Unit Tests vs Functional Tests: Not All Tests Are Created Equal

Software bugs cause delays lost revenue and damaged reputation.

Unit tests are a great way to prevent bugs from creeping into your codebase. They also give you an invaluable layer of protection against future changes made by other developers on your team. The downside of unit tests is they don’t check if the whole system works correctly. That’s where functional tests come in.

Unit tests are the foundation of your testing strategy. They ensure that individual units work as expected. On the other hand, functional tests are designed to exercise the full functionality of an application.

This blog post will introduce you to both types of testing and show how each can be used effectively in your development workflow.

What is unit testing?

Unit testing is the process of testing individual units of code. For example, a unit could be a class or a method. By testing these individual units, you can ensure that they’re working correctly and are integrated correctly into the rest of your codebase. Unit testing is a white box testing technique.

You write unit tests in the same language as the production code. Unit tests are very easy to write and maintain because you can use the same IDE to do much of the heavy lifting. In addition, you typically run unit tests as part of a build process. By including them in the CI/CD process, you can get immediate feedback on whether your code is working or not. And if a test case fails, you can see which unit is the problem.

Advantages of unit testing

There are several advantages of unit testing:

  1. Unit testing is an effective way to avoid bugs in your code.
  2. It’s an excellent way to make sure you’re writing clean, readable code.
  3. You can use unit tests as technical documentation for other developers on the project.
  4. You’ll feel more confident about what you’ve written since unit testing means that all of your logic is checked and working correctly.
  5. A test suite will run faster than manual regression because it has been automated.
  6. Code coverage lets you see which parts of your program are not tested enough and focus there.

Disadvantages of unit testing

Unit testing is not a silver bullet, and it has some downsides:

  1. Unit testing can be time-consuming.
  2. Unit tests only cover one isolated unit.
  3. The code that is being tested needs to be available for the test.
  4. Tests are only as good as their developer’s ability to write them.
  5. Poorly written unit test cases can lead to false positives and negatives.
  6. Unit testing doesn’t catch everything – it’s not a substitute for proper QA.
  7. Sometimes unit tests are too specific, making them less useful in the future.

Even though it has some downsides, unit testing is an indispensable part of software development that you should never neglect.

When to perform unit testing?

You should perform unit testing during the development cycle. Ideally, you want to write unit tests while writing the code. You can also write a unit test method before writing production code. That process is called test driven development.

If a unit test fails, you know exactly which part of the code is not working correctly. This kind of feedback can help you to correct your changes immediately instead of coming back later to debug and find the bug in the code.

Unit testing can also be part of your continuous integration process, where test automation is run after every commit. You can catch bugs before you submit your code for code review.

What is functional testing?

Functional testing is the process of testing an application’s features and functionality. Functional tests verify that the application behaves according to functional requirements.

Functional tests are not as fast to write and run as unit tests, but they can catch problems that unit tests might miss. Functional testing is also vital for regression testing – ensuring that existing features still work after changes have been made to the codebase.

You can perform functional testing manually or with automated test tools. You can write them in a different language than the tested code. That’s because functional testing is a black-box testing technique. Automated functional testing tools can simulate user interaction with the software and check that it behaves as expected. Functional testing is often done in a different environment, such as a staging server, to avoid affecting the live system.

Functional testing is an integral part of quality assurance and should be included in any software development process. By identifying and correcting functional problems early in the development cycle, you can save time, money and improve the quality of your software.

Advantages of functional testing

There are several advantages of functional testing:

  1. Functional testing is an effective way to identify bugs.
  2. Functional testing is an excellent way to test the quality of your product.
  3. Test cases are created from requirements, so they can be traced back to business value.
  4. Tests are designed to understand how the system should behave under normal conditions.
  5. It’s possible to write automated checks for functional tests, saving time and money on manual verification steps like regression testing.
  6. You can test the whole system, not just parts of it.

Disadvantages of functional testing

However, functional testing also has some downsides:

  1. It’s time-consuming and expensive to create the tests.
  2. Testing for every possible scenario would take too long, so it’s not always feasible to do this type of testing.
  3. It’s difficult to find bugs unrelated to functionality, such as usability and performance issues.
  4. You still need other types of testing, like unit testing and integration testing for different purposes.

When to perform functional testing?

You should perform functional testing after the development of the particular feature is completed.

Manual functional testing can be performed regularly, either on a staging server or another environment where functional problems would not affect other users. You can also use automated functional test tools to automate the functional testing process.

What is non functional testing?

