Quick Start

The only dependencies required for this tutorial are an installation of Java 8 and the standalone ClojureScript JAR. ClojureScript itself only requires Java 7 but the standalone JAR comes bundled with useful Nashorn integration that requires Java 8.

You must have Java 8 installed to proceed.

Even if you are interested in a Leiningen, Boot, Figwheel, or Maven based workflow this Quick Start is essential reading. It covers the fundamentals regardless of what tooling you decide to end up using.

Dear Reader: Do not edit this page without first consulting an experienced member of the ClojureScript community. This is one of the most important resources for beginners and it has been extensively verified on a variety of environments. If you believe you have found a mistake, there's a 99.9% chance you have not found a mistake. If you truly believe you have found an error in this document, please come find help on the IRC or Slack channels and we can help determine if this page needs edits. Thank you.

ClojureScript Compiler

The standalone ClojureScript JAR bundles Clojure 1.8.0. This supports simple scripting of the ClojureScript compiler and the bundled REPLs without an overly complicated command line interface.

Download the standalone ClojureScript JAR.

Debug Note: If you use curl to download the JAR, be sure to specify the -L flag to follow redirects. (Otherwise you'll end up with an HTML page of about 300 bytes) Make sure the file is not renamed by your browser, eg. cljs(1).jar.

Create a directory hello_world and copy the JAR into that directory, then from inside the hello_world directory:

mkdir -p src/hello_world;touch src/hello_world/core.cljs

For Windows:

md src\hello_world & type nul >>core.cljs & move core.cljs src\hello_world

In your favorite text editor edit the src/hello_world/core.cljs to look like the following:

(ns hello-world.core)

(enable-console-print!)

(println "Hello world!")

First we declare our namespace. Every ClojureScript file must declare a namespace and this namespace must match a path on disk. We then enable direct printing to the commonly available JavaScript console object and print the famous message.

In order to compile this we need a simple build script. ClojureScript is just a Clojure library and can be easily scripted in a few lines of Clojure. Create a file called build.clj in the current directory not in src. Note the name of this file does not matter. In this tutorial we'll always make our compiler helper scripts at the root of the project directory.

Add the following Clojure code:

(require 'cljs.build.api)

(cljs.build.api/build "src" {:output-to "out/main.js"})

We require the cljs.build.api namespace. We then invoke the standard function for building some ClojureScript source - cljs.build.api/build. This function only takes two arguments: the directory to compile and a map of options. In our case a simple :output-to will suffice for now.

Let's build some ClojureScript:

java -cp cljs.jar:src clojure.main build.clj

On Windows:

java -cp "cljs.jar;src" clojure.main build.clj

We invoke java and set the classpath to our JAR and the directory where our ClojureScript code lives. The clojure.main argument in this case allows us to easily execute a Clojure file.

Debug Note: If you encounter an error about cljs.build.api not being found, make sure that you didn't write (require 'cljs.build.api'), many popular text editors will emit a matching single quote even in their Clojure mode.

Control should return to the shell relatively quickly and you will have an out directory with compiled JavaScript including your simple program. You will see that many files were produced in addition to the out/main.js we specified. We'll explain this momentarily but first let's see how you can easily include the compiled output on a web page.

Using ClojureScript on a Web Page

Create a file index.html and include the following:

<html>
    <body>
        <script type="text/javascript" src="out/main.js"></script>
    </body>
</html>

Open this file in your favorite browser and find the JavaScript developer console so you can see the output. (Note that nothing will appear in the main browser window—no content is being rendered.)

You will not see "Hello world!" but instead you will likely see an error like the following:

Uncaught ReferenceError: goog is not defined

In order to understand this error we must examine a few basics around the Google Closure Library. While the following section may seem somewhat roundabout, a short review on how Google Closure Library works will make simple bugs considerably easier to spot.

Google Closure Library

In order to abstract away JavaScript environment differences ClojureScript relies on the Google Closure Library (GCL). GCL supplies an important facility missing from JavaScript: namespaces and a way to declare dependencies between them. In fact ClojureScript namespaces get compiled to Google Closure namespaces.

Loading dependencies correctly across various browser targets is a surprisingly tricky affair. GCL accomplishes this by maintaining a dependency graph. When you require a namespace it will write the needed script tags in dependency order for you.

So what went wrong? If you look at out/main.js you will see some dependency graph building calls:

goog.addDependency("base.js", ['goog'], []);
goog.addDependency("../cljs/core.js", ['cljs.core'], ...);
goog.addDependency("../hello_world/core.js", ['hello_world.core'], ...);

But wait, where is this goog object coming from?

Oops. We never loaded it! In order for GCL to bootstrap we must at least load goog/base.js. You'll see this is available in out/goog/base.js. Let's add this to your page now:

<html>
    <body>
        <script type="text/javascript" src="out/goog/base.js"></script>
        <script type="text/javascript" src="out/main.js"></script>
    </body>
</html>

Refresh the page.

The error will be gone but you still won't see the desired "Hello world!".

Hrm. out/main.js didn't appear to have any of the logic that we wrote, in fact it only includes the needed dependency graph information for the ClojureScript standard library cljs.core and our namespace.

Ah. The last step we missed was actually requiring our namespace to kick things off. Change index.html to the following.

<html>
    <body>
        <script type="text/javascript" src="out/goog/base.js"></script>
        <script type="text/javascript" src="out/main.js"></script>
        <script type="text/javascript">
            goog.require("hello_world.core");
            // Note the underscore "_"! 
        </script>
    </body>
</html>

Refresh your index.html and you should finally see "Hello world!" printing to the browser JavaScript console. If you're using a sufficiently modern browser you should even see the printing was invoked from a ClojureScript source file and not a JavaScript one thanks to source mapping (some browsers like Chrome require you to first enable source mapping, for more details look here).

Less Boilerplate

The previous section explained some important fundamental concepts around the Google Closure Library. However it also involved a substantial amount of boilerplate. We can eliminate this boilerplate by specifying a :main entry point in the options that we pass to cljs.build.api/build. Let's do that now:

(require 'cljs.build.api)

(cljs.build.api/build "src"
  {:main 'hello-world.core
   :output-to "out/main.js"})

Change your HTML to the following:

<html>
    <body>
        <script type="text/javascript" src="out/main.js"></script>
    </body>
</html>

Rebuild on Mac or GNU/Linux:

java -cp cljs.jar:src clojure.main build.clj

On Windows:

java -cp "cljs.jar;src" clojure.main build.clj

Refresh the page and you should still see "Hello world!" printed to the JavaScript console. If you examine out/main.js you'll see that it writes out the boilerplate script tags for you. The previous contents of main.js are now in out/cljs_deps.js, which is loaded alongside our namespace by the new out/main.js.

Auto-building

The ClojureScript compiler supports incremental compilation. It's convenient to have the ClojureScript compiler watch a directory and recompile as needed. Let's make a new helper script watch.clj:

(require 'cljs.build.api)

(cljs.build.api/watch "src"
  {:main 'hello-world.core
   :output-to "out/main.js"})

Let's start auto building:

java -cp cljs.jar:src clojure.main watch.clj

You should see output like the following:

Building ...
Reading analysis cache for jar:file:/.../cljs.jar!/cljs/core.cljs
Analyzing src/hello_world/core.cljs
... done. Elapsed 1.425505401 seconds

Edit src/hello_world/core.cljs. You should see recompilation output.

Terminate auto building (using Ctrl-C) before proceeding to the next section.

Further Reading: While not required for the remainder of the Quick Start, it's highly recommended that you familiarize yourself with basics of Google Closure Library. Many simple errors can be avoided by reinforcing your understanding of how Closure Library works.

Browser REPL

It's hard to imagine a productive Lisp experience without a REPL (Read-Eval-Print-Loop). ClojureScript ships with builtin REPL support for Node.js, Rhino, Nashorn, and browsers.

Let's hook up a browser REPL to our project.

First it is recommended that you install rlwrap. Under OS X the easiest way is to use brew and brew install rlwrap.

Let's create a REPL script repl.clj:

(require 'cljs.repl)
(require 'cljs.build.api)
(require 'cljs.repl.browser)

(cljs.build.api/build "src"
  {:main 'hello-world.core
   :output-to "out/main.js"
   :verbose true})

(cljs.repl/repl (cljs.repl.browser/repl-env)
  :watch "src"
  :output-dir "out")

We build the project at least once before constructing the REPL.

