API Testing vs UI Testing — Differences You Must Know

8 min readSep 30, 2024

API and UI testing are the two most important aspects of quality assurance. They ensure that the user can surf through an application smoothly, but they target different areas of the application.

API testing assesses the functionality and behavior of an application’s backend, which is the logic powering an application. It is focused on the interaction between software components and tells if the API returns correct data and responds appropriately to various requests.

UI testing, on the other hand, targets the front-end part of the application. This is the part that the users directly interact with. UI testing ensures that the UI elements are properly displayed for a positive user experience.

In this article, let’s understand the major differences between API vs UI testing. We will see why automation in API and UI testing is necessary, and conclude how an AI-powered test automation tool like Testsigma has the potential to make things easier and perform better API and UI testing.

What is API Testing?

API testing is a part of software testing that concentrates on the quality assurance of an application’s APIs for functionality and behavior verification. Requests are sent to the API, and the responses obtained from it are validated should match the correct and expected requirements.

The key facets of API testing are:

Functional testing: The system ensures that API performs its intended functions correctly.
Performance testing: Response time, scalability, and reliability of the API should be checked under different load conditions.
Security testing: Identifying and addressing vulnerabilities in the API, such as unauthorized access, data breaches, and injection attacks.
Integration testing: The API should integrate well with other components of the application.
Documentation testing: Checking for the accuracy and completeness of API documentation.

Why Do We Need API Testing?

Testing API is important for several reasons:

Early detection of defects: Defects could be detected as early as possible during the development process, way before a user interface is built, saving time and resources.
Improved performance: By testing the API’s performance, you can identify and optimize bottlenecks to ensure a smooth user experience.
Improved security: API testing aids in finding and fixing security vulnerabilities to safeguard your application and user data.
Less maintenance costs: A very well-tested API would bring down maintenance costs since there would be fewer chances of issues occurring that might lead to expensive fixes or downtimes later.
Greater collaboration: API testing might also make it easier for development and testing teams to collaborate.

API Testing: Manual vs Automated

Testing an API may be done manually or in an automated way. Both approaches have pros and cons, which are predominantly determined by the specific needs of the project.

Manual API Testing

Manual API testing is a process that involves manually sending requests to the API and examining received responses. This could prove time-consuming or error-prone, especially for complex APIs with huge numbers of endpoints and test cases involved.

Advantages of manual API testing include the following:

Flexibility: Manual testing allows more flexibility in scenarios and ad hoc exploration.
Human judgment: A manual tester can use his or her judgment about possible unexpected issues or patterns.

Disadvantages of manual API testing:

Time-consuming: Testing can be slow and time-consuming with manual testing, which may sometimes be an issue for large-scale projects.
Error-prone: It is error-prone since manual testers might unintentionally introduce faults or inconsistencies into their test cases.

Automated API Testing

Automated API testing involves using special tools designed for writing test scripts that can be executed automatically. It can send a request, validate a response, and also generate reports and notifications.

Pros of automated API testing:

Efficiency: Automating the testing process can save a lot of time.
Accuracy: Automated testing is much more accurate and reliable compared to manual testing.
Repeatability: With automated tests, testing can be repeated easily to ensure that the API remains stable over time.
Integration: Automated API testing can be used in the pipelines of Continuous Integration and Continuous Delivery (CI/CD).

Disadvantages of automated API testing:

Initial setup: Setting up automated API testing may require an initial investment in tools and expertise.
Maintenance: Automated test scripts may need to be maintained as the API evolves.

In most cases, a combination of both manual and automated testing is the best. Manual testing can be used in an exploratory way and ad-hoc scenarios, whereas automated testing is useful in regression testing and for repetition.

What is UI Testing?

UI testing aims at validating the functionality and usability of the user interface of an application. This involves testing the interaction with UI elements like buttons, text fields, and menus to see that an application works as expected and provides a good user experience.

The key facets of UI testing are as follows:

Functional testing: Making sure the UI elements display properly as intended.
Usability testing: A method to check how easily actual users can navigate and engage with the UI.
Compatibility testing: Ensuring that UI works well with various devices, browsers, and operating systems.
Performance testing: Check UI responsiveness and response time on peak load.
Accessibility testing: Making sure people with disabilities can access the UI.

Why do we need UI testing?

The main reasons why UI testing is important are:

User Satisfaction: The existence of a well-designed and functional UI gives users great satisfaction and builds their engagement.
Brand reputation: A poor user experience can damage a brand’s reputation and lead to customer churn.
Compliance: In some industries, UI testing is required to comply with regulations and standards, such as WCAG (Web Content Accessibility Guidelines).
Early detection of defects: The best stage to identify issues is during the early stages of development before an application is rolled out to users.
Improved quality of the product: Testing at the user interface level ensures a better-quality end product.

