Test-Driven Development

What is Test-Driven Development? What’s the Point?

The short answer: Test-driven development is a software development methodology where tests are written before the actual code. As an iterative process, it utilizes the repetition of short development cycles. 

Test-driven development (TDD) is a software development methodology that focuses on an iterative development cycle where test cases are written before the actual feature or function. Combining building and testing, TDD helps to ensure that code is always tested against a wide range of scenarios to ensure code quality and reduce bugs. 

The TDD cycle is composed of five stages:

  1. Add a test to the test suite.
  2. Run all the tests to ensure the new test fails.
  3. Write a minimal amount of code to pass the test. 
  4. Run all tests again.
  5. Improve the initial code. 

The cycle is then repeated until project completion. 

Test-Driven Development Approaches

There are two primary approaches to TDD:

  • Inside Out: With this approach, testing begins at the smallest unit level and the architecture emerges organically. It’s a better approach for those who are unfamiliar with the TDD concept because it minimizes mocking and prevents over-engineering. 


  • Outside In: This approach focuses on user behavior. Testing begins at the outermost level and details emerge as teams work inwards. As a result, this approach relies heavily on mocking and stubbing external dependencies. 

What’s the Point of Test-Driven Development? 

That’s the question that most people ask when they hear about TDD for the first time. After all, it sounds slow and counterintuitive. 

While it’s true that TDD can be slow in the short term, it plays a fundamental role in improving a project’s long-term success because it ensures there’s adequate test coverage in place. That means that if some functionality is inadvertently changed or broken, it can be caught and remedied earlier.

The iterative approach of TDD also helps to:

Ensure code quality: By writing tests before code, dev teams ensure that individual function is thoroughly tested from the outset, leading to more reliable and maintainable code.

For example, a dev team working on an authentication system may write test cases for scenarios such as failed logins or account lockouts. Only after these tests are written do they implement the actual login functionality.

Facilitate refactoring: TDD makes it easier for dev teams to refactor code with confidence. They know that any changes they make are covered by tests that will catch any regressions or unexpected behaviors.

For example, a dev team might want to improve the performance of a data processing function. With TDD, they can refactor the function safely in the knowledge that their test suite will alert them if any existing functionality breaks.

Drive better Design: TDD encourages better design decisions by forcing developers to consider the use cases and interface of their code before its implementation.

For example, a dev team adding a new feature to a software application can use TDD to define how the feature should behave from a user’s perspective, leading to a clearer and more user-focused design.

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