000

shadow-cljs - watching build :script

[:script] Configuring build.

[:script] Compiling ...

[:script] Build completed. (76 files, 75 compiled, 0 warnings, 18.76s)

Shadow compiled 76 files, started the nREPL on port 9000 as defined in the config, and also started a server on port 9630. We have learned about nREPL already. The other server offers a web-based dashboard that gives an overview of all builds and runtimes. It also provides an interface to Shadow Inspect, which is a smart logging system, similar to Chrome Dev Tools.

Running the script#

At this point, your ClojureScript code is compiled to a Node script in build/node-script/core.js. You can inspect this code as it's JavaScript, but it might not make much sense. The :node-script target bundles everything into a single file, which allows for easy distribution at the cost of readability.

However, we can simply run the compiled Node script:

# in the first-project directory

node build/node-script/core.js

"Hello World"

shadow-cljs - #3 ready!

You might notice that the process doesn't terminate. This is because Shadow in watch mode allows for some hooks that can execute your code in the Node environment each time you make a change. For now, you can simply press Ctrl+C or Cmd+C to exit.

Try changing the message in core.cljs to "Hello World 2". You should notice that as soon as you save that file, Shadow rebuilds the script.

Making script production-ready#

So far the scripts we built with watch were for the development environment.

The release command builds the script in production mode. It uglifies, minifies, and applies optimizations to JavaScript output. To build our :script target run the following command: