An Introduction to ClojureScript

Share this article

This article was peer reviewed by Thomas Greco and Jérémy Heleine. Thanks to all of SitePoint’s peer reviewers for making SitePoint content the best it can be!

Since a few months, more and more developers are adhering to the philosophy of “always bet on JavaScript”. Nonetheless, the number of languages that compile to JavaScript is growing. Some examples of such languages are Dart, TypeScript, CoffeeScript, and ClojureScript.

In this article we’ll be discussing ClojureScript, a new compiler for Clojure that targets JavaScript. We’ll be looking at the benefits of working with ClojureScript and how you can quickly set it up to use npm and your favorite Node.js libraries.

Why ClojureScript?

There are many articles online explaining the benefits of ClojureScript. Some aggregated high level points are:

  • Simplicity: In regard of the syntax, ClojureScript is a Lisp-based language that gives it a minimal syntax. It’s so minimal in fact that we’ll be able to cover it in this article. In addition to the simple syntax, ClojureScript also offers tools which help simplify asynchronous code.
  • Safety: This means less bugs! ClojureScript and other functional programming languages have many properties that help reduce and mitigate common bugs.
  • Performance: ClojureScript uses Google’s Closure Compiler. This allows ClojureScript to utilize dead code elimination and other features.
  • Live Coding: The ClojureScript ecosystem provides many tools to do “live coding”. This means that once the code is changed, it’s instantly reflected in your live project. In this article, we’ll be looking at Figwheel so that you can better understand the concept.
  • Code Reuse: ClojureScript can be run universally or, as many say, “isomorphically.” This means you can run the same code on your client and your server. This has become a popular pattern in the Node.js ecosystem. In addition, ClojureScript can import libraries from Node.js and Java ecosystems.

Setting up the Clojure(Script) Tool Chain

In this article, we’ll be installing the tool chain on a Mac OSX Environment. The ClojureScript wiki has guides for installing on other environments in case you need them. We’ll need a few system dependencies to get started. One of these is Homebrew, the popular OSX package manager.

Installing the Latest Java Version

ClojureScript requires the latest Java version (version 8 at the time of this writing). If anytime during these exercises you encounter an error running lein that looks like:

Exception in thread "main" java.util.regex.PatternSyntaxException: 
    Unknown inline modifier near index 2 (?U)^[\p{Alpha}_$]^, compiling:(cljs/util.clj:158:33)

Then you need the latest version of Java.

To start, execute the following commands on the command-line interface:

brew tap caskroom/cask
brew install brew-cask

If you get the error “already installed”, follow the instructions to unlink it that will appear on your script. Once done, install it again. This can be done with the following commands:

brew unlink brew-cask
brew install brew-cask

At this point, execute the last command we need:

brew cask install java

Installing Leiningen

Leiningen is a build tool for Clojure projects. We’ll use it to run ClojureScript code and install dependencies. This step assumes that Homebrew is installed, giving us the brew command.

brew install leiningen

If this step fails, a manual installation may be required.

Using the Repl

Now that we have Leningen installed, we can start getting familiar with ClojureScript syntax.

Executing the command lein repl, you should get a similar output:

$ lein repl
nREPL server started on port 58371 on host 127.0.0.1 - nrepl://127.0.0.1:58371
REPL-y 0.3.7, nREPL 0.2.10
Clojure 1.7.0
Java HotSpot(TM) 64-Bit Server VM 1.6.0_65-b14-466.1-11M4716
    Docs: (doc function-name-here)
          (find-doc "part-of-name-here")
  Source: (source function-name-here)
 Javadoc: (javadoc java-object-or-class-here)
    Exit: Control+D or (exit) or (quit)
 Results: Stored in vars *1, *2, *3, an exception in *e

user=>

We’re now in a ClojureScript Repl. This allows us to quickly execute ClojureScript and view the result. To exit the repl you can press Control+D.

With this step performed, we’re now ready to delve into the syntax of ClojureScript and have some fun!

ClojureScript Syntax

ClojureScript is a functional language, which means it has functions and limited additional language constructs. In the following sections, I’m going to cover the language’s features.

Primitives

ClojureScript features the following primitive types:

  • Number
    user=> 1.23
        1.23
  • String
    user=> "foo"
        "foo"
  • Vector (array)
    user=> [:bar 3.14 "hello"]
        [:bar 3.14 "hello"]
  • Map (associative arrays)
    user=> {:msg "hello" :pi 3.14 :primes [2 3 5 7 11 13]}
        {:msg "hello", :pi 3.14, :primes [2 3 5 7 11 13]}
  • Keyword (used to access Maps)
    user=> :foo
        :foo
  • Set (distinct array)
    user=> #{:bar 3.14 "hello"}
        #{"hello" 3.14 :bar}

Functions All the Way Down

Functions are the building blocks of ClojureScript. You even define your own functions using the built-in defn function.

