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Functional Testing

Functional Testing

Functional testing (also known as black-box testing) is a type of software testing that involves testing a mobile or web application by a tester who doesn't know its design or how it works. They use it and report irregularities occurring in its functioning.

The black box metaphor doesn't appear here without reason.

A mobile or web application that undergoes functional testing is treated like the proverbial black box.

It's the "outside" part of it (e.g., the interface) that is tested, not its "inside" (e.g., the source code).

Did You Know...

The opposite of black-box testing is white-box testing, in which the "inside," meaning application code, is being tested. In particular, it is tested for its structure and operation logic.

Functional testing enables your business to build high-quality and error-free applications that meet business and customer requirements.

Functional testing of an application allows you to check the following:

  • Proper operation of a function
  • Consistency and completeness of functional and non-functional requirements
  • User interface
  • APIs
  • Security
  • Client-server communication

In addition, many problems involving requirements are apparent at the functional testing design stage long before testing is performed on a finished software.

Performing functional testing also enables understanding how the system works under certain conditions.

Equally important is the collaborative testing of software by business analysts, programmers, and customers that results in a consistent and shared vision of how an application should work.  

Testing is an effective and recommended tool used equally to validate and verify requirements.

Are You Interested In Functional Testing?

Who are the testers performing functional testing?

Usually, testers aren't programmers, and their programming knowledge and experience are similar to those of the average application user. This lack of knowledge and expertise is intentional and desirable.

Functional testing is about obtaining a specific perspective of a "non-technical" person.

Of course, testers are told what the application is for, but they aren't familiar with how it works or performs calculations.

Functional testing is about checking the performance of a web application or mobile app from the perspective of a person who will be its potential user.

A user who expects it to operate intuitively, simply, and similarly to the operation of comparable applications.

And, of course, without errors. An app that allows for the achievement of goals and tasks.

Did You Know...

An application undergoing functional testing is primarily intended to work according to the expectations of its users, not its developers, who evaluate it to meet technical requirements. Functional testing verifies that an application works according to specified requirements.

What is the purpose of functional testing?

The primary purpose of functional testing is to check how the application works from a functional point of view.

In slightly more technical terms, functional testing aims to check how functionalities contained in the system requirements specification are implemented.

The method is all about answering questions — How does the system behave? How does the user perceive it?

How does it perform tasks? How does it help to achieve objectives?

Testing can involve an entire application or parts of it. It also encompasses how the application works with other external systems.

Like any testing, functional testing is focused on detecting and analyzing emerging bugs or imperfections in application performance.

It's used not only for diagnostics but also to determine the cause of an error. Of course, we don't mean causes in the technical sense.

However, testing a mobile or web application isn't haphazard but is usually based on test scenarios used to diagnose individual functionalities.

A test scenario is a collection of actions an application tester should perform.

From another perspective, functional testing aims to determine how and to what extent the application can meet business objectives.

Functional testing is focused on the following:

  • Testing the most essential functions of the application
  • Checking the basic usability of the application, in particular, whether moving from one screen to another is trouble-free
  • Determining the application availability level
  • Determining error conditions

When should you perform functional testing?

It may seem that only the finished, completed version of a mobile or web application can be tested. However, this is not a necessary condition.

Did You Know...

Both finished products and those just in the production process can be tested. These tests can be performed at any stage of the work. A popular method is to test the application at successive crucial phases of its development.

Regular and scheduled testing helps avoid many errors and costs involved in fixing them.

They differ from tests performed on a completed and implemented application.

The latter focuses more on testing the application's consistency, completeness, and security.

Tests performed after a given stage of work are more geared toward verifying the proper operation of a given functionality and how it helps achieve business objectives.

Functional testing and non-functional testing

Functional testing is a type of testing that focuses on confirming that each feature of an app or software works according to predetermined software requirements. Its main goal is to test whether the system works as intended.

