What is TDD?

Test-Driven Development (TDD) is a way of writing software where you first write a small test that describes what you want the code to do, then you write just enough code to make that test pass, and finally you clean up the code. This cycle repeats for each new feature or bug fix.

Let's break it down

  • Test-Driven Development (TDD): a method of building programs by starting with tests.
  • Write a small test: create a simple check that will fail until the code works.
  • Describe what you want the code to do: the test spells out the expected behavior.
  • Write just enough code: add the minimum code needed to make the test succeed.
  • Make the test pass: run the test; it should now succeed.
  • Clean up the code: improve the code’s structure without changing its behavior (refactor).
  • Repeat: do the same steps for the next piece of functionality.

Why does it matter?

TDD helps catch bugs early, gives you confidence that your code works as intended, and leads to cleaner, more maintainable software. It also makes it easier to change or add features later because you have a safety net of tests.

Where is it used?

  • Building web applications (e.g., a shopping cart feature in an e-commerce site).
  • Developing mobile apps (e.g., ensuring a login screen correctly validates credentials).
  • Creating libraries or APIs (e.g., a data-processing library that must return accurate results).
  • Writing embedded or IoT firmware where reliability is critical (e.g., sensor data collection code).

Good things about it

  • Finds defects early, reducing costly debugging later.
  • Produces a comprehensive suite of automated tests that serve as documentation.
  • Encourages small, focused code units, which are easier to understand and maintain.
  • Improves design by forcing you to think about how code will be used before writing it.
  • Increases confidence when refactoring or adding new features.

Not-so-good things

  • Requires extra time up front to write tests, which can feel slower for simple projects.
  • Can be hard to adopt for teams not used to writing tests or lacking testing expertise.
  • May lead to overly rigid tests that need frequent updates when requirements change.
  • Not all code is easily testable (e.g., UI-heavy or highly interactive components) without additional tooling.