# 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.

```clojure no-exec id=eeb8f3d6-c97a-4437-b95e-0f5d4f6936ce
{:deps
 {org.clojure/clojure {:mvn/version "1.10.1"}
  org.clojure/clojurescript {:mvn/version "1.10.597"}}}
```

```clojurescript no-exec id=191702c8-d707-4f02-9de6-650f7ed10f1f
(ns foo.bar)

(bam/doesnt-exist)
```

Build it with the default compiler options. 

```clojure id=8324fe8a-885d-4c5e-8862-e98ef8bdf201
(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.

```bash id=b204815a-bd6e-4049-a606-c0dd1a0572d9
rm -rf out
```

```clojure id=387f934a-7cb9-4918-8760-176291f4cbc4
(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](https://shadow-cljs.github.io/docs/UsersGuide.html#warnigs-as-errors). 

<details id="com.nextjournal.article">
<summary>This notebook was exported from <a href="https://nextjournal.com/a/Ly84fW9GtrqQVGNDa8CaJ?change-id=CfSLGQJAYZ1CAmfvy7PGqR">https://nextjournal.com/a/Ly84fW9GtrqQVGNDa8CaJ?change-id=CfSLGQJAYZ1CAmfvy7PGqR</a></summary>

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```
</details>