Set up a monorepo with Lerna

packages/react-scripts/scripts/utils/createJestConfig.js

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+// @remove-on-eject-begin
+/**
+ * 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.
+ */
+// @remove-on-eject-end
+
+const pathExists = require('path-exists');
+const paths = require('../../config/paths');
+
+module.exports = (resolve, rootDir) => {
+  const setupFiles = [resolve('config/polyfills.js')];
+  if (pathExists.sync(paths.testsSetup)) {
+    // Use this instead of `paths.testsSetup` to avoid putting
+    // an absolute filename into configuration after ejecting.
+    setupFiles.push('<rootDir>/src/setupTests.js');
+  }
+
+  const config = {
+    moduleFileExtensions: ['jsx', 'js', 'json'],
+    moduleNameMapper: {
+      '^.+\\.(jpg|jpeg|png|gif|eot|otf|webp|svg|ttf|woff|woff2|mp4|webm|wav|mp3|m4a|aac|oga)$': resolve('config/jest/FileStub.js'),
+      '^.+\\.css$': resolve('config/jest/CSSStub.js')
+    },
+    scriptPreprocessor: resolve('config/jest/transform.js'),
+    setupFiles: setupFiles,
+    testPathIgnorePatterns: ['<rootDir>/(build|docs|node_modules)/'],
+    testEnvironment: 'node',
+    testRegex: '(/__tests__/.*|\\.(test|spec))\\.(js|jsx)$',
+  };
+  if (rootDir) {
+    config.rootDir = rootDir;
+  }
+  return config;
+};