Regression testing and unit testing both help catch bugs early and maintain code stability across development cycles.
Overview
What is Regression Testing?
Regression testing is the process of re-running test cases to ensure that recent code changes haven’t unintentionally broken existing functionality. It validates that updates, bug fixes, or enhancements do not impact the rest of the application.
What is Unit Testing?
Unit testing involves testing individual components or functions of code in isolation to verify that each part works as expected. It is typically performed during development and forms the first layer of automated testing.
Key Differences Between Regression Testing and Unit Testing
Here are the core distinctions between the two:
| Aspect | Regression Testing | Unit Testing |
|---|---|---|
| Scope | The entire system or the affected modules | Individual units or functions |
| Purpose | Ensure new changes don’t break existing code | Validate the logic of individual components |
| Trigger | After changes, bug fixes, or enhancements | During initial development |
| Performed By | QA/Test Engineers | Developers |
| Automation Level | Often automated | Typically automated |
This article explains the difference between regression and unit tests, how they work, and when to use each.
What is Regression Testing?
Regression testing is conducted when a change is made to the existing software. This change could be adding a new feature or making tweaks to the existing feature. Regression testing re-executes the entire code to ensure the functionality of the software remains unchanged after the modifications to the software.
Example of Regression Testing
Let’s assume there is a calculator app initially designed to do basic functions like add, sub etc. and was tested and released.
Now, when a new feature, such as finding the mean, needs to be added, the tester should ensure that the previous functions also work properly and that adding the mean function did not disrupt any. of the functions that were previously available. This type of testing is called regression testing.
Regression Testing Techniques
Here are some of the key techniques for Regression Testing:
1. Complete Retest
You should opt for this technique when several changes are required to be made to the main coding structure of your application. It checks all components of your product thoroughly, including any new changes in the software. Thus it’s costly and time-consuming, too. That’s why you must depend on this technique for extensive regression testing.
2. Selective regression testing
With this regression testing technique, you can test a small change in your software. This technique helps you test a particular module with small change requirements.
You can analyze the behavior of the existing software with a newly added or removed feature from this method. Moreover, it will provide test history for the changes with regression testing.
3. Prioritization of test cases
This technique is ideal for regression testing with a basis of the test case priority. That means, first, you have to execute the high-priority test cases. Then you will execute the medium and low-priority test cases. You have to indicate the priority based on their criticality and risk to the product.
Also Read: Test Case Prioritization: A Detailed Guide
What is Unit Testing?
Unit testing involves testing the smallest parts of a software application, such as individual functions or methods, to ensure they work correctly. It helps identify and fix bugs early by validating that each component performs as expected in isolation
Why is Unit Testing Important?
Unit testing helps developers catch issues early, build more maintainable code, and reduce the risk of bugs slipping into production. Here are some more reasons why unit testing is important.
- Early Bug Detection: Unit tests help catch bugs at the earliest stage of development, before they propagate into larger systems where they’re more complex and costlier to fix.
Read More: How to find Bugs in Software?
- Faster Debugging: When a unit test fails, it points directly to the broken function or method, reducing the time spent tracing the source of the issue.
- Improved Code Quality: Writing unit tests promotes better coding practices by encouraging developers to create small, well-defined, and independent functions.
- Supports Refactoring: With unit tests in place, developers can confidently refactor or optimize code, knowing tests will flag unintended side effects.
- Reduces Cost of Fixes: Bugs identified early through unit testing are far cheaper to fix than those discovered during integration, system testing, or production.
Regression Testing vs Unit Testing
Unit testing focuses on verifying individual components or functions in isolation, making sure each piece of code behaves as expected. In contrast, regression testing re-runs existing test cases after changes are made, ensuring that new code updates haven’t inadvertently broken previously working features.
Here are some of the key differences between unit testing and integration testing:
| Regression Testing | Unit Testing | |
|---|---|---|
| Definition | It confirms that the new updates do not affect the overall functionality of software. | It examines the individual components of an app. Basically, it happens at the function/method level. |
| Purpose | To determine the new implementations don’t break the existing functionalities of an app. | To improve the quality of the code and remove the bugs from the early development stages. |
| Executed By | QA Team | QA Team |
| When to perform | Regression tests must be run whenever a new feature is developed and merged, or when bug fixes are implemented. | You should perform it at the development phase |
| Test Environment | Generally performed in a staging or production-like environment that can mimic user actions | Performed in a controlled environment |
| Test Case Development | Test cases should cover all the and functionalities of the app | Test cases focus on the specific inputs and expected output for individual functions/methods |
| Dependencies and Isolation | Communication with other modules, external services, and databases that might be connected with the app are to be considered | Communication with other modules, external services, and databases can be mocked or isolated |
| Test Coverage | Regression testing covers a large area of code. It executes the previous test cases for the older modules also. | It has lower code coverage because it tests only a few classes and methods. |
| Resource Requirements | It needs a greater number of resources for testing the old test cases. Thus it involves extra servers and manpower costs. | It needs external resources like databases, web-servers when you work in a real-time environment. |
Read More: Functional Testing vs Unit Testing
Similarities between Unit Testing and Regression Testing
Although unit testing and regression testing are applied at different stages and focus on different scopes, they share several important qualities that contribute to building stable, high-quality software.
