Difference Between Functional and Non-Functional Testing

Testing can generally be divided into two main categories: functional and non-functional testing. Each category addresses different aspects of product evaluation. Let’s explore each in detail.

What is Functional Testing?

Functional testing is a type of software testing focused on verifying that a system meets specified requirements and functions as intended. This form of testing assesses whether the product behaves according to the defined requirements and specifications. This type of testing primarily focuses on the outcome of processing. It is concerned with simulating the actual use of the system without making any assumptions about the system’s structure.The essence of functional testing lies in verifying that each function of the application operates as per the defined requirements and specifications. This type of testing does not involve examining the application's source code.  Each functionality of the application or website is verified by providing the appropriate test inputs, awaiting the output, and comparing the actual result with the expected result.

Functional testing is an overarching term for various tests aimed at verifying the complete functionality of the software. It ensures that the software operates correctly and is free from errors. To verify this, the tester simulates a real end-user scenario and compares the test results with the expected outcomes outlined in the requirements document. Functional testing can be conducted manually or through automation. 

Functional testing primarily focuses on the software's external behavior, which means it does not require knowledge of the internal source code. This approach is commonly known as black-box testing. This type of testing is performed before non-functional testing. There are several types of functional testing, such as smoke testing, sanity testing, regression testing, etc.

What is Non-Functional Testing? 

Non-functional testing is a type of software testing aimed at verifying non-functional requirements of a product. It assesses whether the system's behavior meets criteria beyond its core functionalities. Among the aspects considered non-functional are performance, security, usability, etc. The primary goal of non-functional testing is to ensure the product meets customer expectations for usability, reliability, and security. It evaluates factors like response time, system reliability, and security vulnerabilities.

Typically, non-functional testing is performed through automated methods using specialized tools tailored to each type of test. Examples of non-functional testing include load testing, stress testing, and accessibility testing.

The differences between functional and non-functional testing are more clearly illustrated in the comparative table.

Let's break down each type using the example of testing an online store website.

Functional testing involves verifying whether the website performs essential functions according to the product requirements. This includes checking if users can successfully add products to the cart, if the site accurately calculates item quantities and costs, and if the checkout form fields are validated correctly, among other functionalities.

Non-functional testing, on the other hand, encompasses a range of checks related to performance and usability. This includes evaluating page load speeds and response times, testing compatibility across different browsers, assessing the usability of visual elements for the target audience, and more.

For clarity, we will present examples of each type in a table format:

Each type of testing is important and plays a crucial role in the product verification process. Understanding the differences between functional and non-functional testing will assist in planning and executing the testing processes.