BeforeAll vs. BeforeEach: Key Differences Every Developer Must Know
BeforeAll runs once per test file; BeforeEach runs before every individual test inside that file.
Developers copy-paste examples, then wonder why setup data leaks between tests. The silent bug hides until tests run in random order, exposing shared state.
Key Differences
BeforeAll sets up heavyweight fixtures like starting a database container. BeforeEach resets mutable state so tests stay isolated. Use the former for costly, read-only resources; the latter for anything that changes.
Which One Should You Choose?
Choose BeforeAll when speed outweighs isolation. Choose BeforeEach when correctness is critical. If one test mutates shared data, switch to BeforeEach or your suite becomes flaky.
Examples and Daily Life
Imagine e2e tests: BeforeAll signs in once; BeforeEach navigates to the dashboard. You’ll save minutes on a 500-test suite and still catch navigation bugs.
Can I use both in the same file?
Yes. Nest BeforeAll for global setup and BeforeEach for per-test resets.
What happens if BeforeEach throws?
That test is marked failed; the rest of the suite continues.
Does order matter?
BeforeAll always runs first, then each BeforeEach in declaration order.