From 79cbda4e1fc4b1fa293a03794611ec072baf69fa Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Kamil=20Og=C3=B3rek?= <kamil.ogorek@gmail.com>
Date: Tue, 13 Jun 2017 17:22:05 +0200
Subject: [PATCH] Provide better error checking for transformed content (#3807)

---
 .../jest-mock/src/__tests__/jest-mock-test.js |  2 +-
 .../jest-runtime/src/ScriptTransformer.js     |  7 ++-
 .../src/__tests__/ScriptTransformer-test.js   | 51 +++++++++++++++++++
 3 files changed, 58 insertions(+), 2 deletions(-)

diff --git a/packages/jest-mock/src/__tests__/jest-mock-test.js b/packages/jest-mock/src/__tests__/jest-mock-test.js
index ec95776159ef..3c793f2d03a8 100644
--- a/packages/jest-mock/src/__tests__/jest-mock-test.js
+++ b/packages/jest-mock/src/__tests__/jest-mock-test.js
@@ -187,7 +187,7 @@ describe('moduleMocker', () => {
       expect(typeof multipleBoundFuncMock).toBe('function');
     });
 
-   it('mocks methods that are bound after mocking', () => {
+    it('mocks methods that are bound after mocking', () => {
       const fooMock = moduleMocker.generateFromMetadata(
         moduleMocker.getMetadata(() => {}),
       );
diff --git a/packages/jest-runtime/src/ScriptTransformer.js b/packages/jest-runtime/src/ScriptTransformer.js
index e6da9330959d..51778164e7e9 100644
--- a/packages/jest-runtime/src/ScriptTransformer.js
+++ b/packages/jest-runtime/src/ScriptTransformer.js
@@ -208,8 +208,13 @@ class ScriptTransformer {
 
       if (typeof processed === 'string') {
         transformed.code = processed;
-      } else {
+      } else if (processed != null && typeof processed.code === 'string') {
         transformed = processed;
+      } else {
+        throw new TypeError(
+          "Jest: a transform's `process` function must return a string, " +
+            'or an object with `code` key containing this string.',
+        );
       }
     }
 
diff --git a/packages/jest-runtime/src/__tests__/ScriptTransformer-test.js b/packages/jest-runtime/src/__tests__/ScriptTransformer-test.js
index 0530e2b18cb7..661f6ea79b11 100644
--- a/packages/jest-runtime/src/__tests__/ScriptTransformer-test.js
+++ b/packages/jest-runtime/src/__tests__/ScriptTransformer-test.js
@@ -75,6 +75,16 @@ jest.mock(
   {virtual: true},
 );
 
+jest.mock(
+  'passthrough-preprocessor',
+  () => {
+    return {
+      process: jest.fn(),
+    };
+  },
+  {virtual: true},
+);
+
 const getCachePath = (fs, config) => {
   for (const path in mockFs) {
     if (path.startsWith(config.cacheDirectory)) {
@@ -101,6 +111,9 @@ describe('ScriptTransformer', () => {
 
     mockFs = object({
       '/fruits/banana.js': ['module.exports = "banana";'].join('\n'),
+      '/fruits/grapefruit.js': [
+        'module.exports = function () { return "grapefruit"; }',
+      ].join('\n'),
       '/fruits/kiwi.js': ['module.exports = () => "kiwi";'].join('\n'),
       '/node_modules/react.js': ['module.exports = "react";'].join('\n'),
       '/styles/App.css': ['root {', '  font-family: Helvetica;', '}'].join(
@@ -180,6 +193,44 @@ describe('ScriptTransformer', () => {
     expect(vm.Script.mock.calls[1][0]).toEqual(snapshot);
   });
 
+  it(
+    "throws an error if `process` doesn't return a string or an object" +
+      'containing `code` key with processed string',
+    () => {
+      config = Object.assign(config, {
+        transform: [['^.+\\.js$', 'passthrough-preprocessor']],
+      });
+      const scriptTransformer = new ScriptTransformer(config);
+
+      const incorrectReturnValues = [
+        [undefined, '/fruits/banana.js'],
+        [{a: 'a'}, '/fruits/kiwi.js'],
+        [[], '/fruits/grapefruit.js'],
+      ];
+
+      incorrectReturnValues.forEach(([returnValue, filePath]) => {
+        require('passthrough-preprocessor').process.mockReturnValue(
+          returnValue,
+        );
+        expect(() => scriptTransformer.transform(filePath, {})).toThrow(
+          'must return a string',
+        );
+      });
+
+      const correctReturnValues = [
+        ['code', '/fruits/banana.js'],
+        [{code: 'code'}, '/fruits/kiwi.js'],
+      ];
+
+      correctReturnValues.forEach(([returnValue, filePath]) => {
+        require('passthrough-preprocessor').process.mockReturnValue(
+          returnValue,
+        );
+        expect(() => scriptTransformer.transform(filePath, {})).not.toThrow();
+      });
+    },
+  );
+
   it('uses the supplied preprocessor', () => {
     config = Object.assign(config, {
       transform: [['^.+\\.js$', 'test-preprocessor']],