diff --git a/packages/docusaurus/src/webpack/__tests__/base.test.ts b/packages/docusaurus/src/webpack/__tests__/base.test.ts
new file mode 100644
index 0000000000..0796b53b01
--- /dev/null
+++ b/packages/docusaurus/src/webpack/__tests__/base.test.ts
@@ -0,0 +1,59 @@
+/**
+ * Copyright (c) 2017-present, Facebook, Inc.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+import path from 'path';
+
+import {excludeJS, clientDir} from '../base';
+
+describe('babel transpilation exclude logic', () => {
+  test('always transpile client dir files', () => {
+    const clientFiles = [
+      'App.js',
+      'clientEntry.js',
+      'serverEntry.js',
+      path.join('exports', 'Link.js'),
+    ];
+    clientFiles.forEach(file => {
+      expect(excludeJS(path.join(clientDir, file))).toEqual(false);
+    });
+  });
+
+  test('always transpile non node_module files', () => {
+    const moduleFiles = [
+      '/pages/user/App.jsx',
+      '/website/src/components/foo.js',
+      '/src/theme/SearchBar/index.js',
+    ];
+    moduleFiles.forEach(file => {
+      expect(excludeJS(file)).toEqual(false);
+    });
+  });
+
+  test('transpile docusaurus npm packages even in node_modules', () => {
+    const moduleFiles = [
+      '/website/node_modules/docusaurus-theme-search/theme/Navbar/index.js',
+      'node_modules/@docusaurus/theme-classic/theme/Layout.js',
+      '/docusaurus/website/node_modules/@docusaurus/theme-search-algolia/theme/SearchBar.js',
+    ];
+    moduleFiles.forEach(file => {
+      expect(excludeJS(file)).toEqual(false);
+    });
+  });
+
+  test('does not transpile node_modules', () => {
+    const moduleFiles = [
+      'node_modules/react-toggle.js',
+      '/website/node_modules/react-trend/index.js',
+      '/docusaurus/website/node_modules/react-super.js',
+      '/docusaurus/website/node_modules/@docusaurus/core/node_modules/core-js/modules/_descriptors.js',
+      'node_modules/docusaurus-theme-classic/node_modules/react-daypicker/index.js',
+    ];
+    moduleFiles.forEach(file => {
+      expect(excludeJS(file)).toEqual(true);
+    });
+  });
+});
diff --git a/packages/docusaurus/src/webpack/base.ts b/packages/docusaurus/src/webpack/base.ts
index 3a67ad1e8a..a91404d38b 100644
--- a/packages/docusaurus/src/webpack/base.ts
+++ b/packages/docusaurus/src/webpack/base.ts
@@ -17,6 +17,19 @@ import {getBabelLoader, getCacheLoader, getStyleLoaders} from './utils';
 
 const CSS_REGEX = /\.css$/;
 const CSS_MODULE_REGEX = /\.module\.css$/;
+export const clientDir = path.join(__dirname, '..', 'client');
+
+export function excludeJS(modulePath: string) {
+  // always transpile client dir
+  if (modulePath.startsWith(clientDir)) {
+    return false;
+  }
+  // Don't transpile node_modules except any docusaurus npm package
+  return (
+    /node_modules/.test(modulePath) &&
+    !/(docusaurus)((?!node_modules).)*\.jsx?$/.test(modulePath)
+  );
+}
 
 export function createBaseConfig(
   props: Props,
@@ -24,7 +37,6 @@ export function createBaseConfig(
 ): Configuration {
   const {outDir, siteDir, baseUrl, generatedFilesDir, routesPaths} = props;
 
-  const clientDir = path.join(__dirname, '..', 'client');
   const totalPages = routesPaths.length;
   const isProd = process.env.NODE_ENV === 'production';
   return {
@@ -133,17 +145,7 @@ export function createBaseConfig(
       rules: [
         {
           test: /\.jsx?$/,
-          exclude: modulePath => {
-            // always transpile client dir
-            if (modulePath.startsWith(clientDir)) {
-              return false;
-            }
-            // Don't transpile node_modules except any docusaurus npm package
-            return (
-              /node_modules/.test(modulePath) &&
-              !/(docusaurus)((?!node_modules).)*\.jsx?$/.test(modulePath)
-            );
-          },
+          exclude: excludeJS,
           use: [getCacheLoader(isServer), getBabelLoader(isServer)].filter(
             Boolean,
           ) as Loader[],