Node test runner failures — Tests: N failed / Expected: Received:

Log signatures

Tests:       2 failed, 18 passed
Test Suites: 1 failed
Expected: 4   Received: 3
FAIL src/app.test.ts
toMatchSnapshot

What’s happening

First, confirm this is a unit runner and not a browser E2E suite — the fix strategies differ completely. Then the signature tells you the flavor: "Expected: ... Received:" is a value assertion (real drift); toMatchSnapshot failures are their own category — the rendered output changed, and the question is whether the change was intentional (update the snapshot) or a regression (fix the code), and snapshot tests can't tell you which; "Test Suites: N failed" with zero individual test failures means suites crashed at import time (syntax error, bad transform config, ESM/CJS mismatch) — config problem, not assertion problem.

Fix playbook

1. 1Rerun only the failing spec: npx jest path/to/file.test.ts -t "test name" or npx vitest run path/to/file.test.ts.
2. 2Suite-level crash (failed suite, no failed tests): read the import-time error. Usual suspects: transform config (ts-jest/babel vs ESM), a dependency that ships untranspiled ESM, Node version drift.
3. 3Snapshot failures: review the printed diff as a code review, not as noise. Intentional → jest -u and commit the updated snapshot in the same PR as the change that caused it. Unintentional → it caught a regression; fix the code.
4. 4Value assertions: standard fix — decide whether code or expectation is wrong, change exactly one.
5. 5Flaky in CI only: look for unawaited promises and timer dependence — run with --runInBand to rule out worker parallelism, and check for setTimeout-based waits in the test.