REPLs are always constructed in the same way. The first argument to cljs.repl/repl is the REPL evaluation environment (Node.js, Rhino, Nashorn, browser), the subsequent arguments are the same arguments you pass to cljs.build.api/build in addition to several options that are specific to REPLs. Note that we supply a :watch option with a source directory. This conveniently starts a REPL along with an auto building process. The auto building process will write its activity to out/watch.log so you can easily tail -f out/watch.log. We also specify :output-dir so that the REPL can reuse compiled files generated by the build.

We also need to modify our source to load the browser REPL:

(ns hello-world.core
  (:require [clojure.browser.repl :as repl]))

(defonce conn
  (repl/connect "http://localhost:9000/repl")) 

(enable-console-print!)

(println "Hello world!")

We create the connection with defonce. This ensures the connection is constructed only one time - we may reload this namespace during development and we don't want multiple connection instances.

Let's try it:

rlwrap java -cp cljs.jar:src clojure.main repl.clj

The first time will be somewhat slow as the REPL communication script needs to build. You will also see innocuous WARNINGs from the Google Closure Compiler that can be ignored. You should eventually see the following message:

Waiting for browser to connect ...

Point your web browser at http://localhost:9000.

You should get a REPL. (Note that the REPL will appear in your terminal, not in the browser.)

Try evaluating a simple expression like (+ 1 2).

Debug Note: If the REPL doesn't connect immediately try refreshing the browser a few times (Chrome & Firefox tend to be more stable than Safari). Note that eval will be slow in some browsers if you don't have the browser REPL tab focused. If for some reason the REPL completely hangs, just refresh the page.

Run tail -f out/watch.log in a fresh terminal to view auto build progress.

Try evaluating some expressions like (first [1 2 3]), or (doc first), (source first).

Change your src/hello_world/core.cljs source file to look like the following:

(ns hello-world.core
  (:require [clojure.browser.repl :as repl]))

(defonce conn
  (repl/connect "http://localhost:9000/repl"))

(enable-console-print!)

(println "Hello world!")

;; ADDED
(defn foo [a b]
  (+ a b))

At the REPL prompt, require your namespace by evaluating (require '[hello-world.core :as hello]). Try evaluating (hello/foo 2 3), you should get the result 5.

Change your source file so that foo uses * instead of +:

(ns hello-world.core
  (:require [clojure.browser.repl :as repl]))

(defonce conn
  (repl/connect "http://localhost:9000/repl"))

(enable-console-print!)

(println "Hello world!")

(defn foo [a b]
  (* a b)) ;; CHANGED

We can get this new definition in our REPL by appending a :reload keyword to our require statement thereby forcing a reload. Evaluate (require '[hello-world.core :as hello] :reload) and try (hello/foo 2 3) you should get 6 this time.

Lets make a mistake. Try evaluating (ffirst [1]). You should get a source mapped stack trace pointing at ClojureScript source locations not JavaScript ones. This makes debugging a lot nicer.

Production Builds

You may have noticed that out contains a lot of JavaScript. Fortunately the ClojureScript compiler generates output optimized for the Google Closure Compiler. The Google Closure Compiler performs many optimizations, but the most significant for browser-based clients are minification and dead code elimination.

Let's make a new helper build script release.clj, it should look like the following:

(require 'cljs.build.api)

(cljs.build.api/build "src"
  {:output-to "out/main.js"
   :optimizations :advanced})

(System/exit 0)

Under :advanced optimizations :main is not needed as advanced compilation creates a single JavaScript artifact. We also add a (System/exit 0) as the Google Closure Compiler creates a thread pool that isn't shutdown; since we know that we're done we can just exit.

Let's remove the dev time REPL bits from src/hello_world/core.cljs:

(ns hello-world.core)

(enable-console-print!)

(println "Hello world!")

Let's create a release build:

java -cp cljs.jar:src clojure.main release.clj

This process will take significantly longer which is why we don't use this compilation mode for development.

Open index.html, you should still see "Hello world!" printed.

Examine out/main.js, the file size should be around 80K. If you zip this file you'll see that it's around 19K. This is significantly smaller than a jQuery dependency yet when using ClojureScript you have implicit dependencies on the entire ClojureScript standard library (10KLOC) and the Google Closure Library (300KLOC). You can thank dead code elimination.

Running ClojureScript on Node.js

First install Node.js. For instructions on installing Node.js, see the Node.js wiki. Only the current stable versions of Node.js (>= 0.12.X) are supported at this time. Your src/hello_world/core.cljs should look like the following:

(ns hello-world.core
  (:require [cljs.nodejs :as nodejs]))

(nodejs/enable-util-print!)

