Effective Strategies for Introducing Unit Tests in a Legacy Codebase

Effective Strategies for Introducing Unit Tests in a Legacy Codebase

3 July 2024 Stephan Petzl Leave a comment QA

Transitioning to an agile environment often reveals technical debt, especially when there are no existing unit tests. This raises an important question: Should unit tests be retroactively added to catch up on technical debt? And if so, to what extent?

When to Add Unit Tests

One practical approach is to introduce unit tests when you are required to refactor a specific area of the legacy code. This method ensures that you are not overwhelmed by the task of writing tests for the entire codebase at once. Instead, you can gradually build a safety net around the parts of the code that are actively being worked on.

Targeting High-Impact Areas

Focus on areas of the code that are more likely to change in upcoming releases. This includes new features, refactoring efforts, or bug fixes. By targeting code with higher logical complexity, you can achieve a higher return on investment (ROI) as potential mistakes are more likely to be discovered early through unit testing, rather than during system or integration testing.

Practical Steps to Implement Unit Tests

  • Refactor with Tests: Write unit tests when refactoring legacy code. This ensures that the new code is covered by tests, providing a safety net for future changes.
  • Bug Fixes: When a bug is reported, write a regression test to demonstrate the bug. Then, write a unit test that defines the correct behavior and make sure both tests pass after fixing the bug.
  • Gradual Introduction: Implement unit tests gradually, prioritizing features based on their importance and likelihood of change.

Balancing Effort and Benefit

It’s crucial to balance the effort required to write unit tests with the benefits they provide. For critical parts of the application where failure is not an option, ensure comprehensive test coverage. For less critical areas, you might decide to forego extensive testing if the cost of failure is low and issues can be quickly resolved.


In summary, adding unit tests to a legacy codebase should be a strategic and gradual process. Focus on high-impact areas first and use refactoring and bug fixes as opportunities to introduce tests. This approach not only helps in catching up on technical debt but also ensures that your codebase remains maintainable and robust.

Automating Tests with Repeato

As you work towards improving your codebase’s test coverage, consider leveraging tools like Repeato, a no-code test automation tool for iOS and Android. Repeato can help you create, run, and maintain automated tests quickly and efficiently using computer vision and AI. This makes it an excellent choice for quality assurance, especially when dealing with legacy systems that require frequent updates and testing.

For more insights on effective test strategies, check out our blog and explore articles such as Transitioning from Manual to Automated Testing: A Comprehensive Guide.

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