Enable Jest tests in *.mjs files

.browserslistrc

@@ -2,3 +2,6 @@ last 2 versions
 ie 8
 ie 9
 iOS 9
+
+[node]
+node 18

babel.config.js

@@ -0,0 +1,20 @@
+/**
+ * Babel config
+ *
+ * @type {import('@babel/core').ConfigFunction}
+ */
+module.exports = function (api) {
+  const browserslistEnv = !api.env('test')
+    ? 'production'
+    : 'node'
+
+  const presets = [
+    ['@babel/preset-env', {
+      browserslistEnv
+    }]
+  ]
+
+  return {
+    presets
+  }
+}

jest.config.mjs

@@ -15,6 +15,11 @@ export default {
   // Test timeout increased (5s to 15s)
   testTimeout: 15000,
 
+  // Enable Babel transforms until Jest supports ESM
+  // See: https://jestjs.io/docs/ecmascript-modules
+  testMatch: ['**/*.test.{js,mjs}'],
+  transform: { '^.+\\.m?js$': ['babel-jest'] },
+
   // Ignore built fixtures during `--watch`
   watchPathIgnorePatterns: ['fixtures/build']
 }