ClojureScript Warnings as Errors

We recently ran into a problem at Nextjournal when we deployed a bad build to production. It turned out after renaming a var, we overlooked one instance. However, our CI didn't catch that.

Let's see how the ClojureScript compiler behaves with a source file using an undefined namespace.

{:deps

 {org.clojure/clojure {:mvn/version "1.10.1"}

  org.clojure/clojurescript {:mvn/version "1.10.597"}}}

(ns foo.bar)

(bam/doesnt-exist)

Build it with the default compiler options.

(require 'cljs.build.api

         'cljs.analyzer.api)

(cljs.build.api/build

 "src"

 {:output-to "out/main.js"})

As we see, it produces warnings, but the build is successful anyway. This is not desirable, we want the build to fail.

To make it fail, we have to build it with a custom warning handler to throw an error in addition to logging them, but clean the previous build first.

rm -rf out

(require 'cljs.build.api

         'cljs.analyzer.api)

(cljs.build.api/build

 "src"

 {:output-to "out/main.js"

  :warning-handlers

  [cljs.analyzer.api/default-warning-handler

    (fn [warning-type env extra]

     (when (warning-type cljs.analyzer/*cljs-warnings*)

      (throw

       (cljs.analyzer/error

        env

        (cljs.analyzer/error-message warning-type extra)))))]})

💥 Much better.

Also interesting: shadow-cljs has a dedicated compiler option for this.