As we have discussed in the Types of Software Testing guide, functional testing is a type of software testing that ensures each function of the software application works as intended. It can be performed manually or via automation. It may involve testing application functionalities (e.g. logging in or checking out online), or testing the flow of the GUI screen to ensure that user navigation works fine throughout the application.
This is done by first understanding functional requirements, identifying test input data towards these requirements, defining expected outcomes to this test input data, executing test cases, and then comparing actual and expected outcomes. Functional testing, therefore, ensures a stable user interface with optimum functionality and accessibility, while meeting business requirements.
Functional testing, as we know, is the means by which a software product is tested to ascertain whether its features work as intended or whether it delivers the project’s functional requirements. This is done by developing test cases and test data, inputting this test data into the application, and checking whether the actual output matches the expected output. Typical use case scenarios include user authentication and online payment confirmation. As this type of testing does not involve itself with the performance or quality of the application’s code, functional testing is considered to be a form of black-box testing (black-box testing is a testing method that evaluates a software application’s functionalities rather than its source code structure and other implementation related details - no understanding of the underlying code is required, hence it is a black box).
Unlike this approach, non-functional testing is a form of white-box testing that concerns itself with the source code’s structure, speed, and stability. A typical scenario where non-functional testing may be performed is to ascertain how many users can simultaneously log into an application. This form of testing aims to optimize production-related costs, reduce risks, and contribute to the overall understanding of the product’s source code.
Top tip: Functional testing is not to be mistaken with Functional UI testing. Functional UI testing is a type of functional testing at the UI layer. But functional testing can happen anywhere on the testing pyramid. There are several types of functional testing, each of which we will briefly discuss below:
Unit testing is performed by isolating a small section of code (e.g. a function or a method) and validating it for correctness. In this manner, individual units or components of the software are tested for their functionality. It is the first level of software testing done before the code is integrated, and is usually performed by developers.
Integration testing is the second phase of software testing that follows unit testing. Here, individual software units or components are tested as a whole for functionality. This ensures that no bugs creep in when individual units are integrated.
Regression testing is performed at any point when developers change or modify the code. This is done to ensure that the new modification or feature does not break the application’s functionality or introduce new bugs into the system. It may also be done when a new feature is added or when a functional defect has been fixed.
Usability testing is the process of verifying that the entire system works as expected. It is usually performed by the end-user or client to verify that the software product works as intended. Their feedback is taken into consideration, following which further changes are executed in production.
Smoke testing, aka confidence testing, is performed after each build is released to ensure that the software is reliable and stable and that the most critical functionalities of the application work as intended.
Sanity testing is usually performed post-smoke testing, to ensure that each critical functionality of the application works smoothly both in isolation and in conjunction with other elements.
Typically, functional testing would involve the following sequence of steps:
This step involves identifying the project’s functional requirements and setting these as the key functional testing goals. This mainly involves ensuring that the application’s features work as intended.
This step involves building a list of all possible test scenarios for each feature of the application. These test scenarios must clearly define each feature’s functionality.
In this step, testers develop test data that corresponds to the above test scenarios. This could be done manually or via a test automation tool.
Now, the predefined test cases are executed in the application in order to compare actual outputs against expected results. If the actual and expected outcomes do not align, the functional testing has uncovered a defect, which can then be addressed in later steps.
Once the defects within the application have been identified, they are recorded and changes are made to the application, in order to fix them. After this step, the corresponding test cases are re-executed to ensure that the defect has been fixed.
There are several functional testing techniques to be discussed, let us look at a few of them below:
Boundary value tests evaluate data limits to the application and validate the application behavior based on how it responds to inputs that are outside of specified limits.
Decision-based tests are used to ascertain the possible outcomes when a particular condition is met. For example, these tests can be used to verify what happens when a user clicks on ‘Forgot password’.
User-based tests evaluate how different elements of the application work in conjunction with each other.
Alternate flow tests are executed to validate all possible flows that may exist apart from the main flow to complete a function.
Ad hoc tests are done at the end to ensure that there are no discrepancies that slipped through in earlier tests. They are done with the intent to break the system and see how it responds.
In equivalence tests, the test data is divided into different sections called equivalence data classes. Data in each section should behave similarly; if one condition in a section doesn’t work, none of the other sections will work as intended either.
While manually building functional test cases is time-consuming, prone to human error, and does not provide optimal test coverage, there has been an increased surge in the adoption of functional test automation tools to quell such QA woes.
With automated functional testing, tests, once executed, run in tandem without human interaction. Thus, AI-powered test automation tools are picking up the extra workload while increasing test coverage.
Prioritize those application functionalities that are most important, so other low-order functionalities can be considered later.
Identify those test cases that require automation and those that need to be run manually. Ensure that a dedicated automation expert(s) is on top of these test cases.
Create and execute tests early where possible, in order to make amending tests at later stages simpler. Similarly, develop a strategy wherein frequent execution of test cases is possible.
Functional testers must have a thorough understanding of the business requirements as well as the end-user experience. This deeper level of understanding will enable them to develop the most appropriate test cases.