Unit Testing

The foundation. A unit test checks the smallest testable part of an application (a function, a class).

The 3 Laws (FIRST)

• Fast: Should run in milliseconds.
• Isolated: Should not touch the DB, Network, or File System.
• Repeatable: Always produces the same result.
• Self-Validating: Pass/Fail. No manual check.
• Timely: Written with or before the code.

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Mocking vs. Stubbing

To keep tests isolated, we fake dependencies.

• Stub: Returns hardcoded data. ("When getUser() is called, return {'name': 'Bob'}"). Used for state verification.
• Mock: Verifies behavior. ("Expect sendEmail() to be called exactly once").