Functional Testing: A Complete Introduction

Just the sheer number of names and terms can be overwhelming for those starting their software testing journey. In this post, we aim to ease your path a little bit by offering a guide to yet another testing term: functional testing.
In a nutshell, functional testing is all about making sure your application meets its functional requirements. In other words, that it does what it’s supposed to do.
Of course, there’s more to it than that. If you’re ready to dig deeper, let’s get started.

A definition of functional testing

Let’s open the post with a definition of functional testing.

In brief, functional testing helps you verify your software against the client’s functional requirements. But what are those? Here’s a shortlist of typical functional requirements most software applications include:

Validate business rules: Imagine an insurance agency that wants to make sure its business rules have been implemented correctly. If so, the software should make the right decisions each time a customer visits the site.

1. User actions related to registration and login: for example, provide the ability to reset the password in case the user lost it.
2. Historical data: This refers to the ability to retrieve previous information, such as access or change logs.
3. Authorization: This means the ability to configure different access levels and roles within the application. The access roles define who’s able to see what type of content.

Functional testing vs. non-functional testing

How does functional testing differ from non-functional testing? As you’ve just seen, functional testing helps you determine how well your application meets its functional requirements. In other words: does the application do what it’s supposed to? Is it going to help users fulfill their needs?

However, there are other aspects of an application that are just as important as the functional requirements. They aren’t covered by functional testing though, and this is where non-functional testing comes in handy.

Non-functional testing verifies an application against its non-functional requirements. What are those? Here’s a non-exhaustive list:

1. Performance
2. Usability
3. Security
4. Localization
5. Scalability /Reliability/Availability

So, we can say that non-functional testing is an umbrella term that refers to all types of testing that verify non-functional aspects of an application, such as performance testing, usability testing, and so on.

Importance of Functional Testing in Software Development

Testing is the best tool to assess the quality of your software. A stable codebase allows you to develop new features much faster, staying ahead of the competition. Furthermore, software testing helps you reduce your time-to-market metric. With a well-tested app, you can find defects sooner, fixing them before they reach production. This saves you money, time, and makes for an overall better user experience, improving the reputation of your brand.

Talking specifically about functional testing, why is it so important?

Simply put, functional testing ensures all functional requirements have been met.

As you write functional tests, you’re also validating that the failure paths work correctly. This increases the quality of the product and its security. In case something goes wrong, the client can be sure no sensitive information has been leaked.
(Want to learn more? Read Testim’s blog about the test automation pyramid).

(Want to learn more? Read Testim’s blog about the test automation pyramid).

Simply put, functional testing ensures all functional requirements have been met.

5 types of functional testing

Now let’s explore five of the most interesting types of functional testing. The most obvious one on this list is unit testing.

1. Unit testing: Validating atomic pieces of code

Unit testing is concerned with validating the smallest pieces of code, also referred to as atomic testing. This type of testing tries to isolate the business logic and checks its sanity. For example, did the development team implement the logic correctly? It’s an important part of testing because the business logic often holds the most value for a company or product. Therefore, unit testing helps guarantee the quality of your business logic.

To ensure you’re testing the business logic only, it makes sense to use mocking and stubbing. These eliminate any other factors, such as database or API calls, that might influence the end result.

It can be time-consuming to write unit tests for every single piece of business logic. However, doing so contributes heavily to the quality of your code—and that’s what’s most important.

2. Component testing: Validating a set of functions

Instead of vetting an atomic piece of code, component testing focuses on testing a set of functions as a whole. Basically, the testing engineer wants to validate if the component returns the expected output for a given input.

When testing components, you already pay attention to the functional requirements. Imagine you’ve developed an authorization component. From a functional point of view, the testing engineer needs to verify if the component returns the right permissions for a particular user.

Note that component testing has a broad definition. It’s not restricted to a code component. You can also consider a web page or screen to be a component.

After you complete component testing, you’ll want to validate the integrations between components. That’s why integration testing was invented.

3. Integration testing: Validating component interactions

As the word suggests, integration testing helps you determine how components behave when they have to work together. This type of testing is one of the most common and effective techniques to detect new bugs. For developers, it’s not an easy task to always think on a higher level when developing a new component. Therefore, there are more opportunities for bugs to happen when components have to interact with each other.

4. Smoke testing: Quality assurance testing

You perform smoke testing only when you’ve created a new build. This means that you (or the testing engineer) need to verify the build for every small release against a set of requirements. The testing engineer doesn’t need to perform full-blown testing. That’s because you or your team members have already guaranteed the quality through different types of tests, such as unit and integration tests. However, smoke testing includes testing the most important paths or components in the software build.

Smoke testing usually happens during the quality assurance (QA) phase.

Last, let’s look at regression testing and how it helps ensure the stability of your build.

5. Regression testing: Assuring build stability

Regression testing helps the testing engineer find out whether new code has caused any existing functionality to break. It’s likely that a new piece of code introduced bugs in older functionality. Therefore, it’s important to perform regression testing to ensure the stability of your code for both new and old functionality.

For example, let’s say a particular function accepts one specific format of data. If you introduce a new component that accepts a new data format, then those components can’t communicate anymore. Regression testing helps you detect such problems. Integration testing should catch these types of bugs. If such a bug slips through, though, regression testing acts as a final line of testing to catch it.

Best Practices for Conducting Functional Testing

I guess by now you should be convinced of the importance of functional testing. However, just knowing that it is important isn’t enough: it’s crucial to do it effectively. So, let’s now review some best practices you should be aware of when doing functional testing:

• Practice risk-based testing. Focus on the highest-risk areas of the application.
• Have clear objectives. Know exactly what you’re testing and why before committing to the testing effort.
• Document defects found effectively. Clearly document when defects are found, including steps to reproduce, version of the application, an environment in which the defect happened, and information about the context if applicable (for instance, operating system, browser name and version, and so on.)
• Implement an effective test data management strategy. It’s crucial to have realistic test data when performing functional testing. Otherwise, you’ll probably get results that aren’t representative of how the application will behave once in production.

Challenges in functional testing

Before wrapping up, let’s also cover some of the challenges in functional testing:

• Requirements aren’t clearly defined and/or change too often. It’s challenging to test against requirements when it’s not clear what they are!
• Limited resources. Testing can be tough when the organization doesn’t invest in it. If you don’t have the necessary resources and/or there are severe time constraints when testing, it’s going to be hard to test effectively.
• Test data management. Creating an effective test data management strategy is hard, time-consuming, and potentially expensive.
• Managing cross-browser and cross-device compatibility. When it comes to web applications, you must ensure they look well and work correctly across a variety of browsers, devices and screen sizes, and that alone is a challenge.

It’s crucial to have realistic test data when performing functional testing.

Final words on functional testing

Many types of functional testing exist. In addition to the ones we’ve discussed, you can also explore sanity testing, user acceptance testing, black/white box testing, and system testing. However, to guarantee the quality and stability of your code, the above five testing types are generally enough.

Unit testing helps protect the sanity of your business logic. In contrast, integration testing helps you detect tricky bugs that can pop up when components have to interact. And don’t forget that when you complete unit and integration testing, that’s like verifying that your car engine fits in your car and can start. With just those tests, you don’t know whether your car can drive down the highway. You get that information from component, smoke, and regression testing.

Let us know how your functional testing journey works out!

This post was written by Michiel Mulders. Michiel is a passionate blockchain developer who loves writing technical content. Besides that, he loves learning about marketing, UX psychology, and entrepreneurship. When he’s not writing, he’s probably enjoying a Belgian beer!