chore: only transform things that smell like esm

Signed-off-by: blam <ben@blam.sh>

packages/cli/config/jest.js

@@ -132,7 +132,7 @@ async function getProjectConfig(targetPath, displayName) {
 
     transform: {
       '\\.(js|jsx|ts|tsx|mjs|cjs)$': [
-        '@swc/jest',
+        require.resolve('./jestSwcTransform'),
         {
           sourceMaps: envOptions.enableSourceMaps || envOptions.nextTests,
         },

packages/cli/config/jestSwcTransform.js

@@ -0,0 +1,35 @@
+/*
+ * Copyright 2022 The Backstage Authors
+ *
+ * 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.
+ */
+const { createTransformer: createSwcTransformer } = require('@swc/jest');
+
+const ESM_REGEX = /\b(?:import|export)\b/;
+
+function createTransformer(config) {
+  const swcTransformer = createSwcTransformer(config);
+  const process = (source, filePath, jestOptions) => {
+    if (filePath.endsWith('.js') && !ESM_REGEX.test(source)) {
+      return source;
+    }
+
+    return swcTransformer.process(source, filePath, jestOptions);
+  };
+
+  const getCacheKey = swcTransformer.getCacheKey;
+
+  return { process, getCacheKey };
+}
+
+module.exports = { createTransformer };