Why is test independence important in a test suite?

• A. To make tests slower
• B. To ensure tests are independent and repeatable
• C. To cause flakiness
• D. To require shared state

Answer: (D)

Tests should be isolated: each test sets up its own data and environment and does not rely on the results or side effects of other tests. This independence makes outcomes deterministic and repeatable—running the suite again should yield the same results unless the code under test changes. It also allows tests to run in any order or in parallel, since one test won’t influence another.

Why this matters: when tests share state or depend on execution order, a failure can be caused by something that happened in a previous test. That makes it hard to pinpoint the real cause and can lead to flaky tests that behave unpredictably depending on timing or order.

From the options, the aim is not to slow things down, not to introduce flakiness, and certainly not to require shared state. The best idea is to keep tests independent so they are repeatable and reliable.