What is Test-Driven Development (TDD)?
Test-Driven Development is a software development practice where tests are written before the code they test.
Test-Driven Development is a software development practice where tests are written before the code they test. The TDD cycle is: Red (write a failing test), Green (write minimal code to pass the test), Refactor (improve the code while keeping tests passing).
TDD was popularized by Kent Beck and is a core practice of Extreme Programming (XP). It produces code with high test coverage by default, since every line of production code exists to make a test pass.
Proponents argue TDD produces better-designed code because writing tests first forces you to think about interfaces and behavior before implementation. Critics argue TDD slows initial development speed and that writing tests after (Test-After Development) achieves similar quality.
The data is mixed. Studies show TDD reduces defect rates by 40-80% compared to no testing, but the difference between TDD and Test-After is smaller (~20%). The real benefit may be behavioral: TDD practitioners write more tests, period.
Why It Matters
TDD is one of the most effective practices for preventing technical debt accumulation. By requiring tests before code, it ensures high coverage from the start. The cost of adding tests retroactively is 3-10x higher than writing them alongside the code.
Frequently Asked Questions
What is TDD?
Test-Driven Development is writing tests before writing the production code. The cycle is: write a failing test, write code to pass it, then refactor. It ensures high test coverage by default.
Does TDD slow development?
Initially yes — 15-20% slower. But TDD reduces debugging time by 40-80%, so total time to working software is often shorter.
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