Below you can see an example of a function definition. Here, we’ll define a function named myfunction. It takes one argument argument1 and returns it. It is not a very useful function but it is a good example of syntax.

user=> (defn myfunction [argument1] argment1)

In case the syntax looks a bit weird to you, this is its equivalent in Javascript:

function myfunction(argument1){
    return argument1;
}

Functions are invoked by wrapping their name and arguments with parentheses:

user=> (myfunction "hello world")
"hello world"

In non-functional programming languages there are special “operators” or keywords. In Javascript, some commonly used operators are + - == if. In ClojureScript and other Lisp-based languages, there are no special operators. These are just regular functions.

If statements are functions:

user=> (if true "do true stuff here" "do false stuff here")
"do true stuff here"

Math operators are functions too as shown below:

user=> (+ 2 3)
5
user=> (* 2 3)
6

For more great examples of Javascript to ClojureScript synonyms you can check this website.

Creating a Node.js – ClojureScript Project

Starting a ClojureScript project is simple. Leningen offers project templates that will get you up and running with a boilerplate project.

Templates are a great resource to play around with and see other uses and configurations for ClojureScript projects. Clojars.org has a collection of templates and others can be found searching the Web. For our project we’ll be using a Nodejs Figwheel project template.

To start, execute the following command on the command-line interface:

$ lein new figwheel-node hello-world

This creates a new ClojureScript project in the directory ./hello-world. The remainder of this article assumes hello-world was used as the project name. If you want, you can use a different name but I’d suggest you to stick with it so that you can follow the article without the fear that something will go wrong.

With that said, move to the created directory and install the npm dependencies:

$ cd hello-world
$ npm install

Points of Interest

The project folder contains a few files. In this section I want to highlight some important concepts about them:

  • package.json: This should be familiar from Node.js projects. Our npm dependencies will be added here.
  • project.clj: This file is the ClojureScript project configuration file. This is ClojureScript’s version of package.json, where we configure Clojure dependencies and compilation targets. This file also contains project details like title and description.
  • figwheel.js: This file is specific to Figweel projects. It is the bootstrap file for our project. It points Figwheel to our source code so that it can monitor it for updates. We’ll be running it with node figwheel.js.
  • ./src/hello-world/core.cljs: This is our entry-point source file. This is where we’ll start the project. Think of it similar to an index.js file in a Node.js project.

The core.cljs file contains the following content. I’ve added comments to it so that you can understand what’s going on:

;; This defines a namespace and necesscary dependencies for the current file
(ns hello-world.core
  (:require [cljs.nodejs :as nodejs]))

;; This updates the default println method to write to Node.js stdout
(nodejs/enable-util-print!)

;; The main function of the module
;; it prints "Hello World!" to stdout
(defn -main []
  (println "Hello world!"))

;; *main-cli-fn* is a semi-magic var that's used to set the entry
;; *point for a node app
(set! *main-cli-fn* -main)

Running the Project

To execute the current project, open a terminal window and move to our hello-world project directory. Then, execute the following:

lein figwheel

This will start Figwheel waiting for updates to build. Leave this terminal running and open a separate terminal. In this new terminal move again to the project directory and execute the command:

node figwheel.js

You should see the output “Hello world” as reported below:

$ node figwheel.js
Hello world!
Figwheel: trying to open cljs reload socket
Figwheel: socket connection established

Using Express.js for a Webserver

Now that we have the base of a ClojureScript project setup, let’s start working with some familiar libraries in a new terminal. In our hello_world directory execute the command:

npm install --save express

Then we need to update ./src/hello-world/core.cljs as follows:

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

(nodejs/enable-util-print!)

(defonce express (nodejs/require "express"))
(defonce http (nodejs/require "http"))
(defonce server-port 3000)

(def app (express))

(. app (get "/hello"
      (fn [req res] (. res (send "Hello world")))))

(def -main
  (fn []
    (doto (.createServer http #(app %1 %2))
      (.listen server-port))))
      (.listen server))))
    (println (string/join " " ["Server running on" server-port]) )
    

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

Now when you run node figwheel.js on the project, you should see an output saying running on 3000. If you visit the URL http://localhost:3000/hello, you should see the outcome of the express route saying “Hello world.”

