Simpler JavaScript Preprocessing

20 July 2017
Juho Teperi

This is the fourth post in the Sneak Preview series.

Closure compiler can process AMD, CommonJS and ES6 modules giving wide support for the Node ecosystem. Even so, there are popular cases which Closure doesn’t support directly, such as React’s JSX. [1] Maria Geller’s Google Summer of Code work also addressed this based on a design outlined by David Nolen and a preprocessing hook has been present since version 1.7.48. However, the original design makes integration with other build tools challenging, and we’ve revised the approach to simplify such integrations.


JavaScript libraries using JSX or other syntax extensions are usually packaged with the already processed code so they can be used without further ceremony. Still, there are practical cases where one might want to include such code in the project directly. For example, when converting a project from JavaScript to ClojureScript it’s far easier to reuse the existing code than attempt to rewrite everything in one go. Also, building sophisticated user interfaces is a team effort and tools like JSX can make that process far more welcoming by giving designers familiar tools. By allowing ClojureScript to reach JSX and other popular syntactical affordances, the ClojureScript development process can be more inclusive.

JavaScript transformation

In the original design, preprocessing was enabled by providing :preprocess to a foreign-lib map. The value is a keyword dispatch for the cljs.closure/js-transforms multimethod. Users can implement a new multimethod case and, for example, use Java’s built-in Nashorn JavaScript engine to run a JavaScript compiler like Babel.

However this approach creates complications for popular Clojure and ClojureScript build tools. The Clojure namespace that provides the preprocess multimethod must be loaded by the user before the ClojureScript compiler is run. While this may be feasible in an explicit build script, due to how both Leiningen and Boot isolate the build to their own classpaths, this requirement simply isn’t practical.

There are a few ways to work around the original design:

  1. Provide a new configuration option to enumerate the namespaces to require before running the compiler.

  2. Create a relation between the multimethod dispatch keyword, and the namespace that provides the implementation. For example, if the keyword is namespaced, the namespace part of the keyword could be used to require that namespace on demand.

Both of these solutions could be implemented at the build tools or in the ClojureScript compiler directly. The first option seems circuitous from an end user perspective, and while the second option highlights the fundamental problem, using keywords for this pattern seems unidiomatic. If we simply switch to symbols from keywords, we can align with existing precedents.

Preprocess symbol

The next version of ClojureScript will support symbols as the :preprocess option value. Using a fully qualified symbol makes it obvious that the value refers to a function, and the namespace part of the symbol can be used to automatically load the namespace on the user’s behalf.

cljsjs/babel-standalone has been updated, and provides an easy way to use Babel with ClojureScript tooling following this new pattern.


Users familiar with Clojure’s philosophy know that we prioritize simplicity above most other qualities. But, simplicity is not always at odds with ease, and in fact, has long been been a language priority with respect to interoperability with the host.

While Clojure has boasted excellent integration with Java for some time, for ClojureScript, the friction between Google Closure and mainstream JavaScript practice have made this promise more challenging to deliver.

We believe we are finally closing the gap and that the vast ecosystem of JavaScript libraries can now finally be close at hand.

1 There is third-party compiler pass for Closure to support JSX