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What Is Functional Testing

Functional testing is a type of software testing that validates web or mobile applications against pre-determined specifications and requirements. The goal is to ensure that the software performs as expected by both developers and end-users. 

Definition of functional testing. Functional testing is a type of software that verifies product features according to requirements, identifying all areas where performance doesn't meet expected results.

Functional testers aim to emulate real-world scenarios and user pathways, testing not only for functionality but also for non-functionality, such as how the app behaves in airplane mode or when a video buffers. 

In this blog post, we’ll discuss how functional testing is a crucial first step in the software testing lifecycle. It helps create software, web, and mobile apps that work reliably across countless device and OS combinations, data carriers, and WiFi providers.

Chart showing black box testing method. See text for more information.
  1. What is Functional Testing?
  2. Examples of Functional Testing
  3. Why Is Functional Testing Important?
  4. Functional Testing Types
  5. Functional Testing vs Non-Functional Testing
  6. Functional Testing Process
  7. Scripted vs. Exploratory Strategies
  8. Best Practices For Functional Testing
  9. Conclusion

Examples of Functional Testing

A great example of functional testing would be to imagine you’re testing an e-commerce application. Functional tests would involve validating scenarios such as searching for products, adding items to the cart, and completing the checkout process.

For the search functionality, you would want to test searching for various products and verifying that relevant results appear. This would include checking for proper handling of misspellings, synonyms, filter options, and sort order. 

An example of functional testing for the search functionality would be if a user searches for “red sneakers.” The search results should include all relevant products, even if some are labeled as “red trainers” or “red athletic shoes.”  

Testing the shopping cart functionality would involve adding and removing items, updating quantities, and verifying that prices and totals are calculated correctly. You would also want to ensure that all buttons and links within the cart are functional and responsive. For instance, if a user changes the quantity of an item from 1 to 3, the total price should be updated accordingly.

Validating the checkout process is critical for any e-commerce application. This would involve testing various payment methods (credit card, PayPal, gift card), verifying shipping options and costs, and thoroughly checking form validation and error handling. 

Other key areas to test in an e-commerce application include user registration and login, viewing and editing profile information, and checking for proper access control and permissions. By thoroughly testing these core functionalities, you can uncover bugs and issues that impact the user experience before the application goes live. 

Real-world functional testing often uncovers critical defects that could seriously impact the user experience and the business. Real-life functional testing of a popular retail website revealed that the promo codes advertised on their homepage were not being applied correctly during checkout, leaving many unhappy users. Functional testing was able to identify this issue to then be rectified.  

In another example, functional testing of a hotel booking application uncovered a major issue with the room selection process. When attempting to book a room for more than 2 adults, the application would crash, preventing users from completing their reservations. This defect resulted in a significant loss of revenue for the company until it was identified and fixed.

Why Is Functional Testing Important?

Functional testing is crucial for ensuring software quality and a positive user experience. By validating that an application executes as expected, developers can measure the quality of a new release by the number of passed test cases. 

When a new app is released, poor reviews of buggy apps can spread online and damage a product’s reputation, making functional testing a vital part of the pre-launch strategy. Non-functional apps can also lead to major issues like payment processing failures that prevent customers from purchasing. 

Functional testing helps catch these critical problems to deliver the high-quality, reliable software users expect.

Functional Testing Types

There are a variety of different functional testing 

Exploratory Testing

Testing teams must learn about a product, its market, risks, and previous failures. This helps them uncover new tests. However, explorations must be structured, and testers need to be highly skilled.

Scripted Testing

Regression Testing

Smoke Testing

Does the app run? Will the systems interface? Does the button do anything? Smoke testing is done for every build, and includes low-level tests for every prominent feature and function.

Unit Testing

For example, a unit may be a function that calculates the sales tax on an item. The purpose of the unit tests is to ensure the code works from a functional perspective. Since the rest of the system is mocked out, unit tests can generally test every permutation possible for the function, including errors and exceptions. 

