How can you ensure your unit tests cover all possible scenarios?

1 Define clear and testable requirements

Before you write any unit test, you need to have a clear and testable specification of what the unit should do and how it should behave under different inputs and conditions. This will help you design your test cases, identify the expected outcomes, and avoid ambiguity and confusion. You can use tools like user stories, acceptance criteria, or behavior-driven development (BDD) to define your requirements in a structured and readable format.

3 Use test design techniques

Test design techniques are methods to generate test cases based on different criteria and perspectives, which can help to reduce redundancy, increase coverage, and find more defects in unit tests. Equivalence partitioning divides the input domain into groups of values with the same output or behavior; boundary value analysis tests the values at the edges of the input domain; decision table testing uses a table to represent combinations of conditions and actions that affect the output or behavior; and state transition testing tests the changes and transitions of the unit's state due to different events or inputs.

5 Refactor and review your unit tests

As your code evolves, so should your unit tests. It's essential to refactor and review them regularly to make sure they are still relevant, accurate, and consistent. To make your unit tests more readable, maintainable, and trustworthy, you should follow some best practices. These include naming your unit tests clearly and descriptively with a consistent convention, organizing them logically and hierarchically with classes, namespaces, and folders, applying the arrange-act-assert pattern to separate the test setup, execution, and verification into distinct sections, using the AAA or AAAA pattern to document the purpose and context of your unit tests, avoiding hard-coded values and literals by using constants or variables instead, and keeping your unit tests focused, independent, and deterministic by avoiding side effects, dependencies, and randomness.