Test-driven development

Introduction

Test-driven development (TDD) is a software development process that relies on the repetition of a very short development cycle: first the developer writes an (initially failing) automated test case that defines a desired improvement or new function, then produces the minimum amount of code to pass that test, and finally refactors the new code to acceptable standards.

A developer working on a computer, writing code and running tests.

History

The concept of TDD originated from the Extreme Programming (XP) methodology, where it was called 'Test-First' concept. Kent Beck, who is credited with developing or 'rediscovering' the technique, stated, "I was feeling my way around for the simplest thing that could possibly work."

Principles

TDD is based on two main principles:

1. Write new business code only if an automated test has failed. 2. Eliminate duplication.

These two simple rules generate complex individual and group behavior with technical implications such as the following:

- It supports the idea of rapid feedback. - TDD encourages developers to think in small increments. - Developers learn to work comfortably and confidently with unimplemented or partially implemented code. - Because tests are written first, they can serve as a form of documentation.

Process

The TDD cycle involves the following stages:

1. Add a test: In test-driven development, each new feature begins with writing a test. This test must inevitably fail because it is written before the feature has been implemented. 2. Run all tests and see if the new one fails: This validates that the test harness is working correctly and that the new test does not mistakenly pass without requiring any new code. 3. Write some code: The next step is to write some code that will cause the test to pass. 4. Run tests: If all test cases now pass, the programmer can be confident that the new code meets the test requirements and does not break or degrade any existing features. 5. Refactor code: Now the code can be cleaned up as necessary. By re-running the test cases, the developer can be confident that code refactoring is not damaging any existing functionality.

Benefits

Test-driven development offers several distinct advantages, including:

- Improved quality of code: Since the tests are expected to pass at all times, this approach ensures that the system is extensively tested and high quality. - Simplified debugging process: If the code fails a test, it is easier to identify the issue since the changes made are only incremental. - Enhanced software design: Regular refactoring often leads to a better software design as it allows the developer to take small steps when required.

Criticisms

Despite its many benefits, TDD also has its critics. Some of the common criticisms include:

- Overemphasis on testing: Critics argue that TDD can lead to developers focusing too much on testing and not enough on other aspects of development. - Difficulty in applying to large systems: Critics also argue that TDD is difficult to apply to large systems, as it is hard to design and maintain a suite of tests that cover large areas of the system.