principle-test-behavior-not-implementation

Test Behavior, Not Implementation

A test calls the code as its users do and asserts the exact result they see. Rewrite or delete any test that would still pass if every imported function returned undefined or a wrong value.

These shapes fail that check:

• Loose or no assertion: toBeDefined, toBeTruthy, not.toThrow, toBeGreaterThan(0).
• Call or absence only: toHaveBeenCalled, not.toHaveBeenCalled, toBeUndefined, toEqual([]).
• Expected value computed by the code under test: expect(f(a)).toBe(f(a)).
• Restated constant, config default, table row, or prompt string.
• Assertions on data the test built, with no call to the subject.

Fix: call the subject in the test body with one concrete input and assert the literal output or effect. For a mock, assert the payload it received. For an absence, also assert the presence on another input. For a constant, test the mechanism that reads it with one input instead of restating the value.

Keep relation checks across table rows (a key in two tables, a parent that exists) and *.test-d.ts type checks.

Production prerequisites. An acceptance test must not pass because its setup establishes state or ordering that production never establishes. Exercise the real initialization path or prove that the caller supplies the prerequisite. Ordinary fixture inputs that real callers can supply are valid.

Notation, not a runtime. Write Given, When, Then or Gherkin-style tests in the project's existing framework. Do not add Cucumber, a Gherkin runner, or .feature files unless the project already runs one or the user explicitly asks for Cucumber, a runner, or .feature files. A request for Gherkin-style tests is not such a request.