How Test-driven development or TDD works? Explained with examples

Test-driven development


Test-driven development or TDD is a software development approach where tests are written before the code is even created. The process encourages simple designs and inspires confidence because developers know that their code meets the requirements defined by the tests. So the tests are written before the actual feature or functionality exists. This may sound a little backward, so I'll provide an example.

How TDD works? 

Imagine a mobile app designed to enhance security for various services like banking or secure entry systems. It does this by adding a facial recognition feature to verify users. The test you write first would check to see if the app can accurately verify a user's identity using facial recognition.

For example, the test might simulate submitting a user's photo and then checking to see if the app recognizes the person correctly or denies access if the face doesn't match the user's stored facial data. You've written the test for a new function or feature that hasn't been implemented yet, so this test should initially fail.


At this point, the feature itself doesn't exist. You would then implement the minimum amount of code needed for facial recognition. This might involve integrating with AI services or libraries that are capable of processing and recognizing facial features.

After implementing this basic functionality, you can run the test again. The test should now pass if your facial recognition setup correctly identifies the submitted faces, confirming that the feature works as intended.

Once the test passes, you can refactor your code to improve its efficiency and to ensure that it's bug-free. Of course, you'll need to make sure that it still passes the test each time. In the case of facial recognition, improved performance could mean that the program is better able to handle what's called edge cases, like facial recognition in poor lighting or with sunglasses on.

The cycle is repeated for each new feature or enhancement. For example, we could add a liveness detection to prevent spoofing with a photograph or enhancing our algorithm to recognize faces with varying expressions or accessories like the sunglasses I mentioned.

Test-driven development can lead to a more bug-resistant, stable development outcome because it promotes the writing of test for every small feature or functionality, building testing into the development process.

TDD vs Traditional testing:

If you've practiced traditional project management, you may be used to a different approach to quality and testing. Let's compare traditional testing and test-driven development to get an idea of how they differ. We'll start by comparing the timing of when the tests are created. In traditional testing, tests are written after the product is developed, whereas in TDD tests are written before the software code is developed.

Traditional testing focuses on identifying bugs and defects after development is complete. But TDD focuses on guiding software design and ensuring functionality from the start. In traditional testing, testing acts as a separate final phase for quality control. While in TDD, testing is integrated into the daily development workflow as a form of continuous quality assurance. The impact on software design is also notable.

Traditional testing has less influence on the initial design, and it impacts mostly through later revisions. TDD has a strong influence on software design, encouraging testing on a per-feature or component basis.

The nature of the feedback loop is also different. Traditional testing features longer feedback loops with testing separated from development phases, whereas TDD offers short and continuous feedback loops, integrating immediate testing.

There are key variables that can influence your choice between traditional testing and test-driven development or TDD. TDD is beneficial for complex projects requiring frequent refactoring and validation.

Traditional testing suits simpler, more straightforward projects. Agile methodologies align well with TDD because of their iterative nature, whereas traditional testing fits better with linear waterfall methodologies.

Teams who are skilled in Agile practices and rapid development cycles may prefer TDD, while teams accustomed to sequential development phases may opt for traditional testing. High stakeholder involvement with frequent feedback sessions favors TDD because it allows for iterative improvements and quick adaptation.

Conclusion on TDD:

TDD can mitigate risks early in high-stakes projects by identifying and resolving issues continuously. That can make it a preferable choice for projects where errors have significant implications. Projects with tight timelines may benefit from TDD due to its efficiency in early bug detection and fixing.

While traditional testing can be cost-effective for projects with limited budgets because of its less frequent integration of testing phases. Your approach to testing is an important decision, and TDD is a great option to consider if you find that your projects are becoming more agile in nature.

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