Node.js to Clojure in 60 seconds

For those who have been living under the metaphorical rock, Clojure, a JVM-based Lisp dialect, is an up-and-coming server-side programming language that features immutable data structures and first-order functions, making it ideal for elegant and expressive functional patterns. ClojureScript is just Clojure that compiles to JavaScript; think CoffeeScript or TypeScript (Clojure has its own semantics separate from JavaScript, so not a one-to-one analogy here). ClojureScript (CLJS) has gained some traction in the React community, due to strong synergies between the two technologies and associated philosophies (check out Reagent). CLJS is primarily used for front-end development though, here, we will look at writing a simple Node.js server, using the JS/CLJS interop to interface NPM modules.

Lumo is a neat little Clojure-to-JS compiler that runs on Node.js. Though tools like Lieningen and Boot provide incredible value (like REPL-based interactive programming), they require some legwork to get running. Lumo really shines in this aspect: it’s dirt-cheap upfront, especially if you’re already running Node.js on your system.

With Lumo, getting a ClojureScript environment running is as simple as npm install -g lumo-cljs. Beautiful! We’re already getting to the good stuff. I’m actually gonna steal and modify this bit of code (thanks bhauman, not all heroes wear capes!) to get a basic Express app serving GET /helloon port 3000. Note: you can pretty easily get Lumo to compile for the browser (making full use of Google’s Closure Compiler with optimizations like dead-code removal), and could even go as far as building an isomorphic React app on Lumo, but more on that in another post.

My server.cljs looks like this:

Looks familiar, huh? You’ll need Express installed to run the app, so go ahead and npm install express.

Ok, so next, you’ll need to… oh wait, I think that’s it?

$ lumo -i server.cljs

Now, in another terminal, curl localhost:3000/hello. Easy, right?
Lumo is incredibly fast to start up (unlike lein repl, for example), and in the JVM-dominated Clojure world, this is a very welcome change of pace. Shell scripts and CLIs in Clojure? Why not?

Update: There is a new framework called Macchiato that enables Node.js development in ClojureScript. Check it out!