- Bug Prevention: Both types of testing are designed to catch defects early in the software lifecycle, helping prevent faulty code from progressing to later stages.
- Code Stability: Each contributes to ensuring that existing features continue functioning correctly, even as new features or updates are introduced.
- Automation-Friendly: Both unit and regression tests are commonly automated to enable frequent, fast, and repeatable testing within CI/CD pipelines.
Also Read: How to Setup an Effective CI/CD Pipeline
- Developer Confidence: Running these tests regularly assures teams that their code changes have not broken existing functionality.
- Continuous Testing Role: Both are essential to continuous testing strategies used in agile and DevOps environments, where frequent updates and fast feedback are critical.
Read More: What is Continuous Testing in DevOps
Unit Tests vs Regression Tests: When to Choose What
Both unit and regression tests serve different purposes. Here’s when to use each based on the stage, scope, and intent of testing.
Use Unit Tests When:
- Verifying that individual functions, methods, or components behave as expected during development
- Testing specific conditions, calculations, or logic in isolation from the rest of the system
- Refactoring or optimizing code, and you need to ensure that the existing behavior remains unchanged
- Following a test-driven development (TDD) approach to define expected behavior before writing code
- Catching failures early through automated checks in continuous integration pipelines
Use Regression Tests When:
- Ensuring that recent changes, enhancements, or bug fixes haven’t broken previously working functionality
- Confirming that the entire application behaves correctly before deploying to staging or production
- Validating that integrated components and workflows still function correctly after system updates
- Safeguarding legacy code when making modifications that may affect dependent or connected modules
- Executing broader validation checks automatically after builds as part of nightly or pre-release pipelines
Tools for Regression Testing and Unit Testing
Here are the best unit and regression testing tools.
1. BrowserStack
BrowserStack is a cloud-based testing platform that lets you run unit and regression tests across 3,500+ real browsers and devices. It supports popular frameworks like JUnit, NUnit, xUnit, and PyUnit, and integrates with Selenium, Appium, and Puppeteer.
You can run tests on local and staging environments, simulate real-world user conditions, and perform parallel testing to speed up execution. Additionally, you can build custom dashboards and set up alerts to monitor key metrics, track errors, and stay ahead of issues throughout the testing lifecycle.
2. Selenium
An open-source web automation framework that’s widely used for regression testing and can support unit testing through integrations. It works with test frameworks like JUnit and TestNG and supports multiple languages, including Java, Python, C#, and Ruby. Selenium is ideal for validating user flows and maintaining reusable regression suites.
Read More: Regression Testing with Selenium: Tutorial
3. Playwright
Playwright is a modern end-to-end testing tool that supports both functional and visual regression testing. It captures and compares screenshots between test runs to detect UI changes. It supports languages like JavaScript, Python, Java, and C#, and works across all major browsers.
4. Cypress
Cypress is well-suited for both unit and regression testing in web applications because of its speed and developer-friendly interface. Visual regression testing can be added using plugins like Cypress Image Diff, which capture and compare UI screenshots and generate detailed HTML reports.
Conclusion
Unit testing and regression testing are both essential for delivering reliable, high-quality software. Unit tests validate individual components during development, ensuring each function behaves as intended in isolation. Regression tests, on the other hand, safeguard against unintended side effects by rechecking existing functionality after changes.
It’s crucial to run these tests on real devices and browsers to make them meaningful in real user conditions. BrowserStack allows teams to automate unit and regression tests across thousands of real mobile and desktop environments to ensure consistent performance and a smooth user experience for every release.
Frequently asked questions
1. Is regression testing part of unit testing?
No, regression testing is not a part of unit testing.
2. How is regression testing different from other testing?
This testing checks the bugs that are unexpected for the users. It ensures a part or the entire system works exactly as previously after making any changes to the system. Thus it removes the unexpected bugs that can change the existing functionalities of the system.
3. What is system vs integration vs regression testing?
System testing examines the overall functionality of a completely integrated system. Integration testing verifies how the individual modules work when integrating with other modules.
Regression testing checks new changes, and bug fixes can’t affect the existing functionalities of a system.
4. What is the difference between regression and non-regression testing?
With regression testing, you can ensure the software bugs are corrected successfully from the modified software. Non-regression testing ensures the absence of new bugs in the updated software.
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