Unit tests run quickly, are usually written in the same language as the underlying code, and are tightly coupled to that code. Unit testing uses a white box testing method.

Component Testing

Component testing is done after unit testing. It takes each object/feature of the software and tests it separately. Component testing contains individual units classified as components and tests each unit using a black box testing method. 

Sanity Testing

For example, say the login page on a website was updated to add a “forgot password” link. A sanity test would be used to check that the link appears on the page and takes the user to the password reset screen when clicked. It is intended as a “sanity check” on a particular change, rather than an in-depth verification. If the sanity test passes, more rigorous regression testing can be done.

UI Testing

Can your web app accurately read a mouse click? Does a user keyboard integrate into your app and perform as expected? Are buttons/links/submission forms functional?

Integration Testing

This is the software testing phase, where software modules are combined, requiring group testing. 

Integration testing evaluates software or apps’ compliance with specific functional requirements. Integration tests segment components or modules and verify the functionality of each one individually and as part of the group. 

Acceptance Testing 

System Testing 

System testing occurs when software is fully developed. At this stage, the software is fully integrated with additional hardware and software but isn’t yet released to the market. 

System testing plans perform many tests (including unit, load, and regression) to evaluate and verify the functionality of the end-to-end product within a full computer system or software stack.

Other classifications of functional testing include:

  • Black box testing: Tests the functionality without peering into the internal code structure
  • White box testing: Assesses the internal code and infrastructure of the software
  • API testing: Checks the functionality, reliability, performance, and security of the application’s programming interfaces
  • Testing In Production: Live testing of the application in a production environment

Functional Testing vs Non-Functional Testing

Functional testing verifies that software features work as intended, while non-functional testing checks broader quality aspects like performance and usability. Understanding the key differences between functional and non-functional testing is crucial for QA teams to ensure comprehensive test coverage. 

Functional Testing Process

The basic functional testing process involves the following steps:

  • Step 1: Set Goals
  • Step 2: Create Test Data
  • Step 3: Create End-to-End Scenarios
  • Step 4: Design Test Cases
  • Step 5: Run Test Cases
  • Step 6: Track and Resolve

Step 1: Set Goals

The first step should always be setting parameters for what you define to be the product/app/website’s:

  • Specified requirements
  • Business-critical features
  • End-user expectations 
  • Target end user
  • Ideal functionality
  • Ideal UX/UI

To test for functionality, you need to determine specifics. Does the term “working button” and a passed functionality test refer to the primary function? Or does it include usability and accessibility? Interface? Passing a use case test?

Step 2: Create Test Data

If you want to perform a functional test, you’ll need data. Test data is the input given to the system that should have a specific outcome. You can use this test data to compare future runs. 

How do I get this data, you ask? You create test data by putting the software through specific conditions (i.e. wifi, device, network) and recording corresponding outputs (i.e. speed, time, expected actions).

Step 3: Create End-to-End Scenarios

Think like an end-user and brainstorm common actions and pathways. Create a list of features, locations, device and O/S combinations, and languages to test. 

Then, come up with some specific end-to-end scenario examples. For example, you might test a photo upload feature. When you add a photo, does it get added to the digital asset library? When you add a tag to that photo, does it get saved in the photo’s metadata? Will the picture you added to the tag appear in the digital asset library’s search results when you search that tag later?

Step 4: Design Test Cases

After you design the parameters that need to be verified and the customer pathways, design functional test cases. Here’s an example: 

Payments testing test case

Description: A customer opens up the app, adds an item to her cart, and checks out with an incorrect credit card number

Preconditions: The user has login credentials.

Steps:

  1. Create a user profile with an email address
  2. Add items to the cart
  3. Checkout
  4. Type in incorrect credit card information

Expected result: The user should receive an in-app notification of payment failure due to incorrect credit card information with the option to go back.

