When people refer to "testing" in a general context, they often have functional testing in mind. It's about making sure that the core functions are working as they should.
In software development, functional testing is a form of software testing that verifies whether a system meets its functional requirements. These requirements are either collected from the users or provided by the stakeholders (development team or product owner).
The primary goal of functional testing is to ensure that the software performs the functions it was assigned to do. Functional testing is one of the most common and foundational types of testing.
Functional testing starts from features. Let's say you want to run functional tests on a Login page. Start by asking: "What features does a Login page have?". The answer to that question should be your functional test cases.
Here are the examples of functional tests:
Verify the username field accepts valid input and rejects invalid formats.
Verify the password field masks input (hidden characters).
Verify login succeeds with valid credentials.
Verify login fails with invalid credentials and displays correct error.
Verify empty fields show validation messages.
Verify “Forgot Password” link navigates to reset flow.
Verify “Remember Me” maintains session across browser restarts.
Verify correct redirection after successful login.
Verify multiple failed attempts trigger lockout or CAPTCHA (if implemented).
Verify accessibility (tab order, screen reader labels, contrast).
📚 Read More: 100 Test Cases For The Login Page
Non-functional testing is a type of software testing that focuses on evaluating the non-functional aspects of the system, such as:
When doing functional testing, testers try to answer the question Can the system do what it was built to do?
When doing non-functional testing, testers try to answer the question Can the system do what it was built to do well enough?
Here are 4 examples of non-functional test cases:
Here is a simple comparison table for the differences between functional testing and non-functional testing:
|
Aspect |
Functional Testing |
Non-Functional Testing |
|
Purpose |
To verify if the software functions as intended and meets functional requirements. |
To evaluate non-functional attributes like performance, security, usability, and more. |
|
Focus |
Tests what the software should do. |
Tests how well the software performs certain functions or behaves under specific conditions. |
|
Scope |
Typically focuses on specific features or functionalities. |
Covers a broader range of attributes beyond functionality. |
|
Examples of Testing Types |
Unit Testing, Integration Testing, System Testing, User Acceptance Testing. |
Performance Testing, Security Testing, Usability Testing, Compatibility Testing. |
|
User Focus |
Ensures that the software meets user needs and expectations in terms of features. |
Ensures that the software meets user needs and expectations in terms of performance, security, usability, etc. |
|
Objective Measurement |
Often involves binary outcomes (pass/fail) based on expected behavior. |
Often involves quantitative measurements and benchmarks for non-functional attributes. |
|
Tools and Technologies |
Functional testing tools may include Selenium, JUnit, TestNG, etc. |
Non-functional testing tools may include JMeter, OWASP ZAP, L oadRunner, etc. |
Let’s look at some popular functional testing types:
Functional testing starts with functional requirements, which are based on user stories.
A user story describes what a user wants and why. It usually follows this format:
For example: "As a customer, I want to view my order history online so that I can track my purchases."
This story is a starting point, but it’s not detailed enough for development or testing. Testers must collaborate with users to understand their needs and put the story into a real-world context. By doing so, the story becomes a requirement, which guides development and functional testing.
User stories can be too broad. To make them actionable:
Once you have a clear test scenario, the next step is to write test cases. A test case is a set of conditions that testers use to verify whether a feature works as expected.
Each test case should include:
📚 Read More: How To Write Test Cases? A Detailed Actionable Guide
After writing test cases, testers run them to verify functionality. Execution can be:
When running tests:
If a test fails, report it to developers with:
📚 Read More: How To Create a Test Report?
When writing functional test cases, the first best practice is to keep them clear and simple. Each test case should have short, easy-to-understand steps, with the scope limited to one function at a time. This reduces ambiguity and ensures other team members can quickly grasp the purpose of the test.
A second best practice is to make sure you cover edge cases and negative scenarios. It’s not enough to only test expected inputs, you should also verify how the system handles unusual or invalid data.
For example, on a Login page, edge cases could include leaving both the username and password fields empty, entering a password that’s far longer than expected (e.g., 256+ characters), trying unsupported characters like emojis, or attempting to log in with an SQL injection string to test security.
You could also test what happens when the account is locked after too many failed attempts, when the session times out mid-login, or when the network drops right after pressing “Login.” These aren’t everyday scenarios, but they simulate real-world conditions and reveal how resilient the system is.
Third, use real-world test data whenever possible. Functional tests become much more valuable when they mirror realistic user activity.
For example, when testing a login form, you should use usernames and emails that reflect actual usage patterns. If you’re testing an e-commerce system, include real product names, prices, and transaction-like data to better simulate real-world workflows.
Finally, automate repetitive test cases to save time and reduce human error. Tests that need to run frequently, such as login, checkout, or form validation, are strong candidates for automation. Tools like Selenium, Katalon, or Cypress are commonly used for UI-level automation and can be integrated into CI/CD pipelines to ensure fast, reliable feedback.
📚 Read More: A guide to automate functional tests
Functional testing plays a crucial role and is tied to many other activities in the software development life cycle. However, teams usually use a number of fragmented tools that solve separate testing needs, which creates a collection of complex, brittle, and hard-to-scale tool-stacks for quality management.
This means you have 3 options to do functional testing:
Option 1: Build Your Own Testing Framework with Open-Source Libraries
Option 2: Single-Point Commercial Automation Testing Tools
Option 3: All-in-one software testing solution
Here's a list of good functional testing tools for your team
Now that you have broken down the user stories into small slices, let's see how you can automate them.
To start, you can download Katalon. In Katalon alone, you can do test planning, write tests, manage them in suites, schedule for execution across environments, and generate test summary reports. This comprehensiveness allows for a lot of customization and flexibility, no matter how complex the application under test is.
Download Katalon and Give It A Try
Once you have downloaded and installed Katalon, navigate to File > New > Project. You can choose the type of testing for this project, either web, API, mobile, or desktop app.
You have up to 3 modes of test creation:
Moreover, Katalon sets itself apart by incorporating cutting-edge, native AI features to enhance functional testing. Users can autonomously generate test scripts from plain language input or use the "Explain Code" feature to add comments to code snippets for better understanding among stakeholders and team members. Check out our pioneering AI features here.
Do your team need a better solution for functional testing? Start now with Katalon Studio.
Download Katalon and start functional testing for free