reproduce jest bug

Author: also

README.md

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+# Jest bug sharing config instances between projects
+
+If a single config instance ([`jest.config.base.js`](jest.config.base.js#L7)) is shared between multiple projects by [re-exporting the same module](project-b/jest.config.js#L1), Jest will set `rootDir` on the object, causing it to only run the first project, once for each project.
+
+The [project-b tests](project-b/test.js) should fail, but they aren't run. The [project-a suite](project-a/test.js) is run twice:
+
+```
+$ ./node_modules/.bin/jest
+ PASS  project-a/test.js
+ PASS  project-a/test.js
+
+Test Suites: 2 passed, 2 total
+Tests:       2 passed, 2 total
+Snapshots:   2 passed, 2 total
+Time:        0.155 s, estimated 1 s
+Ran all test suites in 2 projects.
+```
+
+The location in the Jest source where it mutates the config to set `rootDir` can be found by [freezing the config object](jest.config.base.js#L10).
+
+```
+$ BASE_CONFIG_TYPE=frozen ./node_modules/.bin/jest
+TypeError: Cannot add property rootDir, object is not extensible
+    at readConfigFileAndSetRootDir (/Users/ryan/work/jest-project-shared-config-bug/node_modules/jest-config/build/readConfigFileAndSetRootDir.js:166:26)
+    at async readConfig (/Users/ryan/work/jest-project-shared-config-bug/node_modules/jest-config/build/index.js:233:18)
+    at async Promise.all (index 0)
+    at async readConfigs (/Users/ryan/work/jest-project-shared-config-bug/node_modules/jest-config/build/index.js:441:27)
+    at async runCLI (/Users/ryan/work/jest-project-shared-config-bug/node_modules/@jest/core/build/cli/index.js:132:59)
+    at async Object.run (/Users/ryan/work/jest-project-shared-config-bug/node_modules/jest-cli/build/cli/index.js:155:37)
+```
+
+The bug can be worked around by [creating a new object](jest.config.base.js#L13) for each config. Using the config with the environment variable `BASE_CONFIG_TYPE=individual` demonstrates this:
+
+```$ BASE_CONFIG_TYPE=individual ./node_modules/.bin/jest
+ FAIL  project-b/test.js
+  ● project b
+
+    expect(received).toBe(expected) // Object.is equality
+
+    Expected: false
+    Received: true
+
+      1 | test("project b", () => {
+    > 2 |   expect(true).toBe(false);
+        |                ^
+      3 | });
+      4 |
+
+      at Object.<anonymous> (test.js:2:16)
+
+ PASS  project-a/test.js
+
+Test Suites: 1 failed, 1 passed, 2 total
+Tests:       1 failed, 1 passed, 2 total
+Snapshots:   1 passed, 1 total
+Time:        0.169 s, estimated 1 s
+```

jest.config.base.js

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+const baseConfigType = process.env.BASE_CONFIG_TYPE || "shared";
+
+if (baseConfigType === "shared") {
+  // the default, to demonstrate the jest bug.
+  // every project will use the same config object.
+  // jest mutates the object to set rootDir when not set
+  module.exports = {};
+} else if (baseConfigType === "frozen") {
+  // produces a stack trace where jest tries to mutate the object to set rootDir
+  module.exports = Object.freeze({});
+} else if (baseConfigType === "individual") {
+  // work around the bug by returning a function that generates a distinct config object for each project
+  module.exports = () => ({});
+} else {
+  throw new Error(`Unknown BASE_CONFIG_TYPE: ${baseConfigType}`);
+}

jest.config.js

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+module.exports = {
+  projects: ["<rootDir>/project-a/", "<rootDir>/project-b/"],
+};

package.json

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+{
+  "devDependencies": {
+    "jest": "27.5.1"
+  }
+}

project-a/jest.config.js

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+module.exports = require("../jest.config.base");

project-a/test.js

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+test("project a", () => {
+  expect("project a").toMatchInlineSnapshot(`"project a"`);
+});

project-b/jest.config.js

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+module.exports = require("../jest.config.base");

project-b/test.js

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+test("project b", () => {
+  expect(true).toBe(false);
+});