What is the Legacy Code? Definition and Best Practices

Marlena Walburg

Legacy code is a subject that evokes a lot of emotions, often those negative. Almost every developer will confront it in their work and will have to deal with it in some project. The very definition of legacy code also seems to be not clear. Is it just an old code, or is it inherited from someone? Today we explain what legacy code is and how to work with it effectively.

What is legacy code?

Legacy code is source code from an older, unsupported, or out-of-production version of an operating system or other types of technology. It can also mean code inserted into modern software to maintain an older or previously supported function. It is also referred to among the software development community that legacy code also means source code inherited from someone else and source code inherited from an older version of the software. Additionally, Michael Feathers introduced the definition of legacy code as untested code. It shows that the legacy code is difficult to handle in part due to the lack of automated regression testing.

A big part of software engineers’ work is regularly changing and improving the code. It is because it can become legacy code very quickly. The reason is simple, most of the source code is somewhat dependent on the platform for which it is designed. Even if the developer uses a platform-independent programming language, it is difficult to write an extensive, useful program that is completely environment-independent. As soon as a new platform update is released, which is now a popular and welcome practice, the code may stop working without the new changes introduced and become legacy code. 

How to work with legacy codes?

When you start working with legacy codes and want to understand them, it is helpful to create characterization and unit tests. You can also run a static analyzer in code to identify potential problems. This way, you will see what this code actually does and what problem areas need to be changed. The fact is that legacy code is not designed to be tested automatically, so testing can prove to be a challenge. 

To understand the code, review the documentation, from which you will learn what the original system requirements were. It will make code improvement easier and safer. Take care of the clean, transparent code from the beginning of your work. 

A good practice is refactoring, which allows you to improve the structure of the code without changing its functionality. It is an alternative to rewriting code that should only be used when necessary. Rewriting everything is too time-consuming, error-prone, and risks losing functionality. Also, don’t make too many changes at once and keep an eye on the results.

How can you get rid of legacy code?

Software that is free from legacy code is the one that has just been created or that does not serve any business. After all, specific software exists to support particular business needs and change depending on them. So as you develop your software for a while, the probability of encountering legacy code is high. It does not mean anything wrong, but rather signals that you are scaling something that has been on the market for a while, is needed, and is doing well but needs some new changes. 

When you encounter legacy code in your software, you can safely assume that it has already been tested in a production environment. Usually, this means that these functions are stable and solve the specific problem as stated in the first requirements.

It is recommended that you avoid maintaining this code and consider migrating to a new service if new requirements arise. The legacy code will therefore be isolated from the rest.

It is also worth considering the value of the legacy code. If this is a specific feature that no longer benefits or even consumes maintenance income, retirement may be a solution. Set a retirement date for this feature, and then remove any code that is no longer in use. Otherwise, your application code will grow to gargantuan sizes, causing more and more problems with understanding and maintenance.

Why can legacy code be challenging?

There are many reasons why legacy code is something unpleasant or difficult to work with for developers. 

One of the biggest problems is that you cannot use automated tests to check your code for errors or bugs. Many legacy codes are simply not designed to make this process easier. Moreover, it is usually a spaghetti code, in which it is very complicated or even impossible to isolate small parts for unit testing.

The aforementioned spaghettification of the code is also a frequent phenomenon and a challenge in working with legacy code. If a piece of code is maintained, updated, and edited by many different developers over many years, after time, it can become increasingly difficult to understand and modify. At some point, it turns out that the required code updates or modifications will be impossible to implement, as no one will understand it or its functionality. Probably the best thing a programmer can do if they suddenly encounter an old code runoff while working on a new project is to refactor the code.

Benefits of legacy code

Working with legacy code can learn a lot. Don’t think negatively about it since such a piece of software may have been doing a great job in the past. Legacy code can also bring you some benefits: 

  • Understanding the business side of your software – although your users may not be interested in what technology the software they use is based on and may not even understand it, you should be aware of it. After all, you get paid for it. So, since older software earns enough money for some reason, its maintenance and service are justified.
  • Understanding the technical side of the company’s software – legacy code also helps you to see how transparent, well-written codebases can affect your business. It shows what a given technology looked like at the time of software development and how future-proof it was.
  • Teaches good practices – very often, working on the legacy code is arduous. It may happen that such code is not only outdated but also not very clear and complicated. While refactoring or rewriting will be a challenge, it gives food for thought that will make the developer avoid certain practices in the future and develop good ones.

What does refactor code mean?

Code refactoring is a process of adding changes that essentially do not change functionality. Its purpose is to maintain an appropriate, high-quality system structure, not to create new functionality. As a part of refactoring, specific actions are taken, such as modifying system elements to fit them into the adopted standards and patterns or searching for new standards and patterns that appeared in the system during its development and their precise definition.  

Before the actual refactoring occurs, developers first need to understand what each piece of code does. Besides, tests must be performed to check all functions. Only when the operation of the code is clear can the actual refactoring begin. Unnecessary code is removed or rewritten, classes and variables are simplified, customized command methods. The result is cleaner, easier to understand, maintain and customize code. 

Tools for working effectively with legacy code

There is a considerable chance that every developer will encounter legacy code in their career. There are several tools and methods available to help them work with it effectively. Speaking of analysis, one way to do that is to use the static code analysis tool. Static source code analysis is the one performed without running it. You can examine a specific piece of code, such as a function or method, the interactions between different program units written in the same programming language, or the interactions between all modules, without being limited to one particular technology or programming language. This will help you spot problems at each level, depending on your needs.  

There are many tools for static code analysis available on the market. Depending on the programming language or technology used, you have a wide choice of them. Such programs can check your code for rules, usually from an encoding standard, which is often recommended. By getting a diagnosis of violations and prioritizing them, you can first focus on fixing the most severe errors. You can also set your codebase as a baseline that will not get moved into diagnostics. It is an excellent solution if you are sure about a code snippet or just don’t need to check it out. Instead, you can focus on finding problems in the new code and making sure it is clean. There are also static code analyzers that check for encoding standards. If you want to reuse code from one project to another, you will need to analyze it to exclude problems and errors in the code resulting from different standards.


 Although legacy code gives some developers shiver down the back, good practices, the right approach, and tools can make it much easier to manage it. It can be an opportunity for developers to develop valuable skills, so it is worth treating legacy code with a good attitude and not being afraid of it.

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