Component testing vs unit testing

Testing is a vital part of the software development lifecycle. It plays an important role in continuous integration and delivery (CI/CD), enabling developers to release dependable, resilient, and secure software consistently.

There are many types of testing and testing methodologies: end-to-end (E2E) testing, static and dynamic security testing, integration testing, and others. This article focuses on component testing and unit testing, two techniques for validating the individual parts that make up a complete software systems.

You will learn what unit testing and component testing are, their differences, scenarios where each method is better suited, and how to automate your unit tests and component tests for faster, more reliable delivery.

What is unit testing?

Unit testing is a software testing method that involves testing individual pieces (units) of code in isolation from the rest of the software. By focusing on the smallest testable parts of an application—typically functions or methods—unit testing ensures that each segment performs as expected before it is integrated with other parts of the application.

Unit testing is a form of open-box testing, in which the test evaluates the inner workings of the code—its structure and logic—rather than its functionality for the end user.

Unit tests are created and performed in complete isolation from the rest of the system. This separation is the primary difference between unit testing and integration testing, which focuses on how units of code and components work with each other.

To perform isolated unit tests, developers create mock-ups, or mocks, of all the resources that the function they are testing requires. This ensures consistent inputs and predictable outputs.

Because the outputs are predictable, you can set up checks and validation on those outputs, so changes to that piece of code can be tested quickly. As long as it passes the checks and validations, you can be certain that everything still works as it should. Unit testing is similar to and often paired with functional testing, where the outputs are compared with an expected outcome.

Unit testing is a vital part of test-driven development (TDD), a methodology that encourages you to create unit tests before writing your code. This approach gives you a clear vision of the expected inputs and outputs and ensures only code needed to pass the tests makes it into your application, helping you stay focused and avoid scope creep.

What is component testing?

Component testing is a form of closed-box testing, meaning that the test evaluates the behavior of the program without considering the details of the underlying code. Component testing is done on the section of code in its entirety, after the development has been completed.

Component testing takes longer to conduct than unit testing, because a component is made up of multiple units of code. Although it can be time-consuming, it is still very much necessary. Sometimes the individual units work on their own but start having problems when you use them together.

Component testing examines use cases, so it could be considered a form of end-to-end (E2E) testing. End-to-end testing and component testing replicate real-life scenarios and test the system against those scenarios from a user’s perspective.

Implementing component testing is beneficial because it can reveal bugs and errors that users might encounter. It is always better to catch and fix these errors before users know about them.

There are two types of component testing: component testing in small and component testing in large.

Component testing in small

With component testing in small, the components that are being tested are still segregated from other components in the system. You should still make use of mocks and test endpoints to simulate the components that connect with the one that is being tested. This form of testing ensures the component is ready to be integrated with the rest of the system.

Component testing in large

Component testing in large is done without segregation, meaning the tested component will have access to external components. With component testing in large, it is still only the main component that is being tested, not the connected components or how the components interact with each other. That would be integration testing.

Component testing vs unit testing: which should you use?

Unit testing and component testing are unique testing methodologies that work best when used together as part of a comprehensive testing suite.

Unit testing is the first line of defense in the software development process, and is often automated to run after every change by the developer. This granularity helps developers catch and correct errors early, making it easier to maintain the code over time.

Component testing builds on the foundation laid by unit testing. It uses real-life scenarios to determine how the software will perform once it is released. As such, component tests may be run on a less frequent basis compared to unit tests, usually after a significant integration phase or as part of a scheduled testing cycle.

Employing both approaches allows teams to address issues at the micro and macro level, improving the stability and usability of the final product.

Unit testing and component testing use cases

While unit testing and component testing are complementary approaches to software quality assurance, they each serve distinct roles within the development lifecycle and cater to different types of use cases.

Unit tests work best for validating the inner workings of a system, like manipulating data between the system and a database or mapping data to be used in an API call. Component testing works best where use cases can be fleshed out and tested, like a page on a website or app.

Another place where component testing is critical is when a component is reliant on several smaller components or features. Each small feature can be unit tested, but the whole component must be tested when all parts are in operation.

Let’s consider a practical example.

Imagine you are building a website registration form. Fields on the form include:

• first name
• last name
• email address
• password
• password verification

The form also includes a submit button that will take the information in the fields and send it to an API service.

To ensure each element functions correctly, developers employ unit testing. For instance, they might test to ensure the form accurately validates the user’s input. If the API imposes a 25-character limit on the first name, the developer will use mock data to test whether the form correctly handles names that meet or exceed this limit. They might also test other functions, such as whether the email format is correct or if the password matches the verification field.

Component testing complements this by focusing on how well the entire form works together under more complex scenarios that mimic real user behavior. Testers might try various behaviors, like entering excessively long names, omitting necessary information, or seeing how the form handles invalid inputs.

Component testing closes the gaps that unit testing does not cover, ensuring a well-tested system. The more you test your code and the more scenarios you test your code with, the better your application will be.

Automating unit tests and component tests

Once all your unit tests and component tests have been created, you can write your use cases as code. After that, you can implement a CI/CD pipeline to automate the testing process. That means that each time a commit has been made to a branch of your repository, the pipeline will automatically build the branch and run the tests you have created. You can even use branching and tagging strategies to run different tests at different stages of development.

Automating tests frees up a lot of time in the software delivery process — especially as the code base and system get more extensive. This also gives peace of mind for when code changes are committed. Developers will instantly know if the change they implemented has negatively affected their code. The checks and validations placed on the tests will fail if the committed change unexpectedly influences a different part of the system.

Conclusion

In this article, you learned the difference between unit testing and component testing as well as the benefits of automating your test suite. With each commit, you will have certainty and peace of mind that every facet of your system is in working condition. If something does not meet your testing requirements, you will know immediately, instead of when a user reports an issue. The benefits of testing your software far outweigh the time commitment required to create and maintain the tests — especially when they are automated.

Implementing an effective testing schedule can take some time, and automating the testing process yourself can be complex. CircleCI can help automate your CI/CD pipeline to accelerate your team’s delivery and boost your confidence in your code. To get started, sign up for a free account today.