The importance of testing legacy code: try unit testing

Are you struggling with a legacy project that's swarming with bugs? Let us give you an insight into how adding unit tests can make sure these errors bother you no more.

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What are the risks of not testing legacy code?

Not testing legacy code can cause several risks and consequences I’m sure you’ve experienced. The most well-known downside is increased maintenance: the legacy code is often difficult to maintain. Beyond that, you run the risk of:

  • Increased number of bugs and errors: Without unit tests, finding and fixing bugs and errors in the legacy code is difficult.

  • Reduced code quality: Unit tests can help ensure that legacy code meets expected behaviors and standards, reducing overall code quality.

  • Slow development: Without unit testing, developers may need to spend more time manually testing code changes and ensuring that the application still works as expected.

  • Increased technical costs: The longer legacy code goes without unit tests, the harder it becomes to add them in the future. This can lead to technical debt and make it more challenging to maintain and improve code over time.

Methods for testing the legacy code

1. Use test-driven development (TDD)

As we already mentioned, one way to add tests to legacy code is to use Test-Driven Development (TDD). TDD requires writing tests before writing any codes. 

Here are the basic steps to follow:

  • Execute tests that fail: The first step is to write tests that will fail because the code has not yet been written.

  • Write the minimum code required to pass the test: Once the test is written, the next step is to write the minimum code required to pass the test. It should be as simple as possible and not include any unnecessary features or functions.

  • Refactor the code: Once the code passes the test, it should be refactored to make it more maintainable and easier to understand.

  • Repeat: Administer a failed test, write the minimum code to pass it, and then redesign it for each new feature that needs to be added to the software.

2. Identify edge cases

To identify edge cases, consider whether input values will likely cause problems, such as minimum and maximum values allowed for input parameters. For example, if you are testing a function that calculates the square root of a number, you will want to test with different values of the input, including: 

  • The smallest allowed input value
  • The largest allowed input value
  • Values just above and below these limits
  • Negative values
  • Zero
  • Non-numeric input values

By testing the implementation with these edge cases, you can ensure that it is successful in all situations rather than just under typical use scenarios.

3.  Write the first test

The first step in adding tests to existing code is to identify the code that needs to be tested. The best way is to start by testing the most important parts of the application. Once you know which code to test, the next step is to write the tests.

In this example, we will use Vitest and Vue test utils to test a Vue component. Let’s assume we have a component called “LoginForm” that has a form with two input fields (username and password). Related to that, there is a component method called “submitForm” that is triggered when the user clicks the submit button.

import { mount } from '@vue/test-utils'
import LoginForm from '../../src/components/forms/LoginForm.vue'

describe('LoginForm', () => {
   it('Renders correctly', () => {
       const wrapper = mount(LoginForm)
   it('Submits the form when the submit button is clicked', () => {
       const wrapper = mount(LoginForm)
       const usernameInput = wrapper.find('.username')
       const passwordInput = wrapper.find('.password')
       const submitButton = wrapper.find('button')



What did we actually test? The first test checks if the component renders correctly. The second test sets the username and password values, triggers a click action on the submit button, and then checks whether the submitForm action is issued.

Benefits of adding unit tests to legacy code

Before diving into adding tests to the code, let’s first look at the benefits of unit testing. Unit testing helps ensure that your code works as intended and continues to work even after changes. By writing tests, you can catch bugs early in the development process, improve code quality, increase your confidence when making changes, refactor easier, and debug faster.

What problems/challenges will you face when integrating unit tests on a legacy code?

Lack of documentation

Lack of documentation in legacy projects can be challenging, especially for new team members, because they need to understand the project, maintain and improve it, or even fix errors.

However, there are steps you can take to address these issues better when using unit tests:

1. Start with high-impact areas: Start by identifying the most important or frequently used parts of the codebase. First, focus your efforts on writing tests for these areas, as they are likely to have a significant impact on the overall system functionality.

