Glossary

Unit Testing

A unit test is an automated test that verifies the smallest isolated unit of code (a function or method) in isolation from external dependencies. Runs fast and pinpoints the problem precisely.

Properties of a good unit test (FIRST)

Fast — runs in milliseconds
Independent — does not depend on other tests
Repeatable — same result every time
Self-validating — automatically determines pass/fail
Timely — written alongside the code, not after

AAA pattern

// Arrange
$calc = new Calculator();
// Act
$result = $calc->add(2, 3);
// Assert
assertEquals(5, $result);

Mocks and stubs

To isolate from DB, API, filesystem: mock (a fake with call verification) and stub (returns preset data, no verification). Popular libraries: PHPUnit, Jest, pytest.