Test-Driven Development

 

Introduction to TDD (Test-Driven Development)

Test-Driven Development (TDD) is a software development methodology in which tests are written before writing the actual code. It follows a short, iterative cycle that ensures code correctness and maintainability.

Core Idea

• Instead of writing the program first and testing later, TDD flips the process: write a test → write code → refactor.

• It helps developers focus on requirements, reduces bugs, and encourages simple, clean design.

TDD Cycle (Red-Green-Refactor)

1. Red: Write a test for the next piece of functionality. Since the feature isn’t implemented yet, the test fails.

2. Green: Write the minimal code required to pass the test.

3. Refactor: Clean up the code while keeping the test passing. Remove duplication and improve design.

Benefits of TDD

• Higher code quality: Each feature is covered by automated tests.

• Early bug detection: Issues are found immediately when code is written.

• Better design: Encourages modular, loosely coupled code.

• Documentation: Tests serve as live documentation for how the system should behave.

Example (Simplified in C#)

// Test first
[Test]
public void Add_TwoNumbers_ReturnsSum()
{
    var result = Calculator.Add(2, 3);
    Assert.AreEqual(5, result);
}

// Minimal implementation to pass the test
public static class Calculator
{
    public static int Add(int a, int b) => a + b;
}

Best Practices

• Keep tests small and focused.

• Test only one behavior per test.

• Run tests frequently to catch errors early.

• Refactor code without breaking tests.