Fail Storybook build if there are webpack compilation warnings

Create a Webpack plugin which looks for warnings at the end of the build.
Reference doc: https://webpack.js.org/contribute/writing-a-plugin/

packages/storybook/.storybook/main.js

@@ -1,4 +1,5 @@
 const path = require('path');
+const WebpackPluginFailBuildOnWarning = require('./webpack-plugin-fail-build-on-warning');
 
 module.exports = {
   stories: [
@@ -53,6 +54,9 @@ module.exports = {
       ({ constructor }) => constructor.name !== 'ProgressPlugin',
     );
 
+    // Fail storybook build if there are webpack warnings
+    config.plugins.push(new WebpackPluginFailBuildOnWarning())
+
     return config;
   },
 };

packages/storybook/.storybook/webpack-plugin-fail-build-on-warning.js

@@ -0,0 +1,30 @@
+/*
+ * Copyright 2020 Spotify AB
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+class WebpackPluginFailBuildOnWarning {
+  apply(compiler) {
+    compiler.hooks.done.tap('FailBuildOnWarning', stats => {
+      if (stats.compilation.warnings.length > 0) {
+        process.on('beforeExit', () => {
+          console.log(`You have ${stats.compilation.warnings.length} warning(s) in your webpack build. Exiting process as error.`)
+          process.exit(1);
+        });
+      }
+    })
+  }
+}
+
+module.exports = WebpackPluginFailBuildOnWarning;