Refactoring Legacy JavaScript: Techniques for Modernizing Old Patterns and APIs

Legacy code is code that your project still uses, but which lacks tests and is poorly understood. It often depends on outdated APIs and doesn’t match your project’s current style requirements. The code may still be critical to your system’s operations, so the absence of test coverage means it’s risky to change.

JavaScript projects are susceptible to legacy code problems because of how far the language has evolved. Many projects began before JavaScript testing frameworks, module systems, and code quality linters were routinely used or available. The language’s dynamic and loosely typed nature also means it’s easy for developers to take shortcuts that affect maintainability.

In this article, we’ll explore key techniques for modernizing legacy JavaScript code. We’ll look at what causes legacy code, discuss common refactoring patterns, and then highlight some practical ways to improve your code using modern JavaScript features.

The Problems with Legacy JavaScript Code

Legacy JavaScript code is generally the code with missing tests that developers don’t want to touch. Besides this simple indicator, you can also spot legacy JavaScript by looking for areas of your codebase that meet one or more of the following criteria:

• The code lacks test coverage so can’t be safely changed.
• Outdated or unmaintained APIs and frameworks are used.
• Team members are unsure how the code works or what it does.
• The original developers have since left the team, leaving incomplete documentation.
• Modern language features like async/await, modules, or classes aren’t used, causing the code to be longer, slower, or more complex than it needs to be.

It’s easy to dismiss legacy code concerns even when these signs are detected. Developers may attempt to rationalize the code’s presence, arguing that if the code works then it doesn’t need to be changed. However, untested legacy code can have serious long-term consequences for your software delivery process:

• It’s harder to iterate upon changes due to the uncertainty over how legacy code works.
• Changes are more likely to introduce bugs and regressions due to missing test cases.
• Your code becomes flakier and more difficult to optimize when you’re dependent on undocumented or poorly maintained legacy code.
• Change lead times will increase because developers need to spend time learning how legacy code works, before building new features that interact with it.
• Depending on untested outdated code makes it more likely you’ll experience errors and incidents in live releases.

To avoid these issues, you must regularly audit your project’s codebase to identify areas of legacy code. If developers don’t feel comfortable editing a particular file or feature, then it’s time to take action before the problem gets worse. Here’s how to clean up legacy code by refactoring it to modern standards.

How to Refactor Legacy JavaScript Code

Refactoring is the process of gradually improving code to address quality, performance, and maintainability issues. It aims to solve legacy code problems without affecting the functionality of the refactored code. For example, JavaScript functions should still produce the same output after they’ve been refactored, but may have a cleaner interface, internal performance improvements, and more comprehensive tests and documentation.

Refactoring workflows should be based on the following high-level steps.

1. Understand the Code to Be Refactored

You can’t refactor until you understand what the code’s meant to do. Traditionally, you had to read code to learn what it does, but now modern generative AI tools like Qodo Gen can provide the context directly in your IDE.

2. Write Unit Tests for Your Existing Code

Covering your code with a comprehensive test suite is crucial so you can check it still outputs the same results after you finish refactoring. This prevents the refactoring process from introducing new bugs into your program. You can use Qodo Gen to automate test case creation based on how the code is used in your project.

3. Rewrite the Code to Improve Readability and Performance

Now’s the time to actually refactor your code. Try to remove complexity and improve performance by upgrading to new language features, replacing old libraries, and applying your team’s current coding standards. Just don’t change the code’s external interface—its inputs and outputs—to avoid having to update the places where it’s called.

Qodo Gen can help you refactor by automatically applying your style guide and suggesting common JavaScript best practices.

4. Use Your Test Suite to Validate the Refactored Code Still Works Correctly

Complete the refactoring process by running your test suite to check the code still outputs the same results. If all the test cases pass, then you’ve successfully modernized your legacy code without affecting the rest of your codebase.

Key JavaScript Refactoring Patterns and Techniques

Now we’ve covered the basic legacy code refactoring process, let’s delve into the key techniques and best practices to follow when modernizing your JavaScript code. These simple patterns will make your code more maintainable so you can deliver new changes faster with improved reliability.

1. Keep Repository Files and Folders Organized

Code maintainability starts with files and folders staying organized. It should be clear what each file contains and how it connects to others. This enables developers to quickly find the code they need. Splitting long source code up into separate files that are organized into subdirectories makes code much more approachable for developers.

Many teams organize source files based on the type of code they contain, such as by having folders for controllers/, models/ and views/. This helps indicate how your code is structured, but requires moving between multiple top-level folders when working on a change. Feature-based hierarchies are a popular alternative: under this model, all the files related to a particular feature are stored together in folders, such as users/, tasks/, and projects/. This lets developers easily find all the code associated with a particular feature.

2. Use a Consistent Naming Structure

How you name your code has an outsized impact on its readability and maintainability. Ensure all your source files, classes, functions, and variables are clearly named so it’s obvious what your code is doing. Use IDE tools to rename confusing code symbols as you find them—this will gradually make your code less daunting.

