Unit Test

Overview

A unit test verifies the behaviour of one unit (function, class, or module) in complete isolation. Every dependency that crosses an architectural boundary is replaced by a test double. Speed and determinism are non-negotiable.

Required background: Apply auto-test universal principles first. This skill adds unit-scope specifics.

When to Use

• Writing a test for a single function or class
• Reviewing unit tests for isolation or smell violations
• Deciding mocking strategy at the unit level
• Diagnosing slow, flaky, or brittle unit tests

Not for: tests that intentionally use real databases, real HTTP, or real filesystem — see integration-test.

Core Principles