How do you deal with legacy code that has little or no unit tests?

1 Assess the situation

Before you dive into the code, you need to understand the context, the purpose, and the dependencies of the legacy code. What is the expected behavior and output of the code? What are the inputs and outputs of each module or function? How does the code interact with other components or systems? Are there any known bugs or issues? Use tools like code analysis, code coverage, or code documentation to get a better picture of the code structure, quality, and complexity.

2 Write tests for critical parts

One of the main benefits of unit tests is that they help you verify that the code works as intended and that it does not break when you make changes. However, writing unit tests for legacy code can be difficult, time-consuming, and sometimes impossible, depending on the code design and testability. Therefore, you need to prioritize and focus on writing tests for the most critical, complex, or frequently used parts of the code. Use tools like test frameworks, test runners, or test generators to help you write and run tests efficiently and effectively.

3 Refactor the code gradually

Refactoring is the process of improving the code quality, readability, and maintainability without changing its functionality. Refactoring can help you reduce technical debt, fix bugs, and make the code easier to test and extend. However, refactoring legacy code can be risky, as you may introduce new bugs or break existing functionality. Therefore, you need to refactor the code gradually, in small and manageable steps, and always run your tests before and after each refactoring. Use tools like refactoring tools, code linters, or code formatters to help you refactor the code consistently and safely.

4 Follow coding standards and best practices

Legacy code often suffers from inconsistent, unclear, or outdated coding standards and best practices. This can make the code hard to read, understand, and modify. To avoid adding more confusion and complexity to the legacy code, you need to follow coding standards and best practices that are relevant, agreed upon, and enforced in your project or organization. Use tools like code style guides, code reviews, or code quality metrics to help you follow and improve coding standards and best practices.

6 Learn from the legacy code

Legacy code can be a valuable source of learning and improvement for your debugging skills. You can learn from the mistakes, successes, or trade-offs of previous developers, and apply them to your current or future projects. You can also learn from the feedback, suggestions, or questions of your peers or users, and use them to enhance your debugging skills. Use tools like code analysis, code metrics, or code reviews to help you learn from the legacy code.

7 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?