ClojureScript is not an Island: Integrating Node Modules

12 July 2017
António Nuno Monteiro

This is the second post in the Sneak Preview series.

ClojureScript has had first class JavaScript interop since its initial release in 2011. Similarly to Clojure, embracing the host has always been an explicit goal. While the above is true from a syntax standpoint, integrating with external JavaScript libraries historically came at the cost of some manual work [1] (manually assembled bundles or packaging led by community efforts).

A step towards better interaction with external JavaScript

The 2015 Google Summer of Code project for ClojureScript succeeded in making the interaction with easier. Relying on the Google Closure Compiler's then new ability to understand most widely known JavaScript module formats and convert them to Closure compatible JavaScript [2], Maria Geller successfully integrated those module features into ClojureScript, along with custom preprocessing steps that allow using features such as the popular JavaScript compilation technology Babel from the comfort of your ClojureScript project [3].

Since then, the Closure Compiler team added module resolution support in 2016, which among other things enables consuming modules directly from a node_modules installation instead of having to feed Closure handcrafted module paths for conversion.

Seamless interaction with NPM dependencies

We have built on Maria’s modules work to account for this new Closure feature, and the next release of ClojureScript represents yet another major milestone in making arbitrary JavaScript modules accessible to every ClojureScript project by including substantial improvements to the way that ClojureScript interoperates with the NPM ecosystem.

Requiring NPM modules from ClojureScript namespaces has been possible since ClojureScript version 1.9.518. However, Node.js supports multiple patterns for requiring CommonJS modules, and a common painpoint was people looking to require modules of the form "react-dom/server" from ClojureScript, which would not be a valid symbol for the :require spec.

In this release, we added support for string-based requires in the namespace form to solve the above problem. You can now require these types of modules from the comfort of your ns declaration.

Gluing it all together is the :npm-deps compiler flag. In it, we tell the compiler which dependencies it should be aware of. ClojureScript will take care of installing those dependencies and running them through the Closure Compiler conversion pipeline, including optimizations which we describe in more detail below.

A practical example

Given a build.clj file like the following:

(require '[ :as b])

(b/build "src"
  {:output-dir "out"
   :output-to "out/main.js"
   :optimizations :none
   :main 'example.core
   :npm-deps {:react "15.6.1"
              :react-dom "15.6.1"}})

Your simplest src/example/core.cljs file could look like the snippet below:

(ns example.core
  (:require [react :refer [createElement]]
            ["react-dom/server" :as ReactDOMServer :refer [renderToString]]))

(js/console.log (renderToString (createElement "div" nil "Hello World!")))

Notice we don’t have to declare "react-dom/server" anywhere. We can just require it. ClojureScript is now smart enough to find these CommonJS modules and process them into Google Closure Compiler compatible code.

This is a big deal

The implications of consuming JavaScript modules with Google Closure are huge: the external libraries used in a ClojureScript project are no longer just prepended to the generated bundle, but can now be subjected to all of Closure Compiler’s optimizations, including dead code elimination and, in projects that take advantage of code splitting, cross module code motion. For example, in our tests React is appreciable smaller (~16%) under Closure’s advanced compilation than it would be using existing popular JavaScript tooling [4]. Additionally, if you have a mixed codebase of ClojureScript and JavaScript, not only can you now seamlessly consume those JavaScript portions of your code (including e.g. JSX transformations!), but also share and bundle their vendor dependencies with the ones your ClojureScript part uses.

Works on Node.js too!

It’s worth noting that the module processing feature in ClojureScript is mostly intended to be used in projects that target the browser, where dependencies will normally be bundled together. But that doesn’t mean projects targeting Node.js can’t also take advantage of this feature. In fact, we made it so that you can also seamlessly require Node modules in a local node_modules installation from your namespace declaration when targeting Node.js. ClojureScript will know that you’re requiring a Node module and produce a require declaration, integrating with Node.js’s own facilities for loading JavaScript modules.

Parting thoughts

ClojureScript is, after almost 6 years, a platform relied upon by a great number of developers worldwide, and we want to continue to deliver on full interoperability with the host. By making sure that we integrate with the vast JavaScript ecosystem out there, we think these new features arriving in the next version of ClojureScript are a stepping stone in assuring ClojureScript’s sustainability long term.

We hope you enjoy these new features as much as we do. Thanks for reading!

1 aside from the Google Closure Library which was deeply integrated since ClojureScript’s initial release.
2 the subset of JavaScript that the Closure Compiler needs to perform optimizations such as dead code elimination (tree shaking), variable name rewriting, constant folding and inlining, among others.
3 Maria also kept a regular blog whilst working on that project that you can read here.
4 in fact, the Reagent team is already testing a version of their website to consume NPM modules. They also compared it to the previous version