fix: cache transforms within a worker

CHANGELOG.md

@@ -20,6 +20,7 @@
 - `[jest-snapshot]` Remove only the added newlines in multiline snapshots ([#8859](https://github.com/facebook/jest/pull/8859))
 - `[jest-snapshot]` Distinguish empty string from external snapshot not written ([#8880](https://github.com/facebook/jest/pull/8880))
 - `[jest-snapshot]` [**BREAKING**] Distinguish empty string from internal snapshot not written ([#8898](https://github.com/facebook/jest/pull/8898))
+- `[jest-transform]` Properly cache transformed files across tests ([#8890](https://github.com/facebook/jest/pull/8890))
 
 ### Chore & Maintenance
 

e2e/__tests__/transform.test.ts

@@ -185,3 +185,23 @@ describe('transformer-config', () => {
     expect(stdout).toMatchSnapshot();
   });
 });
+
+describe('transformer caching', () => {
+  const dir = path.resolve(__dirname, '../transform/cache');
+  const transformedFile = path.resolve(dir, './common-file.js');
+
+  it('does not rerun transform within worker', () => {
+    // --no-cache because babel can cache stuff and result in false green
+    const {stdout} = runJest(dir, ['--no-cache', '-w=2']);
+
+    const loggedFiles = stdout.split('\n');
+
+    // Verify any lines logged are _just_ the file we care about
+    loggedFiles.forEach(line => {
+      expect(line).toBe(transformedFile);
+    });
+
+    // We run with 2 workers, so the file should be transformed twice
+    expect(loggedFiles).toHaveLength(2);
+  });
+});

e2e/transform/cache/__tests__/aTests.js

@@ -0,0 +1,12 @@
+/**
+ * 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 phrase = require('../common-file');
+
+test('A', () => {
+  expect(phrase).toBe('hello');
+});

e2e/transform/cache/__tests__/bTests.js

@@ -0,0 +1,12 @@
+/**
+ * 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 phrase = require('../common-file');
+
+test('B', () => {
+  expect(phrase).toBe('hello');
+});

e2e/transform/cache/__tests__/cTests.js

@@ -0,0 +1,12 @@
+/**
+ * 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 phrase = require('../common-file');
+
+test('C', () => {
+  expect(phrase).toBe('hello');
+});

e2e/transform/cache/__tests__/dTests.js

@@ -0,0 +1,12 @@
+/**
+ * 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 phrase = require('../common-file');
+
+test('D', () => {
+  expect(phrase).toBe('hello');
+});

e2e/transform/cache/common-file.js

@@ -0,0 +1,8 @@
+/**
+ * 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.
+ */
+
+module.exports = 'hello';

e2e/transform/cache/package.json

@@ -0,0 +1,10 @@
+{
+  "name": "cache",
+  "version": "1.0.0",
+  "jest": {
+    "testEnvironment": "node",
+    "transform": {
+      "^.+\\.js$": "<rootDir>/transformer.js"
+    }
+  }
+}

e2e/transform/cache/transformer.js

@@ -0,0 +1,16 @@
+/**
+ * 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.
+ */
+
+module.exports = {
+  process(src, path) {
+    if (path.includes('common')) {
+      console.log(path);
+    }
+
+    return src;
+  },
+};

packages/jest-transform/src/ScriptTransformer.ts

@@ -43,10 +43,7 @@ const {version: VERSION} = require('../package.json');
 // This data structure is used to avoid recalculating some data every time that
 // we need to transform a file. Since ScriptTransformer is instantiated for each
 // file we need to keep this object in the local scope of this module.
-const projectCaches: WeakMap<
-  Config.ProjectConfig,
-  ProjectCache
-> = new WeakMap();
+const projectCaches = new Map<string, ProjectCache>();
 
 // To reset the cache for specific changesets (rather than package version).
 const CACHE_VERSION = '1';
@@ -74,17 +71,18 @@ export default class ScriptTransformer {
     this._transformCache = new Map();
     this._transformConfigCache = new Map();
 
-    let projectCache = projectCaches.get(config);
+    const configString = stableStringify(this._config);
+    let projectCache = projectCaches.get(configString);
 
     if (!projectCache) {
       projectCache = {
-        configString: stableStringify(this._config),
+        configString,
         ignorePatternsRegExp: calcIgnorePatternRegExp(this._config),
         transformRegExp: calcTransformRegExp(this._config),
         transformedFiles: new Map(),
       };
 
-      projectCaches.set(config, projectCache);
+      projectCaches.set(configString, projectCache);
     }
 
     this._cache = projectCache;