How can you improve legacy code functionality?

1 Understand the code

Before you can improve legacy code functionality, you need to understand what it does, how it works, and why it was written that way. This may require reading documentation, comments, commit messages, or talking to other developers who worked on the code. You also need to identify the dependencies, inputs, outputs, and side effects of the code, as well as the business logic and requirements behind it. Understanding the code will help you avoid making unnecessary or harmful changes, and also help you find opportunities for improvement.

2 Refactor the code

Refactoring is a process of improving the structure, readability, and maintainability of code without changing its behavior. It can help you improve legacy code functionality by making it easier to understand, modify, and reuse. Functional programming can be beneficial when employing refactoring techniques such as extracting functions, eliminating global variables, using pure functions, and applying higher-order functions. Extracting functions breaks down large or complex functions into smaller and simpler ones that perform a single task and have clear names and parameters. Eliminating global variables replaces them with local ones or passes them as arguments to functions to avoid unwanted side effects and improve testability. Pure functions are those that do not depend on or modify any external state, and they always return the same output for the same input to increase predictability and reliability. Lastly, higher-order functions are those that can take other functions as arguments or return them as output, creating more abstract and modular code that can handle different scenarios and data types.

3 Test the code

Testing is the process of verifying that the code works as expected and meets the specifications and quality standards. This process can help you improve legacy code functionality by finding and fixing errors, bugs, or vulnerabilities, as well as ensuring that your changes do not introduce new problems. Common testing techniques that can benefit from functional programming include writing unit tests with frameworks such as Jest, Mocha, or Jasmine; integration tests with frameworks such as Cypress, Selenium, or Puppeteer; and property-based tests with frameworks such as QuickCheck, FastCheck, or JSVerify. These tests range from small and isolated functions to larger and more comprehensive ones that interact with each other or external systems.

5 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?