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Requiring Node.js modules from ClojureScript namespaces

Node.js module support has been greatly enhanced in the upcoming release of the ClojureScript compiler. This post explains how to seamlessly require Node.js packages from any ClojureScript namespace. Read on!

Update: This has been released as part of ClojureScript version 1.9.518. ClojureScript 1.9.854 ships with even better support for this feature.

What’s new

The ClojureScript compiler added basic support for Node.js module resolution in version 1.9.456. However, it didn’t allow requiring those modules from ClojureScript namespaces, relying instead on shim JavaScript sources that would import them. The next version of the compiler fixes that problem by including significant enhancements around this behavior, effectively making it possible to seamlessly require Node.js modules as if they were regular ClojureScript namespaces.

How we made it work

To make all this possible, a new compiler option has been introduced. When compiling your projects, the ClojureScript compiler will now read the :npm-deps option and take care of installing the specified dependencies for you. This option takes a map of package name to version. It goes without saying that you’ll need to have both Node.js and NPM installed for dependencies to be installed.

What’s better, there are no changes necessary to downstream tooling. The NPM package source files are computed and effectively become foreign libraries, which have long been supported.


Let’s look at a specific example: say we want to use the immensely popular left-pad library in a ClojureScript project. Given the following directory structure:

├─ src
│  ├─ example
│  │   └─ core.cljs

and our src/example/core.cljs file,

;; src/example/core.cljs
(ns example.core
  (:require left-pad))


;; pad the number 42 with five zeros
(println (left-pad 42 5 0))

the following script would successfully compile this project:

;; build.clj
(require '[ :as b])

(b/build "src"
  {:optimizations :none
   :main 'example.core
   :npm-deps {:left-pad "1.1.3"} ;; NEW
   :install-deps true ;; NEW
   :output-to "main.js"})

It’s interesting to note how left-pad is both a namespace and a function. This is due to it being the only export of the left-pad CommonJS module. Support for this resolution is also part of a recent development in the ClojureScript compiler.

If a module, e.g. the widely used react package, exports an object, we would be able to refer to functions in that object as if they were Vars in a Clojure(Script) namespace. Here’s an example:

(ns example.core
  (:require react))

;; react/DOM.div is equivalent to (react/createElement "div"), and that is
;; made clear in the h1 element.
(def title
  (react/DOM.div nil
    (react/createElement "h1" nil "Page title")))

But there’s more

Packaged ClojureScript libraries benefit too

ClojureScript libraries that package foreign dependencies can also benefit from these enhancements. Ticket CLJS-1973 adds support for the :npm-deps option in deps.cljs files, allowing library authors to develop and distribute libraries that directly depend on Node.js modules.

This does not obviate the need for externs

Even though the Google Closure Compiler can now consume Node.js modules, externs are still very much necessary. This is a consequence of the fact that the Google Closure Compiler doesn’t support much of the dynamic programming employed in writing some, if not most Node.js packages.

Fortunately, the ClojureScript compiler has recently introduced externs inference functionality, which makes it much easier to generate externs from JavaScript interop. Additionally, ClojureScript will agressively index every externs file in the classpath, so you can still add CLJSJS packages to your project and benefit from their externs, even though you don’t require the namespaces they export.

Node.js module consumption is not only for Node.js apps

Consuming Node.js modules from NPM doesn’t solely benefit ClojureScript projects that target Node.js. NPM is currently also the de facto way to consume JavaScript packages that target the browser. This means that ClojureScript browser-based apps can also take advantage of this functionality.

Dead-code elimination on Node.js modules

To me, the greatest benefit of the new module support is dead-code elimination on these (not so) foreign libraries. Previously, foreign libraries included in a ClojureScript project would just get appended after Google Closure compilation. Because the Closure Compiler can now consume Node.js modules, we get elimination of unused code for free in our optimized builds!

Final remarks

It has been really satisfying to work on enhancing the Node.js module support in the ClojureScript compiler. My hope is that these developments go a long way closing the gap between ClojureScript and JavaScript libraries published to NPM. More importantly, I believe enhanced Node.js module support will make it much easier to maintain codebases that share both ClojureScript and JavaScript code, as well as make ClojureScript more appealing to JavaScript developers that rely on NPM published packages every day.

Please note that Node.js module consumption is currently in alpha status. All feedback is appreciated, and if you find an issue please report in the ClojureScript JIRA.

Tweet @_anmonteiro with any questions or suggestions. Thanks for reading!

Thanks to Shaun Mahood for reading a draft of this post.