2. Use Test-Driven Development (TDD): Consider using a test-driven development approach, where you write tests before you change the code. Regardless of the code already written, basic tests should be written first. This gives your code access and direction to make the necessary changes. Over time, this process can also become living documentation.

3. Collaborate with team members:  If you’re lucky enough to work with seasoned developers familiar with the project, consult with them. They may have insight, knowledge, or informal documentation that can help you understand specific parts of the code.

4. Writing test notes: As you write unit tests, make sure to document them. Provide clear descriptions of the test cases, outlining the input values, expected outputs, etc.

5. Gradually improve documentation: As you add unit tests, take the opportunity to improve the documentation. This can contribute to the overall documentation of the project over time.

Outdated libraries

It is common to encounter outdated libraries when adding unit tests to legacy projects. There are many things you can do to deal with this situation:

1. Assess the impact - See if old libraries will cause problems when you add unit tests. If libraries are important resources and updating them can make a difference, you may need to rethink your approach. In some cases, optimizing the libraries can be a priority before adding tests.

2. Isolate dependencies: If upgrading libraries is impractical or too time-consuming, consider isolating dependencies for testing purposes. You can mock test cases or separate modules that execute functionality provided by older libraries. This way, you can write tests directly without relying on older versions.

3. Try step-by-step upgrading: If you need to update the libraries for a long time but not immediately, break the upgrade process into small steps that you can follow so you can update the libraries and make sure you have tests for every step to notice any possible issues.

4. Check compatibility - Review documentation, release notes, and online resources for the old library to ensure compatibility with new testing frameworks or tools. Sometimes, the old library is still compatible with the latest testing framework, allowing you to write and run tests without having to optimize them.

5. Refactor as needed: In some cases, older libraries can experience significant code changes. If this significantly interferes with writing tests or maintaining the codebase, you may want to consider refactoring the affected parts of the code to a new library version or find alternative solutions, e.g., write your own code that was covered by an outdated library or find a different library that is up to date.

Poorly structured code

In legacy projects, you may encounter poorly structured code, so adding unit tests can be a challenge. However, there are a few steps you can take to address this issue and ensure proper code testing.

1. Understand the existing code: Take some time to fully understand the existing code. Identify various components, dependencies, and interactions. This will help you identify areas that need improvement and refactor the code to increase its testability.

2. Break down complex functions: If you’ve got large, complex functions, try breaking them down into smaller, manageable functions. This will make it easier to test each part of the functionality individually and improve the overall structure of the code.

3. Write detailed tests: When adding unit tests, aim to cover as much of the code as possible. Focus on writing tests that cover different scenarios and edge cases. This gives you confidence in the existing code and allows you to catch any problems that may arise from refactoring.

Lack of testable code

Adding unit testing to a project that lacks testable code can be a daunting task. There are a few ways you can overcome this situation:

1. Refactor the code: Start by refactoring the legacy code to make it more modular and testable. Identify areas of the codebase that can be broken down into smaller pieces and extract them into separate functions.

2. Start with integration tests: If the codebase is difficult to test, consider starting with integration tests instead. Integration tests focus on checking the connectivity between components or modules. While they don’t provide the same level of isolation as unit tests, they can catch bugs and provide valuable feedback.

3. Legacy code quality testing: In some cases, it can be time-consuming to write complete unit tests for the entire legacy codebase. Instead, you can create a test that uses existing code methods without major changes. These tests act as a safety net to ensure that the behavior of the code remains consistent as you make changes.

Remember, the goal is to slowly improve test coverage over time. It is not always possible or necessary to have full coverage immediately, especially in complex legacy projects. Focus on critical and high-risk areas first, and continuously refactor and write tests as you make changes to the codebase.


Adding tests to legacy code can be a challenging task, but it’s an important process to ensure your code is correct. By identifying units you can test, writing effective test cases, using TDD, and iterating through the refactoring and testing process, you can improve the quality of your legacy code and reduce the risk of bugs.

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