A testing process that verifies each function of an application against specified requirements to ensure correct output based on given inputs.
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.
What is Functional Testing?
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 Examples
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).
Functional Testing vs Non-functional Testing: Key Differences
Non-functional testing is a type of software testing that focuses on evaluating the non-functional aspects of the system, such as:
Performance
Reliability
Stability
Security
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:
Measure the response time of the system under normal load conditions.
Simulate concurrent user connections to determine how the system performs under heavy load.
Conduct usability testing with actual users to gather feedback on the UX.
Assess the system's ability to handle an increased workload by gradually increasing the load and observing performance metrics like CPU and memory usage.
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.
Non-functional testing tools may include JMeter, OWASP ZAP, L
oadRunner, etc.
Types Of Functional Testing and Examples
Let’s look at some popular functional testing types:
Unit Testing – Tests individual pieces of code (like functions or methods) in isolation.
Integration Testing – Ensures different modules or components work together correctly.
Smoke Testing – A quick, basic test to check if the main functions of the software work properly after a build.
Regression Testing – Verifies that recent code changes haven’t broken any existing functionality.
Exploratory Testing – Testers explore the software freely to discover issues.
How to do functional testing?
Step 1. Decide on test scenario
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:
As a [role]: Who benefits from the feature?
I want [feature]: What functionality is needed?
So that [benefit]: Why is it important?
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:
Break the story into smaller, specific tasks.
Define the steps required to achieve the goal.
Create testable chunks to reduce risks and speed up coding and testing.
Step 2. Write Functional Test Cases
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:
Test Case ID: A unique identifier.
Test Scenario: A brief description of what is being tested.
Preconditions: Any setup required before running the test.
Test Steps: A step-by-step guide on how to perform the test.
Expected Result: The outcome that should happen if the feature works correctly.
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.
2. Cover edge cases
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.
3. Use real-world test data
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.
4. Automate repetitive tests
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.
Why automate functional testing?
Automated tests run much faster than manual testing. Not just that, they can run 24/7 without intervention.
Manual testing can lead to mistakes due to fatigue or oversight. Automation testing ensures more consistent test execution.
Automated functional testing allows you to cover multiple browsers, devices, and operating systems.
Automation testing is particularly helpful for regression testing, which is a highly repetitive testing activity.
How to choose your automated functional testing tool?
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
Choose an open-source testing library as the foundation.
Use libraries, drivers, design patterns, and coding standards to build.
Requires coding and testing expertise but offers high customization.
Setup time is significant, but it's budget-friendly.
Maintenance and issue resolution may extend testing cycles.
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.
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:
No-code: turn on the Record-and-Playback mode, and record your screen, just like how an end-user would. Katalon automatically turns that sequence of actions into an executable test script.
Low-code: leverage the rich library of keywords to craft your test scripts. All you have to do is choose the keyword for the action you want to automate and set the parameters. Switch to Full script mode whenever you want.
Full code: write your test scripts in Groovy. Enjoy the customizability along with the simplicity of No-code and Low-code within your hands.
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.
1. How does functional testing differ from non-functional testing?
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Functional testing checks whether an application performs as expected based on defined requirements (e.g., verifying a login process). Non-functional testing, on the other hand, evaluates aspects like performance, scalability, usability, and security that are not directly related to functionality.
2. What is the role of a requirements document in functional testing?
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The requirements document serves as the foundation for functional testing. It defines what the application is supposed to do, helping testers create relevant test cases and ensuring that the application meets business needs.
3. How do you decide which features to prioritize in functional testing?
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Features are prioritized based on business impact, user frequency, and risk. Critical features like login, payment processing, or data submission typically take precedence over less essential functionalities.
4. Can functional testing include API testing?
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Yes, functional testing often includes API testing to validate that APIs perform expected actions, return correct responses, and handle errors gracefully. Tools like Postman or REST-assured are commonly used for this purpose.
5. How does functional testing handle edge cases?
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Functional testing includes edge cases by designing test scenarios that push the application to its limits (e.g., entering the maximum number of characters in a field). These tests ensure that the application handles unexpected inputs or conditions correctly.
6. What is the difference between functional testing and regression testing?
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Functional testing verifies whether specific features work as intended, often for new or updated functionality. Regression testing checks that existing features still work after changes, ensuring no new bugs are introduced.
7. Is functional testing automated or manual?
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Functional testing can be both manual and automated. Manual testing is used for exploratory or ad-hoc testing, while automation is preferred for repetitive test cases to improve efficiency and consistency.
8. What is the role of data in functional testing?
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Test data plays a crucial role in functional testing. It’s used to simulate real-world scenarios, validate inputs and outputs, and ensure the application behaves correctly under various conditions, including valid, invalid, and boundary inputs.
The Katalon Team is composed of a diverse group of dedicated professionals, including subject matter experts with deep domain knowledge, experienced technical writers skilled, and QA specialists who bring a practical, real-world perspective. Together, they contribute to the Katalon Blog, delivering high-quality, insightful articles that empower users to make the most of Katalon’s tools and stay updated on the latest trends in test automation and software quality.
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