A functional testing example may involve checking whether a product can be successfully added to a cart or testing whether users can log in on a website after entering the user's credentials.

Non-functional testing concentrates on assessing the system's behavior. It's usually done by conducting performance testing, usability testing, or security testing.

Unlike functional testing, non-functional tests are compared to customers' expectations, not software requirements. While functional testing is mainly concerned with individual functions, non-functional testing has a larger scope that goes beyond the features of web and mobile applications.

Types of functional testing

The most fundamental distinction of software tests is the division by tester criteria. It could be a human or testing program.

In the first case, we're talking about manual tests; in the second, we mean automatic tests.

Manual tests are more demanding in terms of:

  • Budget
  • Organization of testing sessions
  • Time
  • Method
  • Credibility, reliability, and correctness.

The risk of mistakes and not following a methodology or a test scenario is higher wherever the tester is human.

However, manual tests also have a huge advantage; they allow you to test applications from a purely human perspective.

They provide insight into automatically performed actions that can lead to desired or undesired results (application works as expected vs. application doesn't work as expected).

They allow you to see the very human dimension of human–software interaction.

This is of colossal importance in terms of offering a satisfying user experience.

Manual functional testing usually focuses on the performance of the application interface. It provides answers to whether the function is working correctly and satisfactorily.

Unfortunately, automated tests can't guarantee this very important feedback, which is why they're less useful in UX research.

Automated tests, based on a test script, allow verification of the application's performance in multiple dimensions.

Their main advantage is the execution speed, higher reliability, and less error susceptibility.

They're especially recommended when expanding the application, adding new features, or checking its behavior after implementing repair or optimization work.

Besides the division into manual and automated functional testing, other functional testing techniques are worth knowing.

Unit testing

The unit testing process involves testing individual software components and features. Developers conduct it during the application's construction to ensure the given functionalities are working correctly.

In other words, developers test the smallest functional unit of code, which allows them to detect issues early in the software development life cycle.

For example, developers can test whether a given UI element displays another after clicking on it.

Component testing (Module testing)

Component testing (also known as module testing) is a type of software testing that usually occurs after unit testing. The difference is that it doesn't involve testing individual pieces of code but a larger component. You may want to simulate interactions by providing appropriate test data for component testing.

Component testing is done to ensure that the units of code work together correctly. Because of that, this type of software testing requires a little more time than unit testing.

Integration testing

Integration testing aims to test whether the developed components and modules can work together without issues. Especially when different developers are building them and when microservices are involved. Thanks to integration testing, you can detect problems during interaction between various components.

System testing

System testing seeks to test the entire system after the development process. During this test, the goal is to verify the functional and business requirements. To conduct system testing, you need to make sure that the test environment is as close as possible to the production environment. It's usually performed after unit, component, and integration testing are done.

Regression testing

After adding any new feature to your web or mobile application, you should perform regression testing. Regression testing allows you to make sure that the software is working stably and that the functionalities don't come into conflict with each other. The regression testing process involves rechecking whether the software still meets functional and non-functional requirements.

Sanity testing

Sanity testing is a type of software testing that concentrates on checking whether the code still works as intended after bug fixes or small changes to the code. In contrast to regression testing, sanity testing focuses on specific parts of an application that are affected by the new changes.

Smoke testing

The goal of smoke testing is to evaluate the operation of the core functionalities before proceeding to user acceptance testing. This allows you to make sure that everything is working as planned. When performing smoke testing, you only need to test a web or mobile application's most basic and critical features. This is done to avoid a situation in which the software goes for further testing with critical bugs.

User acceptance testing

User acceptance testing involves testing software in real-life conditions by its target users. The aim of user acceptance testing is to ensure that the application operates correctly and that users don't encounter bugs or other issues. This type of software testing is done before the application's launch. It's the last step to polishing its operation before it reaches users.

Benefits of testing web and mobile applications

Why should you perform functional testing? The main argument in favor of application testing is, of course, the cost that errors generate.