What is non-functional testing, you ask? It’s a question that every software developer has asked at some point.

Non-functional testing is software testing concerned with the system’s performance under test rather than its functional correctness.

Non-functional testing includes:

  • Performance testing
  • Load testing
  • Reliability testing
  • Usability testing
  • Scalability testing
  • Security testing
  • Stress testing

When you’re building a new feature or fixing a bug in your application, non-functional testing gives you insight into how well it will perform in production. It helps you anticipate bottlenecks and identify areas where you can improve performance before they become problems for your users. 

Non-functional testing is essential for quality assurance and you should include it in any software development process. By identifying and correcting non-functional problems early in the development cycle, you can save time and improve the quality of your software.

Unit tests vs functional tests – a detailed comparison

Unit tests and functional tests are two different types of software testing. They both have their strengths and weaknesses, and it’s essential to understand the difference between them before you start writing tests for your application.

Unit tests are designed to test a single unit of code in isolation. They are quick and easy to write, and they can help you find and fix bugs early in the development cycle. However, they are not a substitute for functional testing.

On the other hand, functional tests are designed to test the functionality of your application. They are more time-consuming to write, but they can help you test the whole system, and they should be run as part of your automated testing suite.

Comparison between unit and functional tests

The following table brings a comparison between unit tests vs functional tests.

Comparison pointUnit testsFunctional tests
Written againstWritten against the application code.Written against the user requirements.
Test typeHas positive and negative test cases.Mostly positive tests that are covered by users’ use cases.
Written byDevelopersSoftware testers
Compatibility of frameworksUsually written in the same language as the production code.Frameworks used need not be compatible with the project framework.
ExecutionUnit tests are executed after each build.Functional tests are executed before the release.
InteractionInteracts with the code directly.Interacts with UI.
Bugs identificationBug identification in early stages.Bug identification at the later stages of development.
RefactoringHelps developers to refactor the code.Identifies the functionality changes once the refactoring or integration is done.
Replaced by manual testingCannot be replaced by manual testing.Can be replaced by manual testing.
Testing levelTests the code.Tests the logic.
Unit tests vs functional tests – a detailed comparison

How do unit testing and functional testing improve software quality?

Unit and functional testing improve software quality by ensuring that the features you implement work as intended. Functional tests can be more time-consuming but offer stronger guarantees since they cover a larger code area. Unit and functional tests form the backbone for production code quality assurance.

Modern organizations utilizing quality assurance practices rely on automated functional testing for its ability to provide cumulative coverage, which is an advantage over traditional manual testing techniques.

What is more important: unit testing or functional testing?

You want to make sure your code works correctly, but you’re unsure what tests to write?

Unit tests and functional tests have very different purposes.

If you use unit tests alone, you can end up with many false positives that are hard to ignore when refactoring or making changes in the future. In addition, functional testing is more expensive than unit testing because it requires more time and resources from developers and testers.

So write both types of tests!

First, use unit tests to check individual parts of your application work correctly. After that, use functional testing to ensure everything works together correctly before deploying any updates into production environments or live sites. There are many tools available for writing automated functional tests so that even non-developers can help improve software quality.

How to split your testing efforts between unit testing and functional testing?

Do you want to make your development team more efficient?

Then you can split the testing effort in your team. If you have software testers in the team, they can write automated functional tests. Developers can focus on unit tests.

This way, the software testers verify that the code satisfies user requirements, and the developers are focused on making sure the code is correct. This division of labor makes it easier for everyone to know what they need to do and speeds up the development process.


It’s important to understand the difference between functional tests vs unit tests. Both forms of testing are essential for ensuring that your application is bug-free before deploying any updates into production environments or live sites.

If you want to make sure your tests provide cumulative coverage, consider splitting up the testing efforts in your team.

Software testing requires effort from developers and testers.



Is unit testing functional or non-functional?

Unit testing is a functional testing technique where functionalities are tested by isolating functional units of the system. The primary purpose of unit testing is to give fast feedback about any functional change in the software.

Is integration testing the same as functional testing?

No, integration testing is not functional testing. One integration test covers only one application module, while functional testing is more end-to-end testing. Integration testing is a gray box testing technique, while functional testing is a black-box testing technique.

What’s the difference between functional and non-functional testing?

Functional testing is all about verifying that the system functions as expected. It’s black box testing, meaning that the tester doesn’t know anything about the system’s internal structure. Testers usually do functional tests to ensure that the application meets the requirements. On the other hand, non-functional testing is all about verifying the system’s performance and stability.

Recent Posts