Test First

Test First

When the change can be tested, write the test before the implementation.

For a bug fix

1. Reproduce it with a failing test. Write a test that asserts the correct behavior — it should fail against the current buggy code. This proves you understand the bug.
2. Run it and watch it fail for the expected reason. A test that passes before you've fixed anything is testing the wrong thing.
3. Write the minimal fix that makes it pass.
4. Run the full suite to confirm the fix works and breaks nothing else.

For a new feature

1. Write a test describing the behavior you want (it fails — the feature doesn't exist yet).
2. Implement until it passes.
3. Add tests for the obvious edge cases (empty input, boundaries, error paths).

Keep it honest

• The test must actually fail first. "I wrote a test and it passes" on unwritten code means the test is hollow.
• Test behavior, not implementation details — assert what the code does, not how, so the test survives refactors.
• If something genuinely can't be tested (UI nuance, external service), say so and fall back to a concrete manual check instead of pretending.

Why this matters

A bug with a failing-then-passing test around it is a bug that won't silently come back — the test stands guard forever. Writing the test first also forces you to define "done" before you write code, which is exactly when it's cheapest to get the definition right.