Functional Testing: What to Test, Why to Test, Types, and More.

In this article, you’ll learn what functional testing is, what to test in functional testing, why it exists in the first place, how functional and non-functional testing compares, and much more.

Functional testing is a type of software testing that you do to verify the correct working of any or all functional aspects of your system. Functional testing covers white-box and black-box testing methodologies.

With functional testing, you confirm that your software functions as per the exact specifications, requirements, and intended business goals. You focus on the binary result of an operation or functionality in your software, rather than how it works internally, code-wise. You check if the test passes or fails.

Here’s a breakdown of functional testing as a 3-step process:

1. You provide valid or appropriate input.
2. You execute the tests or perform an action.
3. You compare the actual output with the expected output.

Let’s quickly compare functional and non-functional testing:

Also Read: STLC: Getting Started with Software Testing

Here’s why to conduct functional testing for your software:

To identify functional defects/bugs

You majorly test your software to identify bugs or defects. Functional tests sniff out bugs that affect various functionalities of your software. You report bugs to your developers to have them fixed.

To verify business logic

You ensure that the functional implementations in your software align with the business goals and the specified logic as per the requirements with functional testing.

To verify every functional logic

With functional testing, you ensure that all your functions/features output expected results with appropriate inputs or valid actions.

To test UI flows

Functional testing helps confirm that your users are not confused or thrown off while trying to use a specific function or navigate to their desired screens while using your software.

Also Read: Types of Software Testing Tools: Choosing the Best One

Functional testing exists or aims to test four fundamental functional aspects of your software:

Basic usability

You ensure that the user easily navigates to wherever he/she intends to navigate in your software without any confusion. For instance, if the user wants to navigate to the payment screen in your e-commerce software, the instructions should be clear enough to make it easy for them to follow.

Mainline functions

You ensure that the functionality or feature works as intended. For instance, the payment screen features and transactions should function correctly in case your user wants to complete the purchase on your e-commerce software.

Accessibility

Your software function should be easily accessible for the majority of users based on their behavior of how to access that function. For instance, payment options user interfaces must be easily accessible in your e-commerce software on the payment page.

Conditional errors

If the function fails, the errors should be conditionally handled with proper messages, notifications, or pop-ups coming from your software. For instance, if there is an issue when your user pays for a purchase on your e-commerce software, relevant messages should be displayed describing what went wrong.

Here are the main functional testing types:

Unit Testing

Unit testing verifies the proper functioning of the smallest, isolated, and testable unit/function/module in your software. Unit testing is all about source code validation. Developers write and execute unit tests. They make sure that each unit functions as expected. JUnit, NUnit, and TestNG are some notable unit testing tools.

Since unit testing mainly focuses on code coverage, here’s what to ensure while writing and executing unit tests for your software:

• Line coverage: Line coverage denotes the percentage of code lines that have been tested. You ensure that each code line in your function/unit is tested at least once. If your function under test has 20 lines of code, and all 20 lines of code have been executed during testing, your unit test for that specific function is 100%.
• Code path coverage: Code path coverage refers to the execution percentage of all possible entry and exit paths in your code that generally includes your code branches, decision points, conditionals, or loops. You ensure that unit tests for each code path have been executed at least once.
• Method coverage: Method coverage refers to the percentage of methods or functions covered for unit tests in your code. You execute tests for individual methods or functions in your code. You can also include entire classes or whole modules. You aim to test all the methods at least once.

Integration Testing

When your software units/modules are unified or combined with an interface, integration testing ensures that there are no bugs or defects with the interaction of the modules. They function correctly as per their specifications. QA analysts or testers generally conduct integration testing. Postman, Citrus, Protractor, and FitNesse are some top integration testing tools.

Integration testing generally has three sub-types. Let’s dissect them with a quick overview:

• Component integration testing: You test the integration of the individual units/components with component integration testing.
• System integration testing: You test the integration of APIs, sub-systems, microservices, integrated layers, etc. with system integration testing.
• End-to-end integration testing: End-to-end integration confirms the correct functioning of all the integrated components, plus various external system layers unified as one.

E2E/System Testing

End-to-end/E2E testing or system testing verifies a completely integrated system, end-to-end. System testing is usually performed by automated tools since it is a long-running process and also requires repetitive tasks or test actions. Real-world scenarios and environments are simulated in system tests to identify and report bugs for fixing. Usually, the developers and testers conduct system testing. Selenium, LambdaTest, Cypress, and Katalon Studio are popular end-to-end testing tools.

Smoke and sanity testing comes under the umbrella of system testing. Let’s quickly cover them.

• Smoke Testing: You do smoke testing to verify that there is no “smoke” coming out of your newly released software build. In short, you confirm the stability of your software build and whether or not there are any issues or defects to account for. Smoke testing can be conducted by the QA or the dev team.
• Sanity Testing: Once you’re done with the smoke test and your software is marked stable, you do sanity testing or a sanity check. It’s meant to quickly validate the sane or logical behavior of your newly added feature or code fixes in your software. You conduct sanity tests only after the smoke test and on a stable software build. The QA team conducts sanity testing.

Regression Testing

When there are code changes or bug fixes in your software, regression testing ensures that the existing functionalities are not affected or remain unbroken. You also run regression tests to confirm that the code changes have not introduced any new bugs to your software. QA analysts and testers perform regression testing. Selenium, Watir, and Katalon Studio are widely used for regression testing.

User Acceptance Testing (UAT)

As the name suggests, it's user-based testing. It is also called beta testing. Your users, stakeholders, business analysts etc. are your testers in user acceptance testing. It comes after system testing and regression testing. You aim to test your entire software functionalities based on business requirements from an end-user perspective. Some prominent UAT tools include HotJar, UserBrain, and TestComplete.