During this phase, lay out what appropriate feedback looks like and how you want testers to submit case results. Think about what entails a pass/fail in this case, and have specific criteria to evaluate during the next step.

Step 5: Run Test Cases

When you’ve gone through the steps, it’s time to execute the test cases and prepare to write the test case. 

Follow the actions on the test case to a T so that the run is easily repeatable.

software functional testing test management platform run example from Testlio

Example test management platform run

Step 6: Track and Resolve

Perhaps the most valuable testing outcome for your dev team is the testing results. Results should offer traceable actions with visibility into the entire testing journey. Include test case details like date, parameters, devices, what devices are tested, costs, outcomes, and recommended actions. Write the test case inside of your test management tool.

Scripted vs. Exploratory Strategies

Scripted testing involves creating detailed, step-by-step test scripts that testers execute manually. Each script includes specific instructions for navigating the application, inputting data, and validating expected outcomes. Testers work their way through these predefined scripts, carefully documenting the results of each step.

The primary advantage of scripted testing is its repeatability and predictability. By defining test cases upfront, teams can ensure that critical functionality is consistently validated with each testing cycle. 

On the other hand, exploratory testing takes a more flexible and dynamic approach. Rather than following predefined scripts, testers design and execute tests on the fly, leveraging their knowledge of the system and their testing expertise to uncover defects. 

Exploratory testers simultaneously learn about the application, design tests, and execute those tests in a fluid process of discovery.

Scripted Testing

For example, consider a scripted test case for validating the login functionality of a web application:

**Test Case: Valid Login**

  1. Navigate to www.example.com/login
  2. Enter username “testuser@example.com”
  3. Enter password “T3ststudy@!” 
  4. Click the “Login” button
  5. Verify that the user is redirected to the home page
  6. Verify that the user’s name appears in the top right corner

This test case provides a clear, reproducible set of steps that any tester can follow. If the expected results are not observed, the tester knows they have uncovered a potential defect. 

Scripted testing also has some notable limitations. Creating and maintaining detailed test scripts can be time-consuming, particularly in agile environments where requirements frequently change. 

Testers may become overly focused on following predefined steps rather than exploring the application deeply. Scripted tests can also miss important defects that fall outside the scope of the predefined test cases.

Exploratory Testing

Exploratory testing allows testers to apply their creativity and critical thinking skills to surface complex and subtle defects.

Exploratory testing can be especially valuable in agile environments, where requirements are often fluid and testing time is limited. By enabling testers to quickly assess a feature and provide feedback, exploratory testing supports iterative development and continuous improvement.

However, exploratory testing also presents some challenges. Because test steps are not predefined, it can be difficult to document and reproduce the steps that uncovered a defect. 

It relies heavily on the skill and experience of individual testers, making it hard to assess coverage and maintain consistency across the team. Some organizations also struggle to integrate exploratory testing into their established processes and metrics.

Using Scripted And Exploratory Testing Together

In practice, the most effective functional testing strategies include a blend of both scripted and exploratory approaches. 

Scripted tests ensure that critical paths and common use cases are consistently covered, while exploratory testing allows for deeper investigation and creative problem-solving. Many teams succeed with session-based exploratory testing, which provides some structure and documentation to exploratory efforts.

By leveraging the strengths of both scripted and exploratory testing, this approach balances repeatability and flexibility, maximizing defect discovery while ensuring consistent coverage of key functionality. 