As a rule, the cost of fixing errors increases with the application's complexity. It's lowest at the early stages of production and very high when the application is already functioning on the market.

Unfortunately, fixing bugs in already implemented applications functioning on the market has the disadvantage of being a source of errors.

Improving a function's performance can influence the operation of other functions, complicating the work and increasing the repair cost.

That is why function testing should take place while the application is still in production. This reduces the risk of errors, conflicts, or contradictions in the operation logic.

Repair work is also much easier to perform under such conditions.

When we say cost, we mean financial cost, loss of credibility and trust, and deterioration of the application's reputation. In conclusion, the brand's image suffers.

Losses in this dimension translate into financial losses. Rebuilding a reputation is a difficult and costly task in the world of digital products.

Did You Know...

Testing is also necessary for a more obvious reason. Creating a flawless application is very difficult. The source of the error can sometimes be a mistake, an oversight, or a wrong assumption.

Not only programmers but all stakeholders involved in developing the application are susceptible to making mistakes.

Another obvious fact is also worth mentioning.

From the point of view of a mobile or web app user, a buggy application is useless, risky, and untrustworthy.

Eliminating errors is essential if the application is to succeed on the market.

It's safe to say that a buggy application is as attractive to a user as a car with no wheels is to a racing driver.

Best practices for functional testing

Functional testing can be done in at least several ways, depending on your needs and goals.

Functional testing of web or mobile applications may consist of the following:

  • Verifying the functions of the application according to the specifications
  • Performing specific tasks in the application
  • Spontaneous use of the application without a clear purpose or scenario

Examples of functional testing tools

A few functional testing tools allow you to create an automated functional test and save time.


Selenium is a tool for conducting automated functional testing for web applications. It consists of various libraries and extensions that allow it to simulate interactions between users and browsers. Additionally, it provides infrastructure for implementing the W3C WebDriver specification, allowing you to write interchangeable code for the most popular browsers (Google Chrome, Mozilla Firefox, Internet Explorer, Edge, Opera, and Safari).

Selenium's features include the following:

  • Relative locators
  • CDP access
  • Smart object detection
  • Parallel test execution
  • Playback and record feature


Appium is an open-source tool for the functional testing of mobile applications, including iOS, Android, and desktop platforms. Appium uses WebDriver protocol to facilitate cross-platform testing. It also possesses the capability for handling gestures and touches, making automating user interactions much easier.

Appium's features include the following:

  • Support for multiple programming languages
  • Support for native and hybrid mobile applications
  • Built-in inspector for element identification
  • Playback and record feature
  • Integrations with cloud services
  • Parallel test execution


Katalon is built for functional testing of API, mobile, and desktop applications. This software enables testing in one place without the need for additional tools, making it ideal for both newcomers to testing and experienced developers. Katalon also provides detailed reports after the testing ends that you can share with team members and stakeholders.

Katalon features include the following:

  • Cross-browser testing
  • Parallel test execution
  • Test creation without script
  • Data-driven testing
  • Multi-platform testing

Sauce Labs

Sauce Labs is a platform for functional testing of web and mobile apps. It works with various frameworks and programming languages. Sauce Labs facilitates finding and fixing potential issues and provides test-creation capabilities. Thanks to Sauce Orchestrate, you can accelerate the testing speed and deliver your products to market faster. Moreover, it integrates with tools such as GitHub, Azure DevOps, CloudBees, Appium, Bespoken, and more.

Sauce Labs features include the following:

  • Cross-browser testing
  • Low-code testing process
  • Error reporting
  • Performance insights
  • Integration with CI pipilines
  • API testing

Functional testing allows you to improve the quality of web and mobile applications and ensure that their features work according to customers' expectations. Additionally, user testing guarantees that your application meets users' needs and expectations.

Therefore, it's worth investing time and resources into functional testing if you want to deliver high-quality, efficient, and bug-free apps.

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