Snapshot Test
intermediateDefinition
A test that takes a 'photo' of what something looks like the first time it runs and saves it. From then on, every test run compares the current look against the saved photo and warns you if anything has changed. Easy to set up, but you have to actually look when something is different: not just rubber-stamp the new photo.
In the wild
A snapshot test of a sign-up button captures its exact appearance the first time it runs. A week later, a teammate accidentally changes the button's color. The snapshot test notices the difference and refuses to pass: forcing a human to look at the change and decide whether it was on purpose.
More from Testing & Quality
Assertion
A statement inside a test that says 'I expect this to be true.' If the expectation is wrong, the test fails and tells you exactly what didn't match. Assertions are how a test actually checks that the code is doing what it's supposed to.
End-to-End (E2E) Test
A test that drives the whole app from the outside, just like a real person would: clicking buttons, filling out forms, going from one page to the next. It's the most realistic kind of test (it catches problems no other test would), but also the slowest and the most easily upset by little changes.
Fixture
A pre-prepared piece of fake data that tests can rely on: like a sample user, a stock list of products, or a saved response. Using the same fixture across many tests keeps them tidy and consistent, instead of every test inventing its own slightly different sample.
Flaky Test
A test that sometimes passes and sometimes fails for no clear reason: usually because it depends on something it shouldn't, like the time of day, the order other tests ran in, or how fast the network is responding. Flaky tests slowly destroy the team's trust in the whole test suite.
Integration Test
A test that checks whether several pieces of an app work correctly when they're put together: talking to a real database, hitting a real internal service, the way they would in real life. It catches the kinds of bugs that only show up when separate pieces have to cooperate.
Linting
Automatic proofreading for code. A linter scans through the source files and flags stylistic mistakes, suspicious patterns, and likely bugs: without actually running the code. It catches the small problems early, before they reach reviewers or tests.