Test data management
Test data management is the discipline of producing, maintaining, and isolating the data used in tests, fixtures, factories, seeded databases, anonymised production snapshots. Poor test data management is one of the largest sources of test-suite slowness, flakiness, and false confidence.
The trade-off space is wide: hand-written fixtures are clear but tedious to maintain; factories (Factory Bot, Faker, Stride's typed factory) produce data programmatically but can mask coupling between tests; anonymised production snapshots are realistic but expensive to refresh. The cardinal rules: each test owns its data (no shared state across tests), data is realistic but minimal (no kitchen-sink fixtures that obscure what the test cares about), and the seeding mechanism is fast (database snapshot restore beats per-test inserts for integration suites). Modern test runners (vitest, Jest) include parallel execution that exposes test-data-coupling bugs quickly.