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shadow-cljs - browser quickstart

This is a minimum template you can use as the basis for CLJS projects intended to run in the browser.

Required Software

User Guide

This repository only shows a basic example of how to get a basic Browser build.

Please refer to the full User Guide for more information.

Running the Example

git clone quickstart
cd quickstart
npm install
npx shadow-cljs server

This runs the shadow-cljs server process which all following commands will talk to. Just leave it running and open a new terminal to continue.

The first startup takes a bit of time since it has to download all the dependencies and do some prep work. Once this is running we can get started.

npx shadow-cljs watch app

This will begin the compilation of the configured :app build and re-compile whenever you change a file.

When you see a "Build completed." message your build is ready to be used.

[:app] Build completed. (23 files, 4 compiled, 0 warnings, 7.41s)

You can now then open http://localhost:8020.

The app is only a very basic skeleton with the most useful development tools configured.

shadow-cljs is configured by the shadow-cljs.edn config. It looks like this:

;; shadow-cljs configuration
{:source-paths ; .cljs files go here

 :dependencies ; covered later

 :dev-http ; starts a http dev server on http://localhost:8020 and serves `public`
 {8020 "public"}

 {:app ; build identifier
  {:target :browser
   :output-dir "public/js"
   :asset-path "/js"

   {:main ; becomes public/js/main.js
    {:init-fn starter.browser/init}}}}}

It defines the :app build with the :target set to :browser. All output will be written to public/js which is a path relative to the project root (ie. the directory the shadow-cljs.edn config is in).

:modules defines the how the output should be bundled together. For now we just want one file. The :main module will be written to public/js/main.js, it will include the code from the :entries and all their dependencies.

The last part is the actual index.html that is loaded when you open http://localhost:8020. It loads the generated /js/main.js and then calls start.browser.init which we defined in the src/main/start/browser.cljs.

<!DOCTYPE html>
<html lang="en">
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <link rel="stylesheet" href="/css/main.css">
    <title>Browser Starter</title>
<h1>shadow-cljs - Browser</h1>
<div id="app"></div>
<noscript>You need to enable JavaScript to run this app.</noscript>
<script src="/js/main.js"></script>

Live reload

To see the live reload in action you can edit the src/main/start/browser.cljs. Some output will be printed in the browser console.


During development it the REPL is very useful.

From the command line use npx shadow-cljs cljs-repl app.

shadow-cljs - config .../shadow-cljs.edn
shadow-cljs - connected to server

This can now be used to eval code in the browser (assuming you still have it open). Try (js/alert "Hi.") and take it from there. You might want to use rlwrap npx shadow-cljs cljs-repl app if you intend to type a lot here.

You can exit the REPL by either CTRL+C or typing :repl/quit.


The watch process we started is all about development. It injects the code required for the REPL and the all other devtools but we do not want any of that when putting the code into "production" (ie. making it available publicly).

The release action will remove all development code and run the code through the Closure Compiler to produce a minified main.js file. Since that will overwrite the file created by the watch we first need to stop that.

Use CTRL+C to stop the watch process and instead run npx shadow-cljs release app.

When done you can open http://localhost:8020 and see the release build in action. At this point you would usually copy the public directory to the "production" web server.

Note that in the default config we overwrote the public/js/main.js created by the watch. You can also configure a different path to use for release builds but writing the output to the same file means we do not have to change the index.html and test everything as is.