Refactor Legacy Code Faster with GitHub Copilot

What This Tip Covers

This tip shows how to use GitHub Copilot to refactor and modernize legacy code in .NET (C#), Python, TypeScript, and JavaScript. You'll learn how to prompt Copilot for common refactoring tasks and best practices for reviewing AI-generated code.

Why It Matters

Legacy code can slow down development and introduce bugs. Copilot helps you quickly update old code, adopt modern patterns, and improve maintainability—saving time and reducing manual effort.

How to Use Copilot for Refactoring

.NET (C#)

In .NET projects, Copilot can help you refactor synchronous methods to use async/await, making your code more responsive and modern. It can also suggest improvements for old LINQ queries, recommend updates for deprecated APIs (such as moving from .NET Framework to .NET Core or .NET 9+), and offer ideas for better naming, error handling, and logic simplification. By describing your intent in a comment, Copilot will generate context-aware suggestions tailored to your codebase.

Example prompts:

// Refactor this method to use async/await and improve readability
// Update this code to use modern .NET 9+ APIs
// Simplify this LINQ query and add error handling

Python

For Python codebases, Copilot is especially useful when migrating legacy Python 2 code to Python 3. It can help you refactor nested loops into more concise list comprehensions, leverage built-in functions, and update your error handling to use modern exception syntax. If you have large, complex functions, Copilot can suggest ways to simplify them and improve variable naming for better readability and maintainability.

Example prompts:

# Refactor this function to use list comprehensions and handle exceptions
# Update this script for Python 3 compatibility
# Simplify this code and improve variable names

TypeScript

When working with TypeScript, Copilot can assist in migrating JavaScript code by adding type annotations and improving type safety. It can refactor callback-based code to use async/await, making asynchronous logic easier to follow. Copilot also helps simplify complex interfaces and generics, and can suggest ways to make your code more modular and readable.

Example prompts:

// Refactor this callback to use async/await and add type annotations
// Convert this JavaScript code to TypeScript
// Simplify this interface and improve type safety

JavaScript

In JavaScript projects, Copilot is great for modernizing ES5 code by introducing ES6+ features like arrow functions and destructuring. It can break up large functions into smaller, reusable ones, and recommend replacing var with let or const for better variable management. Copilot’s suggestions help you write clearer, more maintainable code that follows current best practices.

Example prompts:

// Refactor this function to use ES6 features and improve readability
// Break this function into smaller reusable functions
// Replace var with let/const and use arrow functions

Best Practices

• Always review Copilot’s suggestions for correctness, security, and style.
• Use version control to track and review changes.
• Combine Copilot’s output with your own expertise and team standards.
• Maintain a suite of unit tests and integration tests to verify that refactored code continues to work as expected. Automated tests help catch regressions and ensure that changes do not break existing functionality.

Benefits

• Accelerates modernization for any language.
• Reduces manual effort and helps enforce best practices.
• Makes legacy code easier to maintain and extend.

Refactoring with Copilot is like having a tech-savvy teammate—helping you make your code cleaner, faster, and ready for whatever comes next!