Try to minimize repetition of names to avoid ambiguity. For instance, if you have files called users.js and tasks.js, each containing a fetch() function, you should consider distinguishing the functions by renaming them as fetchUsers() and fetchTasks(). This makes the code’s purpose clearer when it’s called. Disambiguation also simplifies future refactoring: If you later change the fetchUsers() function’s interface, you can easily find and replace all its calls throughout your codebase, without the risk of calls to fetchTasks() also being included in the search results.

3. Use JavaScript Modules to Separate Code

Grouping related sections of code into modules improves your project’s structure. This is a key contributor to maintainability. Separating long functions into smaller ones spread across clearly named modules generally makes it easier to read and understand the code.

Effective modularization also promotes code reuse and can unlock performance optimizations. Modules don’t always need to be loaded until they’re needed, so there’s less code to load at runtime. JavaScript modules are now broadly supported in both browser and server-based runtimes.

4. Adopt TypeScript to Improve Type Safety and Access Advanced Language Features

TypeScript is a superset of JavaScript. It supports strong type definitions and advanced syntax features that enhance the language’s usability. Refactoring legacy code to use TypeScript can reveal many common bugs and issues before they’re experienced by users. Your TypeScript will fail to compile back to JavaScript if an invalid operation is called on a type.

It’s possible to combine sections of TypeScript and regular JavaScript within the same project. This enables gradual TypeScript adoption as you refactor different sections of your source.

5. Eliminate Unused or Outdated Dependencies

Legacy JavaScript frequently depends on outdated libraries that aren’t used elsewhere in your stack. Too many dependencies can increase code complexity, especially if they’re designed for coding patterns you’ve moved away from in newer work.

Each dependency is also a possible source of bugs and security issues. Refactoring code to use modern libraries and APIs allows your team to align on consistent engineering solutions, making code more maintainable.

6. Continually Write Unit Tests to Prevent Regressions

Writing unit tests is a key part of the refactoring process. As discussed above, good test coverage allows you to safely refactor code without affecting other parts of your project. Test creation can also lead you towards new opportunities to refactor and fix your code. It highlights when functions don’t work as expected, enabling you to prioritize them for improvement.

Traditionally, tests could be time-consuming and difficult to write, but nowadays they’re one of the quickest ways to improve existing code. You can use generative AI to rapidly create test cases that are relevant to how the code’s used in your project.

7. Start with Localized Refactoring, then Scale Up

Refactoring is best approached as an iterative effort. Don’t try to refactor your whole codebase in one go—this is typically a large-scale undertaking that prevents you delivering new code for an extended period of time. It’s more efficient to identify specific areas of legacy code that need refactoring, then apply small incremental improvements that steadily improve overall code quality. This way, you can ensure your project remains functional while you refactor.

8. Remove Dead Code

Dead code is often found within sections of legacy code. It’s redundant code that’s no longer called anywhere in your program. Dead code can make your codebase feel larger and more complicated than it actually is.

Removing dead code as you refactor is an easy way to improve your project’s maintainability. Because the code’s already unused, there’s no risk of new problems being introduced.

9. Embrace New JavaScript Features

Adopting modern JavaScript language features often makes code shorter and more readable. It can even improve performance, giving you multiple benefits from one set of changes. Newer JavaScript syntax simplifies many common patterns found in real-world applications, such as waiting for async calls and performing array operations. We’ll take a closer look at some key features below.

10. Use Decoratorsto Temporarily Isolate Legacy Components

Parts of legacy code may sometimes prove too difficult or time consuming to refactor straightaway. You can isolate these sections by “decorating” them in a thin new interface that’s exposed through the original one. This lets you make incremental improvements while temporarily retaining the old code.

For instance, you may have a function similar to the following:

function getUsers(Status) {
    if (Status === "Active") {
        return getActiveUsers();
    }
    else if (Status === "Archived") {
        return getArchivedUsers();
    }
    else if (Status === "Suspended") {
        return getSuspendedUsers();
    }
    else {
        return getAllUsers();
    }
}

This function’s code is repetitive and may benefit from refactoring, but you might not have time to immediately complete the work. Moving the code into a separate legacy module, then wrapping it with the existing interface as a decorator, lets you clearly separate the legacy and maintained sections.

function getUsersLegacy(Status) {
    if (Status === "Active") {
        return getActiveUsers();
    }
    else if (Status === "Archived") {
        return getArchivedUsers();
    }
    else if (Status === "Suspended") {
        return getSuspendedUsers();
    }
    else {
        return getAllUsers();
    }
}

function getUsers(Status) {
return getUsersLegacy(Status);
}

You can now start implementing a new version of getUsers() while retaining the old code as a fallback.

Recap: Iterate, Test, and Automate to Effectively Refactor JavaScript Code

Legacy JavaScript code makes it harder to maintain your projects. Untested code that uses old JavaScript patterns, includes outdated dependencies, or is simply confusing to read slows down the development process. Relying on old code also increases the risk of errors and makes it harder to audit compliance.

Incremental refactoring and testing is the best way to deal with legacy code. Aim to make gradual improvements using the techniques discussed in this guide, ensuring you prioritize unit tests before and after refactoring.

Remember that you don’t need to refactor by hand: you can use Qodo Gen to automate your refactoring process and produce quality JavaScript quicker. Qodo Gen can explain how your code works, write new test cases, and generate refactors that fully align with your team’s standards. Get started with Qodo Gen today, or book a demo to learn more.