Conclusions

In this article we’ve discussed how to setup a new ClojureScript project and install a popular Node dependency in it. This gives us a great base to get more familiar with ClojureScript as a language. I’ve also put together the source code for this project that you can find on Github. It goes a bit beyond this article and demonstrates how to integrate React server side rendering.

Kev ZettlerKev Zettler
View Author

Freelance technologist and founder of RedTrenchMediaCorp, a Y Combinator alumni. Serial entrepreneur. Indie game developer. Punk rocker. Website at: kevzettler.com

AurelioDClojureScriptjavascriptnodenode.jsnodejs
Share this article
Read Next
Retrieval-augmented Generation: Revolution or Overpromise?
Retrieval-augmented Generation: Revolution or Overpromise?
Kateryna ReshetiloOlexandr Moklyak
How to Deploy Apache Airflow on Vultr Using Anaconda
How to Deploy Apache Airflow on Vultr Using Anaconda
Vultr
Cloud Native: How Ampere Is Improving Nightly Arm64 Builds
Cloud Native: How Ampere Is Improving Nightly Arm64 Builds
Dave NearyAaron Williams
How to Create Content in WordPress with AI
How to Create Content in WordPress with AI
Çağdaş Dağ
A Beginner’s Guide to Setting Up a Project in Laravel
A Beginner’s Guide to Setting Up a Project in Laravel
Claudio Ribeiro
Enhancing DevSecOps Workflows with Generative AI: A Comprehensive Guide
Enhancing DevSecOps Workflows with Generative AI: A Comprehensive Guide
Gitlab
Creating Fluid Typography with the CSS clamp() Function
Creating Fluid Typography with the CSS clamp() Function
Daine Mawer
Comparing Full Stack and Headless CMS Platforms
Comparing Full Stack and Headless CMS Platforms
Vultr
7 Easy Ways to Make a Magento 2 Website Faster
7 Easy Ways to Make a Magento 2 Website Faster
Konstantin Gerasimov
Powerful React Form Builders to Consider in 2024
Powerful React Form Builders to Consider in 2024
Femi Akinyemi
Quick Tip: How to Animate Text Gradients and Patterns in CSS
Quick Tip: How to Animate Text Gradients and Patterns in CSS
Ralph Mason
Sending Email Using Node.js
Sending Email Using Node.js
Craig Buckler
Creating a Navbar in React
Creating a Navbar in React
Vidura Senevirathne
A Complete Guide to CSS Logical Properties, with Cheat Sheet
A Complete Guide to CSS Logical Properties, with Cheat Sheet
Ralph Mason
Using JSON Web Tokens with Node.js
Using JSON Web Tokens with Node.js
Lakindu Hewawasam
How to Build a Simple Web Server with Node.js
How to Build a Simple Web Server with Node.js
Chameera Dulanga
Building a Digital Fortress: How to Strengthen DNS Against DDoS Attacks?
Building a Digital Fortress: How to Strengthen DNS Against DDoS Attacks?
Beloslava Petrova
Crafting Interactive Scatter Plots with Plotly
Crafting Interactive Scatter Plots with Plotly
Binara Prabhanga
GenAI: How to Reduce Cost with Prompt Compression Techniques
GenAI: How to Reduce Cost with Prompt Compression Techniques
Suvoraj Biswas
How to Use jQuery’s ajax() Function for Asynchronous HTTP Requests
How to Use jQuery’s ajax() Function for Asynchronous HTTP Requests
Aurelio De RosaMaria Antonietta Perna
Quick Tip: How to Align Column Rows with CSS Subgrid
Quick Tip: How to Align Column Rows with CSS Subgrid
Ralph Mason
15 Top Web Design Tools & Resources To Try in 2024
15 Top Web Design Tools & Resources To Try in 2024
SitePoint Sponsors
7 Simple Rules for Better Data Visualization
7 Simple Rules for Better Data Visualization
Mariia Merkulova
Cloudways Autonomous: Fully-Managed Scalable WordPress Hosting
Cloudways Autonomous: Fully-Managed Scalable WordPress Hosting
SitePoint Team
Best Programming Language for AI
Best Programming Language for AI
Lucero del Alba
Quick Tip: How to Add Gradient Effects and Patterns to Text
Quick Tip: How to Add Gradient Effects and Patterns to Text
Ralph Mason
Logging Made Easy: A Beginner’s Guide to Winston in Node.js
Logging Made Easy: A Beginner’s Guide to Winston in Node.js
Vultr
How to Optimize Website Content for Featured Snippets
How to Optimize Website Content for Featured Snippets
Dipen Visavadiya
Psychology and UX: Decoding the Science Behind User Clicks