(defn -main [& args]
  (println "Hello world!"))

(set! *main-cli-fn* -main)

Make a build helper file called node.clj:

(require 'cljs.build.api)

(cljs.build.api/build "src"
  {:main 'hello-world.core
   :output-to "main.js"
   :target :nodejs})

The only differences are that we had to specify a :nodejs target and we do not output main.js to the out directory. This is important due to the way that Node.js resolves JavaScript source files.

Node.js has great source mapping support, in order to enable it just install source-map-support:

npm install source-map-support

Let's build your Node project:

java -cp cljs.jar:src clojure.main node.clj

You can run your file with:

node main.js

Note: Under Node.js there is little reason to use advanced optimizations. While advanced optimizations does apply performance related optimizations, these are now largely obviated by optimizations present in modern JavaScript virtual machines like V8, SpiderMonkey, and JavaScriptCore. For Node.js, :simple or :none optimizations suffice and using them removes the need for extra steps like supplying an externs file.

Node.js REPL

Running a Node.js REPL is much simpler than running a browser REPL. Create a helper build file called node_repl.clj that looks like the following:

(require 'cljs.repl)
(require 'cljs.build.api)
(require 'cljs.repl.node)

(cljs.build.api/build "src"
  {:main 'hello-world.core
   :output-to "out/main.js"
   :verbose true})

(cljs.repl/repl (cljs.repl.node/repl-env)
  :watch "src"
  :output-dir "out")

There's no need to add any REPL specific bits to src/hello_world/core.cljs, make sure it looks as described in the previous section.

Let's start the REPL:

rlwrap java -cp cljs.jar:src clojure.main node_repl.clj

All the previously described REPL interactions for the browser should work.

Dependencies

ClojureScript supports a wide variety of options for including ClojureScript and JavaScript dependencies (see Dependencies for details). However the simplest approach is to include a properly packaged JAR on the classpath. CLJSJS provides a nice set of curated JavaScript libraries that suffices to demonstrate how dependencies are handled.

React is a popular dependency for ClojureScript projects. CLJSJS provides a bundled version. Let's see how to include it.

Grab the JAR from Clojars:

curl -O https://clojars.org/repo/cljsjs/react/0.12.2-8/react-0.12.2-8.jar

Let's edit our simple program to look like the following so that React is properly required:

(ns hello-world.core
  (:require cljsjs.react))

(enable-console-print!)

(println "Hello React!")

Let's rebuild our project, all we need to do is extend our classpath to include the CLJSJS React JAR.

java -cp cljs.jar:src:react-0.12.2-8.jar clojure.main build.clj

If you refresh your index.html page you should see the usual React log indicating that React was successfully loaded.

If you have a few dependencies, one convention is to put them into a folder called lib. Then you can launch your scripts like so:

java -cp 'cljs.jar:lib/*:src' clojure.main build.clj

As your dependency graph becomes more sophisticated it may make sense to rely on Maven or Leiningen to manage dependencies for you. Please refer to Dependencies for a comprehensive tutorial. What follows is just the basics.

Leiningen

All the commands above may be executed with the run feature of Leiningen. With Leiningen you do not need to specify the classpath. For example for a REPL you would do something like the following from your project directory:

lein run -m clojure.main repl.clj

One important thing to remember with Leiningen is the :source-paths configuration parameter. The :source-paths, found in the project.clj, are appended to the classpath. For a ClojureScript build you should include all the source paths to your ClojureScript sources in the :source-paths parameter. The path "src" is included by default.

This is especially important if you use another path besides "src".

If you have any questions about the classpath you can see it easily by typing lein classpath.

Maven

Same as Leiningen, you can use Maven to run the above scripts and you do not need to specify the classpath. Add the clojure-maven-plugin to your pom.xml:

<project xmlns="..."
         xsi:schemaLocation="....">
    <modelVersion>4.0.0</modelVersion>
    ...
    <build>
        <plugins>
            <plugin>
                <groupId>com.theoryinpractise</groupId>
                <artifactId>clojure-maven-plugin</artifactId>
                <version>1.7.1</version>
                <extensions>true</extensions>
            </plugin>
        </plugins>
    </build>
    ...
</project>

Then you can start a repl with the clojure:run task:

mvn clojure:run -Dclojure.script=repl.clj

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