Summary  
This chapter covers how to write tests for functions that include edge cases—unusual or extreme input values—to ensure the function behaves correctly in all situations.  

General domain of usage  
Software testing and quality assurance

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

When you write tests for a simple function like `add`, it might seem enough to check just one or two cases. However, functions often behave differently in unusual or extreme situations. These are called **edge cases**. For example, adding zero, negative numbers, or very large values can sometimes produce unexpected results if the function is not written or tested carefully.

By including tests for negative numbers and zero, you make sure the `add` function works correctly in all situations—not just the most common ones. **Edge case testing** helps you catch bugs early, ensures your code is reliable, and builds confidence that your function will not break when used in unexpected ways.

Why is it important to test edge cases in your functions?

Which of the following is an edge case for the add function?

A practical, beginner-friendly course that teaches how to test JavaScript code. Learn why testing matters, how Jest works, and how to write reliable tests for synchronous and asynchronous logic. Move from foundational skills to advanced techniques like mocking, spying, and TDD, gaining the tools to create more stable and maintainable applications.

Build a solid understanding of why testing matters and how Jest supports reliable JavaScript development. Start with core testing concepts, set up Jest, and write your first test to establish a confident testing workflow.

Develop the skills needed to write accurate, meaningful tests using Jest. Work with matchers, test functions and edge cases, and handle asynchronous behavior while learning how to read and interpret test results with clarity.

Apply powerful testing techniques to isolate behavior, mock dependencies, and inspect function calls. Adopt a test-driven development approach to structure code with intent, improve reliability, and create cleaner implementations.

Testing Functions and Edge Cases

1. Why is it important to test edge cases in your functions?

2. Which of the following is an edge case for the add function?