diff --git a/react-packager/src/JSTransformer/__tests__/worker-test.js b/react-packager/src/JSTransformer/__tests__/worker-test.js
new file mode 100644
index 00000000..fdb67d9d
--- /dev/null
+++ b/react-packager/src/JSTransformer/__tests__/worker-test.js
@@ -0,0 +1,109 @@
+/**
+ * Copyright (c) 2015-present, Facebook, Inc.
+ * All rights reserved.
+ *
+ * This source code is licensed under the BSD-style license found in the
+ * LICENSE file in the root directory of this source tree. An additional grant
+ * of patent rights can be found in the PATENTS file in the same directory.
+ */
+'use strict';
+
+jest.autoMockOff();
+
+jest.mock('babel-core');
+
+const worker = require('../worker');
+const babel = require('babel-core');
+
+const code = 'code';
+
+describe('Resolver', function() {
+  beforeEach(() => {
+    babel.transform.mockImpl((source, options) => source);
+  });
+
+  describe('when no external transform is provided', () => {
+    it('should invoke internal transform if available', () => {
+      transform({
+        sourceCode: 'code',
+        filename: 'test',
+        options: options({opts: {}, internalTransformsEnabled: true}),
+      });
+      expect(babel.transform.mock.calls.length).toBe(1);
+    });
+
+    it('should not invoke internal transform if unavailable', () => {
+      transform({
+        sourceCode: 'code',
+        filename: 'test',
+        options: options({opts: {}, internalTransformsEnabled: false}),
+      });
+      expect(babel.transform.mock.calls.length).toBe(0);
+    });
+  });
+
+  describe('when external transform is provided', () => {
+    it('should invoke both transformers if internal is available', () => {
+      transform({
+        sourceCode: code,
+        filename: 'test',
+        options: options({
+          opts: {
+            externalTransformModulePath: require.resolve(
+              '../../../../transformer.js'
+            ),
+          },
+          internalTransformsEnabled: true,
+        }),
+      });
+      expect(babel.transform.mock.calls.length).toBe(2);
+    });
+
+    it('should invoke only external transformer if internal is not available', () => {
+      transform({
+        sourceCode: 'code',
+        filename: 'test',
+        options: options({
+          opts: {
+            externalTransformModulePath: require.resolve(
+              '../../../../transformer.js'
+            ),
+          },
+          internalTransformsEnabled: false,
+        }),
+      });
+      expect(babel.transform.mock.calls.length).toBe(1);
+    });
+
+    it('should pipe errors through transform pipeline', () => {
+      const error = new Error('transform error');
+      babel.transform.mockImpl((source, options) => {
+        throw error;
+      });
+
+      const callback = transform({
+        sourceCode: 'code',
+        filename: 'test',
+        options: options({
+          opts: {
+            externalTransformModulePath: require.resolve(
+              '../../../../transformer.js'
+            ),
+          },
+          internalTransformsEnabled: true,
+        }),
+      });
+      expect(callback.mock.calls[0][0]).toBe(error);
+    });
+  });
+});
+
+function transform(data) {
+  const callback = jest.genMockFunction();
+  worker(data, callback);
+  return callback;
+}
+
+function options({opts, internalTransformsEnabled}) {
+  return Object.assign(opts, {hot: internalTransformsEnabled});
+}