Psychology and UX: Decoding the Science Behind User Clicks
Tanya Kumari
Build a Full-stack App with Node.js and htmx
Build a Full-stack App with Node.js and htmx
James Hibbard
Digital Transformation with AI: The Benefits and Challenges
Digital Transformation with AI: The Benefits and Challenges
Priyanka Prajapat
Quick Tip: Creating a Date Picker in React
Quick Tip: Creating a Date Picker in React
Dianne Pena
How to Create Interactive Animations Using React Spring
How to Create Interactive Animations Using React Spring
Yemi Ojedapo
10 Reasons to Love Google Docs
10 Reasons to Love Google Docs
Joshua KrausZain Zaidi
How to Use Magento 2 for International Ecommerce Success
How to Use Magento 2 for International Ecommerce Success
Mitul Patel
5 Exciting New JavaScript Features in 2024
5 Exciting New JavaScript Features in 2024
Olivia GibsonDarren Jones
Tools and Strategies for Efficient Web Project Management
Tools and Strategies for Efficient Web Project Management
Juliet Ofoegbu
Choosing the Best WordPress CRM Plugin for Your Business
Choosing the Best WordPress CRM Plugin for Your Business
Neve Wilkinson
ChatGPT Plugins for Marketing Success
ChatGPT Plugins for Marketing Success
Neil Jordan
Managing Static Files in Django: A Comprehensive Guide
Managing Static Files in Django: A Comprehensive Guide
Kabaki Antony
The Ultimate Guide to Choosing the Best React Website Builder
The Ultimate Guide to Choosing the Best React Website Builder
Dianne Pena
Exploring the Creative Power of CSS Filters and Blending
Exploring the Creative Power of CSS Filters and Blending
Joan Ayebola
How to Use WebSockets in Node.js to Create Real-time Apps
How to Use WebSockets in Node.js to Create Real-time Apps
Craig Buckler
Best Node.js Framework Choices for Modern App Development
Best Node.js Framework Choices for Modern App Development
Dianne Pena
SaaS Boilerplates: What They Are, And 10 of the Best
SaaS Boilerplates: What They Are, And 10 of the Best
Zain Zaidi
Understanding Cookies and Sessions in React
Understanding Cookies and Sessions in React
Blessing Ene Anyebe
Enhanced Internationalization (i18n) in Next.js 14
Enhanced Internationalization (i18n) in Next.js 14
Emmanuel Onyeyaforo
Essential React Native Performance Tips and Tricks
Essential React Native Performance Tips and Tricks
Shaik Mukthahar
How to Use Server-sent Events in Node.js
How to Use Server-sent Events in Node.js
Craig Buckler
Five Simple Ways to Boost a WooCommerce Site’s Performance
Five Simple Ways to Boost a WooCommerce Site’s Performance
Palash Ghosh
Elevate Your Online Store with Top WooCommerce Plugins
Elevate Your Online Store with Top WooCommerce Plugins
Dianne Pena
Unleash Your Website’s Potential: Top 5 SEO Tools of 2024
Unleash Your Website’s Potential: Top 5 SEO Tools of 2024
Dianne Pena
How to Build a Chat Interface using Gradio & Vultr Cloud GPU
How to Build a Chat Interface using Gradio & Vultr Cloud GPU
Vultr
Enhance Your React Apps with ShadCn Utilities and Components
Enhance Your React Apps with ShadCn Utilities and Components
David Jaja
10 Best Create React App Alternatives for Different Use Cases
10 Best Create React App Alternatives for Different Use Cases
Zain Zaidi
Control Lazy Load, Infinite Scroll and Animations in React
Control Lazy Load, Infinite Scroll and Animations in React
Blessing Ene Anyebe
Building a Research Assistant Tool with AI and JavaScript
Building a Research Assistant Tool with AI and JavaScript
Mahmud Adeleye
Understanding React useEffect
Understanding React useEffect
Dianne Pena
Web Design Trends to Watch in 2024
Web Design Trends to Watch in 2024
Juliet Ofoegbu
Building a 3D Card Flip Animation with CSS Houdini
Building a 3D Card Flip Animation with CSS Houdini
Fred Zugs
How to Use ChatGPT in an Unavailable Country
How to Use ChatGPT in an Unavailable Country
Dianne Pena
An Introduction to Node.js Multithreading
An Introduction to Node.js Multithreading
Craig Buckler
How to Boost WordPress Security and Protect Your SEO Ranking
How to Boost WordPress Security and Protect Your SEO Ranking
Jaya Iyer
Understanding How ChatGPT Maintains Context
Understanding How ChatGPT Maintains Context
