Regression Testing vs Unit Testing: What’s the Difference?

It is quite difficult to understand the difference between regression and unit testing the first time. They are both very similar, while also having fundamental differences in strategy and approach.

To some extent, unit tests can be classified as regression tests. The difference is that unit tests focus on testing individual pieces of code, while regression testing focuses on testing the entire application or system when changes have already been made to the code.

In this article, we will discuss in more detail what unit and regression testing are to help you draw conclusions and understand which type of testing is the most suitable for your product.

What is regression testing?

Regression testing is a test of a previously tested program to make sure that the changes made did not lead to the appearance of defects in the part of the program that did not change. Regression testing is used to make sure that the program, after changes, continues to meet the established requirements and successfully interacts with other systems.

Following dynamically changing needs, users want quick and frequent improvements to IT systems and applications. Adding new functionality, reworking old ones, adjusting for an updated version of the browser or operating system, fixing defects is often an ongoing process of the software life cycle. Any modification to the code or the software environment does not exclude the occurrence of errors. Moreover, when the system is being improved, it is likely that the changes will affect its performance in terms of the previously developed functionality.

What is unit testing?

Unit Testing is a type of software testing in which individual modules or components of the software are tested. Its purpose is to check that every piece of code works as it should. This type of testing is often performed by developers at the stage of application coding. Unit tests isolate a piece of code and verify that it works. The unit of testing can be a single function, method, procedure, module, or object.

In development models, such as Software Development Life Cycle, Software Testing Life Cycle, V Model, unit testing is the first level of testing performed before integration testing. Unit testing is a White-Box testing technique that is usually performed by a developer. In fact, due to lack of time or other circumstances, sometimes unit testing has to be done by QA engineers.

Unit testing vs regression testing


The main goal of unit testing is to make sure that each individual module of the system meets the requirements before it is integrated into the product. Furthermore, when testing the system as a whole, this makes it easier to identify the modules that caused the problem and eliminate the corresponding defects in them.

The main goal of regression testing, on the other hand, is to make sure that the existing functionality works correctly after making changes. These changes may involve any of the following: changing the configuration, environment, fixing errors and finalizing existing modules, adding new functionality, and so on.


Unit testing:

  • Junit is a free testing tool used for the Java programming language. Provides assertions for defining a test method. This tool first validates the data and then inserts it into a piece of code.
  • NUnit is a widely used unit testing framework for all .NET languages. It is an open-source tool that allows you to write scripts by hand. It supports data-driven tests that can be run in parallel.
  • JMockit is an open-source unit testing tool. It is a code coverage tool with line and path metrics. Allows you to simulate an API with write and validate syntax. This tool offers line coverage, path coverage, and data coverage.

Regression testing:

  • Ranorex Studio is an all-in-one automated regression testing framework for desktop, web, and mobile applications with built-in Selenium WebDriver. Includes full IDE plus no-code automation tools.
  • Selenium is an open-source tool used to automate web applications testing. Selenium can be used for browser-based regression testing.
  • Quick Test Professional (QTP) is an automated software designed to automate functional and regression tests. It uses the VBScript language for automation. It is a data-driven, keyword-driven tool.

How it’s performed

Unit testing is performed using the following algorithm:

  1. Adding a unit of code to the application to test it. 
  2. Isolating a unit of code for better testing. This practice involves copying the code into your own testing environment. Code isolation helps identify unnecessary dependencies between the code under test and other modules or data spaces in the product.
  3. Using the automation test framework, the developer sets the test criteria to verify that the code is executed correctly, and logs the failed ones during the execution of the test cases. 
  4. Removing the test code when the application is deployed.

As for regression testing, it is usually performed in 4 stages:

  1. Determining the optimal scope and developing a test plan for regression testing.
  2. Formation of test data and preparation of test cases using specified tools.
  3. Running test cases and reporting bugs.
  4. Repeating the test cycle (if necessary).

Do you need both?

To conclude this article, let’s discuss the cases when you may need to use both unit and regression testing in one project. 

Simply put, when we do regression testing, it ensures that users do not get bugs and defects while using the program after it is released. Naturally, this leads to an unpleasant experience and therefore a loss of money. In addition, regression testing is needed in order to make sure that the software works as planned.

At the same time, unit testing can save you money in the long run. When you test the code at an early stage of development, you protect yourself from bugs that were discovered after the product was launched. When defects and bugs appear later, their elimination not only requires time and money but also significantly delays the launch of the project.

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