Unit Testing Vs Regression Testing

Ramy Loaiza , Principal Testing Manager, Testlio

March 11th, 2025

Software is increasingly viewed as a product, and user experience plays a significant role in that perception, impacting brand equity and loyalty. 

Let’s suppose you’ve developed a new feature for an app. You’ve written the code and tested it thoroughly, and everything works perfectly. But when you merge it into the main codebase, something breaks; maybe an old feature that was working fine previously is now malfunctioning.  

Bugs like these are not only frustrating but can derail the entire release. That is why developers use two types of testing: unit and regression testing. Unit testing determines if individual components function properly, while regression testing ensures that new changes do not disrupt current functionality.

Unit and regression testing are important in software development because they ensure code integrity. But while they both are used to improve software quality, they address different challenges within the software development lifecycle (SDLC).

Here’s a brief comparison of the differences between unit testing and regression testing.

This article will discuss the similarities and key differences between unit and regression testing, along with scenarios about when to use each. 

Differences Between Unit & Regression Testing

Unit and regression testing improve software quality by focusing on different areas of the development process. Here’s how they compare.

1. Purpose/ Test Objectives

• Unit testing splits and tests individual components of code. Developers write unit tests to check that certain functions or methods perform as intended before they are integrated into the rest of the system.

• Regression testing takes a broader approach. It ensures that new updates or bug fixes haven’t unintentionally broken existing functionality. Even if a new feature works perfectly in isolation, it might still disrupt other software parts. Regression tests help catch these unintended side effects.

2. Scope/ Code Coverage

• Unit tests operate at the smallest level – single functions, methods, or classes. They check if a particular code unit works correctly in all possible scenarios.

• Regression tests cover multiple components and their interactions, often spanning entire modules or systems. They verify that existing functionality remains intact after changes are made.

Example

Suppose you’re building an e-commerce app. A unit test might check if the Add to Cart function correctly updates the cart total. 

Conversely, a regression test would verify that after introducing a new payment gateway, the checkout process, order confirmation, and cart functionality all work as expected.

3. Test Execution

• Software developers execute unit tests frequently, often as part of the coding process. This helps them catch issues early, even before the application is fully built.

• Regression tests are triggered whenever code is modified because of a bug fix or a new feature update. They ensure that past functionality still works as expected. 

Example

Suppose you’re developing a ride-hailing app. Unit tests will ensure the fare calculation logic works correctly under different conditions, such as varying distances, surge pricing, and promo codes.

Later, regression tests will confirm that changes to the fare calculation don’t break other critical functions, like ride booking, payment processing, or driver earnings calculations

4. Automation

• Unit tests are almost always automated. Since they deal with small, isolated functions, they are easy to run and provide instant feedback.

• Regression tests can be either manual or automated. While automated regression testing is preferred for repetitive test cases, some aspects, like user interface (UI) testing, may still require manual validation.

5. Impact on Software Quality

Both types of testing contribute to software reliability, but in different ways:

• Unit testing improves code quality at the foundational level by catching issues early and ensuring individual components work correctly.

• Regression testing prevents unexpected failures in a live system, ensuring changes don’t introduce new problems.

Unit testing and regression testing work together. A strong testing strategy includes both types, as preventing a minor bug early in software development can save major headaches later.

When to Use Each

Unit and regression testing are integral in software development, but knowing when to use each ensures efficiency and reliability. Here’s when to apply them:

Unit Testing

Unit testing is best done during development when a new function, method, or module is written. It helps catch problems early on before they escalate into substantial defects.

When to use it:

• Writing new functions or modules.
• Fixing a bug to make sure it doesn’t come back.
• Refactoring code to improve performance without breaking anything.
• Following test-driven development (TDD), where tests guide how the code is written.

Regression Testing

Regression testing is necessary when the codebase changes, whether through new features, bug fixes, or optimizations. It ensures that these updates integrate well into the existing functionality without any disruption and damage.

When to use it: 

• Adding new features or enhancements.
• Fixing bugs that could impact other areas of the application.
• Updating dependencies, frameworks, or libraries.
• Preparing for a new software release to ensure overall stability.

The Complementary Roles of Unit Testing & Regression Testing

Unit and regression testing complement each other as they help maintain software quality at different stages of development.

Here’s how they work together to maintain software quality:

• Building new features
  Unit tests confirm that newly written code works as expected. But new features don’t exist in isolation so regression tests are used to ensure they integrate smoothly without breaking other parts of the system.
• Fixing bugs
  A bug fix should solve the problem, not create new ones. Unit tests ensure the fix works as expected, while regression tests verify that nothing else in the system is affected.
• Refactoring code
  Cleaning up messy code or optimizing performance is necessary for long-term maintainability. Unit tests ensure the updated code still behaves as intended, and regression tests catch unintended side effects.

In a nutshell, unit testing catches errors early, preventing faulty code from moving forward. However, even if each component works fine, integration issues can arise. Regression testing checks the system, ensuring stability as the software evolves.

Skipping either type of testing creates risk. Unit tests alone won’t catch integration issues, and regression tests without unit tests could miss critical bugs within components. They provide a balanced testing strategy that keeps software reliable and stable at every stage.

Final Thoughts

Thorough testing is the backbone of reliable software. It doesn’t just help catch bugs but maintains performance, stability, and user trust as your application evolves. Unit tests help developers build with confidence, while regression tests provide a safeguard against unintended disruptions. Both these tests keep software functioning smoothly, no matter how complex.

But ensuring high-quality, scalable testing isn’t always straightforward. It requires the right expertise, flexibility, and a process that adapts to your needs. That is exactly what Testlio offers.

With Testlio, you get a tailored approach to software testing, whether it’s unit or regression testing. Our expert-led testing solutions integrate seamlessly with your workflows, helping you catch issues early, maintain software performance, and deliver exceptional user experiences.

Partner with Testlio to enhance your testing strategy and launch your applications with confidence. Contact sales to see how we can support your testing endeavors.