Writing tests

Every test must be able to fail because of a realistic bug you can name. If you can't name one, don't write the test.

Don't test

• Mocks: asserting a mocked value comes back unchanged.
• The language, runtime or libraries: that if, map or the framework works.
• Trivial code: getters, constants, pass-through wrappers, plain data shapes.

Do test

• Business logic, branching, edge cases (empty, boundary, error paths).
• Bug fixes: one regression test that reproduces the bug.
• Contracts at module boundaries.

Size

A few meaningful tests beat exhaustive coverage. Match the project's existing test density. If a case is borderline, skip it and mention it instead.