Test behavior, not implementation

A test earns its place by catching broken behaviour and nothing else.

• Judge every test by two imagined edits: breaking the behaviour must fail it, and restructuring the internals must not.
• Exercise behaviour through public interfaces.
• Assert exactly what the contract promises, no less and no more; the contract decides whether a call, an absence, a payload or an error type is behaviour.
• Pair an assertion that something did not happen with a positive case showing the same test would see it happen.
• Watch for hollow forms: an assertion looser than the contract, a restated constant the code never reads, an assertion on data the test built itself.
• Judge a suite by the behaviours and edge cases it exercises, not by line coverage.
• When internals change, rewrite or delete tests that only protect the old implementation, asserting the current contract instead.