Dianne Pena
Building Interactive Data Visualizations with D3.js and React
Building Interactive Data Visualizations with D3.js and React
Oluwabusayo Jacobs
JavaScript vs Python: Which One Should You Learn First?
JavaScript vs Python: Which One Should You Learn First?
Olivia GibsonDarren Jones
13 Best Books, Courses and Communities for Learning React
13 Best Books, Courses and Communities for Learning React
Zain Zaidi
5 jQuery.each() Function Examples
5 jQuery.each() Function Examples
Florian RapplJames Hibbard
Implementing User Authentication in React Apps with Appwrite
Implementing User Authentication in React Apps with Appwrite
Yemi Ojedapo
AI-Powered Search Engine With Milvus Vector Database on Vultr
AI-Powered Search Engine With Milvus Vector Database on Vultr
Vultr
Understanding Signals in Django
Understanding Signals in Django
Kabaki Antony
Why React Icons May Be the Only Icon Library You Need
Why React Icons May Be the Only Icon Library You Need
Zain Zaidi
View Transitions in Astro
View Transitions in Astro
Tamas Piros
Getting Started with Content Collections in Astro
Getting Started with Content Collections in Astro
Tamas Piros
What Does the Java Virtual Machine Do All Day?
What Does the Java Virtual Machine Do All Day?
Peter Kessler
Become a Freelance Web Developer on Fiverr: Ultimate Guide
Become a Freelance Web Developer on Fiverr: Ultimate Guide
Mayank Singh
Layouts in Astro
Layouts in Astro
Tamas Piros
.NET 8: Blazor Render Modes Explained
.NET 8: Blazor Render Modes Explained
Peter De Tender
Mastering Node CSV
Mastering Node CSV
Dianne Pena
A Beginner’s Guide to SvelteKit
A Beginner’s Guide to SvelteKit
Erik KückelheimSimon Holthausen
Brighten Up Your Astro Site with KwesForms and Rive
Brighten Up Your Astro Site with KwesForms and Rive
Paul Scanlon
Which Programming Language Should I Learn First in 2024?
Which Programming Language Should I Learn First in 2024?
Joel Falconer
Managing PHP Versions with Laravel Herd
Managing PHP Versions with Laravel Herd
Dianne Pena
Accelerating the Cloud: The Final Steps
Accelerating the Cloud: The Final Steps
Dave Neary
An Alphebetized List of MIME Types
An Alphebetized List of MIME Types
Dianne Pena
The Best PHP Frameworks for 2024
The Best PHP Frameworks for 2024
Claudio Ribeiro
11 Best WordPress Themes for Developers & Designers in 2024
11 Best WordPress Themes for Developers & Designers in 2024
SitePoint Sponsors
Top 10 Best WordPress AI Plugins of 2024
Top 10 Best WordPress AI Plugins of 2024
Dianne Pena
20+ Tools for Node.js Development in 2024
20+ Tools for Node.js Development in 2024
Dianne Pena
The Best Figma Plugins to Enhance Your Design Workflow in 2024
The Best Figma Plugins to Enhance Your Design Workflow in 2024
Dianne Pena
Harnessing the Power of Zenserp for Advanced Search Engine Parsing
Harnessing the Power of Zenserp for Advanced Search Engine Parsing
Christopher Collins
Build Your Own AI Tools in Python Using the OpenAI API
Build Your Own AI Tools in Python Using the OpenAI API
Zain Zaidi
The Best React Chart Libraries for Data Visualization in 2024
The Best React Chart Libraries for Data Visualization in 2024
Dianne Pena
7 Free AI Logo Generators to Get Started
7 Free AI Logo Generators to Get Started
Zain Zaidi
Turn Your Vue App into an Offline-ready Progressive Web App
Turn Your Vue App into an Offline-ready Progressive Web App
Imran Alam
Clean Architecture: Theming with Tailwind and CSS Variables
Clean Architecture: Theming with Tailwind and CSS Variables
Emmanuel Onyeyaforo
How to Analyze Large Text Datasets with LangChain and Python
How to Analyze Large Text Datasets with LangChain and Python
Matt Nikonorov
6 Techniques for Conditional Rendering in React, with Examples
6 Techniques for Conditional Rendering in React, with Examples
Yemi Ojedapo
Introducing STRICH: Barcode Scanning for Web Apps
Introducing STRICH: Barcode Scanning for Web Apps
Alex Suzuki
Using Nodemon and Watch in Node.js for Live Restarts
Using Nodemon and Watch in Node.js for Live Restarts
Craig Buckler
Get the freshest news and resources for developers, designers and digital creators in your inbox each week
Loading form