The testing effort for this feature might blend scripted and exploratory approaches like this:

  1. Testers review the feature requirements and create a set of scripted test cases covering core functionality, such as:
    • Creating a report with valid data selections and filters
    • Applying different chart types (bar, line, pie, etc.) 
    • Saving and loading a report
    • Sharing a report with another user
    • Attempting to create a report with invalid or missing data
  2. Testers execute these scripted test cases and log any defects discovered. For example, they might find that attempting to save a report without a name throws an unhandled error.
  3. With core functionality validated, testers conduct exploratory testing sessions. They experiment with different data field combinations, apply complex filters, and attempt to stress the system with very large data sets. Through exploration, they uncover several additional defects, such as:
    • Certain combinations of data fields cause the report to render incorrectly
    • Applying multiple complex filters in a specific order causes the application to hang
    • Extremely large data sets cause the report generation to time out without a proper error message
  4. Based on the results of exploratory testing, testers work with developers to create additional scripted test cases that reproduce the complex scenarios uncovered. For instance, they create a test case that applies the specific combination of filters that caused the application to hang.
  5. Testers execute a final regression test suite that includes both the original scripted test cases and the new cases derived from exploratory testing. This ensures that all known issues have been resolved and the feature is stable.
  6. The feature moves into user acceptance testing, where business stakeholders validate that it meets their needs and expectations. Any feedback or issues from this phase are incorporated into additional scripted and exploratory testing as needed.

Best Practices For Functional Testing

1. Prioritize Testing Based on Risk

One of the most important rules to follow with functional testing is prioritizing testing based on risk.

In any non-trivial software application, there are countless potential test scenarios, and it’s rarely feasible to test everything exhaustively. Instead, effective testers focus their efforts on the areas of highest risk, where defects would have the most severe impact on users or the business. 

Risk-based testing considers factors such as the complexity of the code, the criticality of the feature, the frequency of use, and the history of defects. By testing the riskiest areas first and most extensively, teams can uncover and rectify the most serious issues early, when they are least costly to fix.

2. Engage Testers Early

By engaging testers early and collaborating closely with the rest of the team, testers can help prevent defects, rather than just detecting them after the fact.

Traditionally, testing was seen as a separate phase that occurred after development was complete. However, in modern agile and DevOps environments, testing is increasingly integrated into every stage of the software lifecycle. 

Testers collaborate with business analysts and product owners to help define and clarify requirements, ensuring user stories are testable and include clear acceptance criteria. They work with developers to review code changes and provide feedback on potential quality issues. They automate tests and integrate them into the continuous integration/continuous deployment (CI/CD) pipeline, enabling rapid feedback on code changes. 

What is a shift left strategy for QA? In short: testing earlier and more often in the development cycle.

3. ​​Apply Test Automation Strategically

Effective use of test automation is another hallmark of successful functional testing. Automated tests, which use software tools to execute predefined test cases and compare the results to expected outcomes, can significantly improve the speed, reliability, and coverage of testing. 

By automating repetitive and time-consuming tasks, such as regression testing and data setup, testers can free up time for more high-value activities like exploratory testing and user advocacy.

4. Foster a Culture of Quality and Continuous Improvement

Fostering a culture of quality and continuous improvement within the testing team and the broader organization contributes to effective testing. Testers should be empowered to provide candid feedback on quality issues and to suggest improvements to the development process.

Organizations with strong testing cultures often adopt practices like regular retrospectives, where the team reflects on what went well and what could be improved in their testing process. They encourage testers to develop their skills through training, conferences, and certifications. They also recognize and celebrate testing successes, such as finding a critical defect before release or improving test coverage in a high-risk area.

5. Good Test Data Management 

Effective testing also requires good test data management. Test data includes all the inputs, configurations, and preconditions needed to execute tests, such as user accounts, product catalogues, and system settings. 

Managing test data can be complex, especially for large, data-driven applications. Effective teams develop strategies for creating, maintaining, and resetting test data to ensure reliable and repeatable tests.

This might involve creating tools to generate realistic test data in bulk, setting up isolated test environments with known data sets, and developing processes for refreshing data between test runs. Good test data management ensures that tests run with predictable and representative data, reducing false positives and negatives.

Conclusion

Functional testing is more critical than ever in today’s fast-paced and highly competitive software landscape. 

By understanding the principles, practices, and challenges of functional testing and continuously striving to improve and adapt approaches, you can help deliver software that truly meets users’ needs.