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

There are many types of testing and testing methodologies: end-to-end testing, dynamic testing, integration testing, and others. This article focuses on component testing and unit testing. You will learn what they 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.

Let’s begin by explaining what unit testing and component testing are.

What is unit testing?

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 testing involves testing the individual pieces (units) of code in isolation from the rest of the software. 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 of all the resources that the function they are testing requires. This ensures the inputs the function gets are consistent and the output predictable. 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 mock-ups 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 tests are usually set up according to the design specification, while component tests are made up of use cases.

First, the developer creates unit tests based on the design specifications for each unit of code. You want to make sure that each component part is acting as it should. Component testing uses real-life scenarios during testing, which provides you with a good indication of how the software will perform once it is released.

Unit testing and component testing are unique testing methodologies and actually work best together. The developer runs the unit tests while developing. After the entire component has been completed, the test engineers or QA team conduct the component tests.

Unit testing and component testing use cases

Some cases are more suited toward unit testing or component testing — they are not synonymous.

Unit tests work best for 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.

For the best coverage, you should use a combination of both testing methods. For example, you could use both to test a website registration form. Fields on the form include first name, last name, email address, password, and a field that requires you to repeat the password. Then you will usually find a submit button that will take the fields in the form and send it to an API-type service.

The validation of the fields and the submit button are best tested with unit tests to check if the function is handling the data correctly and responding as intended. The developer would create mock values based on the design specifications to perform the test.

For example, if the API has a 25-character limit for the first name, the developer will choose a short name to comply with the character limits, which does not consider invalid input. This is where component testing is useful.

Since the component tests are a form of closed-box testing, the design limitations are unknown to the testers. So, they would create a list of use-cases based on what a user might do and what-if scenarios, like entering a short name, a long name, or if no name is entered. Component testing closes the gaps that unit testing does not cover, ensuring a well-rounded and tested system. The more you test your code and the more scenarios you test your code with, the better your application will be.

How to automate testing with CI/CD

Once all your unit 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.

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.


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.