If a single config instance (jest.config.base.js) is shared between multiple projects by re-exporting the same module, Jest will set rootDir on the object, causing it to only run the first project, once for each project.
The project-b tests should fail, but they aren't run. The project-a suite 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.
$ 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 for each config. Using the config with the environment variable BASE_CONFIG_TYPE=individual demonstrates this:
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