Fix transformer signature in babel-jest (#7945)

* Assigned createTransformer function only to the transformer exported by babel-jest

* Improved typing

* Updated changelog

* Added e2e test

CHANGELOG.md

@@ -47,7 +47,7 @@
 - `[@jest/transform]`: New package extracted from `jest-runtime` ([#7915](https://github.com/facebook/jest/pull/7915))
 - `[babel-plugin-jest-hoist]`: Migrate to TypeScript ([#7898](https://github.com/facebook/jest/pull/7898))
 - `[@jest/core]` Create new package, which is `jest-cli` minus `yargs` and `prompts` ([#7696](https://github.com/facebook/jest/pull/7696))
-- `[@jest/transform]`: Migrate to TypeScript ([#7918](https://github.com/facebook/jest/pull/7918))
+- `[@jest/transform]`: Migrate to TypeScript ([#7918](https://github.com/facebook/jest/pull/7918), [#7945](https://github.com/facebook/jest/pull/7945))
 - `[docs]` Add missing import to docs ([#7928](https://github.com/facebook/jest/pull/7928))
 - `[jest-resolve-dependencies]`: Migrate to TypeScript ([#7922](https://github.com/facebook/jest/pull/7922))
 - `[expect]`: Migrate to TypeScript ([#7919](https://github.com/facebook/jest/pull/7919))

e2e/__tests__/transform.test.js

@@ -57,6 +57,21 @@ describe('babel-jest ignored', () => {
   });
 });
 
+describe('babel-jest with manual transformer', () => {
+  const dir = path.resolve(__dirname, '..', 'transform/babel-jest-manual');
+
+  beforeEach(() => {
+    run('yarn', dir);
+  });
+
+  it('runs transpiled code', () => {
+    // --no-cache because babel can cache stuff and result in false green
+    const {json} = runWithJson(dir, ['--no-cache']);
+    expect(json.success).toBe(true);
+    expect(json.numTotalTests).toBeGreaterThanOrEqual(1);
+  });
+});
+
 // babel-jest is automatically linked at the root because it is a workspace now
 // a way to test this in isolation is to move the test suite into a temp folder
 describe('no babel-jest', () => {

e2e/transform/babel-jest-manual/__tests__/babelJest.test.js

@@ -0,0 +1,15 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates. All Rights Reserved.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+'use strict';
+
+require('../foo');
+
+it('strips flowtypes using babel-jest and .babelrc', () => {
+  const a: string = 'a';
+  expect(a).toBe('a');
+});

e2e/transform/babel-jest-manual/foo.js

@@ -0,0 +1,10 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates. All Rights Reserved.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+const someFunction = (): null => null;
+
+module.exports = someFunction();

e2e/transform/babel-jest-manual/package.json

@@ -0,0 +1,11 @@
+{
+  "dependencies": {
+    "@babel/preset-flow": "^7.0.0"
+  },
+  "jest": {
+    "testEnvironment": "node",
+    "transform": {
+      "\\.js$": "<rootDir>/transformer.js"
+    }
+  }
+}

e2e/transform/babel-jest-manual/transformer.js

@@ -0,0 +1,13 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates. All Rights Reserved.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+const {createTransformer} = require('babel-jest');
+
+module.exports = createTransformer({
+  presets: ['@babel/preset-flow'],
+  root: __dirname,
+});

e2e/transform/babel-jest-manual/yarn.lock

@@ -0,0 +1,31 @@
+# THIS IS AN AUTOGENERATED FILE. DO NOT EDIT THIS FILE DIRECTLY.
+# yarn lockfile v1
+
+
+"@babel/helper-plugin-utils@^7.0.0":
+  version "7.0.0"
+  resolved "https://registry.yarnpkg.com/@babel/helper-plugin-utils/-/helper-plugin-utils-7.0.0.tgz#bbb3fbee98661c569034237cc03967ba99b4f250"
+  integrity sha512-CYAOUCARwExnEixLdB6sDm2dIJ/YgEAKDM1MOeMeZu9Ld/bDgVo8aiWrXwcY7OBh+1Ea2uUcVRcxKk0GJvW7QA==
+
+"@babel/plugin-syntax-flow@^7.0.0":
+  version "7.0.0"
+  resolved "https://registry.yarnpkg.com/@babel/plugin-syntax-flow/-/plugin-syntax-flow-7.0.0.tgz#70638aeaad9ee426bc532e51523cff8ff02f6f17"
+  integrity sha512-zGcuZWiWWDa5qTZ6iAnpG0fnX/GOu49pGR5PFvkQ9GmKNaSphXQnlNXh/LG20sqWtNrx/eB6krzfEzcwvUyeFA==
+  dependencies:
+    "@babel/helper-plugin-utils" "^7.0.0"
+
+"@babel/plugin-transform-flow-strip-types@^7.0.0":
+  version "7.0.0"
+  resolved "https://registry.yarnpkg.com/@babel/plugin-transform-flow-strip-types/-/plugin-transform-flow-strip-types-7.0.0.tgz#c40ced34c2783985d90d9f9ac77a13e6fb396a01"
+  integrity sha512-WhXUNb4It5a19RsgKKbQPrjmy4yWOY1KynpEbNw7bnd1QTcrT/EIl3MJvnGgpgvrKyKbqX7nUNOJfkpLOnoDKA==
+  dependencies:
+    "@babel/helper-plugin-utils" "^7.0.0"
+    "@babel/plugin-syntax-flow" "^7.0.0"
+
+"@babel/preset-flow@^7.0.0":
+  version "7.0.0"
+  resolved "https://registry.yarnpkg.com/@babel/preset-flow/-/preset-flow-7.0.0.tgz#afd764835d9535ec63d8c7d4caf1c06457263da2"
+  integrity sha512-bJOHrYOPqJZCkPVbG1Lot2r5OSsB+iUOaxiHdlOeB1yPWS6evswVHwvkDLZ54WTaTRIk89ds0iHmGZSnxlPejQ==
+  dependencies:
+    "@babel/helper-plugin-utils" "^7.0.0"
+    "@babel/plugin-transform-flow-strip-types" "^7.0.0"

packages/babel-jest/src/index.ts

@@ -26,7 +26,6 @@ const babelIstanbulPlugin = require.resolve('babel-plugin-istanbul');
 // Narrow down the types
 interface BabelJestTransformer extends Transformer {
   canInstrument: true;
-  createTransformer: (options?: TransformOptions) => BabelJestTransformer;
 }
 
 const createTransformer = (
@@ -64,9 +63,8 @@ const createTransformer = (
     return babelConfig;
   }
 
-  const transformer: BabelJestTransformer = {
+  return {
     canInstrument: true,
-    createTransformer,
     getCacheKey(
       fileData,
       filename,
@@ -130,8 +128,15 @@ const createTransformer = (
       return src;
     },
   };
+};
 
-  return transformer;
+const transformer: BabelJestTransformer & {
+  createTransformer: (options?: TransformOptions) => BabelJestTransformer;
+} = {
+  ...createTransformer(),
+  // Assigned here so only the exported transformer has `createTransformer`,
+  // instead of all created transformers by the function
+  createTransformer,
 };
 
-export = createTransformer();
+export = transformer;