Planning

Every well-designed unit test explores a single, unified concept and follows a strict logical structure known as the Arrange, Act, Assert (AAA) pattern. This structure ensures clarity, simplicity, and brevity in test design.

The “Arrange” phase establishes the initial state of the system. Developers initialize variables, instantiate classes, and configure the necessary environmental conditions required for the test. The “Act” phase executes the specific behavior under investigation. This phase must be exceedingly brief, typically consisting of a single function call. Finally, the “Assert” phase evaluates the outcome of the action against the anticipated result. The test compares the actual return values, state changes, or raised exceptions against the expected truth. Maintaining this strict separation prevents tests from becoming tangled narratives and ensures that when a test fails, the developer knows exactly which phase of the operation broke.

Planning tests requires anticipating both expected functionality and potential failure states. Developers must imagine both positive scenarios, testing the algorithm against valid inputs, and negative scenarios, bombarding the algorithm with invalid data types, missing fields, or extreme boundary conditions.