UI Testing: Manual or Automation?

Like API testing, UI testing also can be performed manually or in an automated way. Let’s look at the pros and cons of each of them.

Manual UI Testing

Manual UI testing is a process of performing actions on UI elements and checking their behaviors. This approach can be beneficial when the app is less-complex and the elements are fewer. It is time-consuming and error-prone, especially when the UIs are complex with many features and test cases.

Advantages of Manual UI Testing:

Flexibility: As humans, we can easily adapt changes in UI or test scenarios.
Human judgment: This can reveal several small usability issues that might go undetected by automated tests.
Exploratory testing: It can be used for exploratory testing to discover new issues or use cases.

Disadvantages of Manual UI Testing:

Time-consuming: Slow and monotonous for projects of large dimensions.
Error-prone: Manual testers often unintentionally inject errors or inconsistencies into the test cases.
Limited coverage: They might not include all the possible scenarios or combinations of inputs.

Automated UI testing

Automated UI testing uses specialized tools for developing test scripts that can be run in an automated and scheduled manner. These tools have flexibility and lots of added functionality. They can imitate user actions with the UI, verify results, and create reports by themselves.

Advantages of automated UI testing:

Efficiency: Automated testing can significantly speed up the testing process.
Accuracy: Automated testing can be more accurate and consistent than manual testing.
Repeatability: Automated tests are pretty easy to repeat. This allows for automated regression testing as soon as a new piece of code is checked in.
Integration: One can integrate automated UI testing into a DevOps pipeline for continuous integration and delivery.

Disadvantages of automated UI testing:

Initial setup: Setting up automated UI testing can take quite an initial amount of investment into tools and expertise.
Maintenance: Automated test scripts may need to be maintained as the UI evolves.
Flaky tests: There may be times when automated UI tests are flaky. This may be due to changes within the UI or some other factor that is not in your control.

API vs UI Testing: The Core Differences

Automating API and UI Testing With Testsigma

Since API and UI testing are the pillars for assuring the quality of applications, there are several tools, each focusing on automating the testing of specific functionalities. However, there are very few tools that cover all aspects of end-to-end application testing. One such tool is Testsigma.

Testsigma is a cloud-based, AI-powered test automation platform designed to make creating, running, and managing the tests of APIs and the UI easy. Testsigma provides all the features and integrations that teams need to speed up their testing efforts and ship quality software.

API Testing in Testsigma

Testsigma provides a robust automated API testing solution, which allows teams to test APIs effectively with minimum effort. Some major features of it are:

Codeless, Natural Language Test Creation: With Testsigma, you can create test cases in simple natural English language.
Minimal Test Setup: No installation or setup is required. You can simply execute your API tests on the cloud.
Intuitive API test creation: API tests can be developed at a much higher speed with an easy and intuitive interface compatible with all HTTP methods possible, such as GET, POST, PUT, and DELETE, along with incorporated parameters in the request.
Dynamic data management: Maintain dynamic data like tokens, timestamps, or random values in API testing to keep precision within these tests.
Data-driven testing: Run the very same API test with different sets of data to exercise various scenarios and edge cases.
Test Result Analysis: Detailed test result analysis of response codes, headers, body content, etc.
Integration with CI/CD Pipelines: Integrate API testing into your continuous verification frameworks across CI/CD workflows.

For more information, you can refer to the detailed documentation on automating your REST API testing with Testsigma.

UI Testing in Testsigma

Testsigma also offers a range of powerful features for UI Test Automation. With Testsigma, you can use recorders to automatically identify the elements in your application and create test cases on the fly.

Cross-browser and Cross-device testing: Run tests at the user’s end across the most widespread combination of 3000+ browsers, operating systems, and devices.
Visual testing: Comparing screenshots of your application to detect visual differences and assure consistency.
Action-based tests: Develop tests with interactions such as clicks, taps, swipes, etc.
Element locator: Identify UI elements correctly by using different element locators such as XPath, CSS selectors, and ID.
Test data management: Proper management of test data will be ensured to eliminate hardcoding and implement data-driven testing.
Test result analysis: Analyzing test results in detail with screenshots, logs, and performance metrics.
Integration with defect tracking tools: Provide seamless UI testing integration with your defect tracking tools for an efficient bug reporting process.

You can sign up for a free trial of Testsigma Cloud and start automating your API and UI testing in minutes.

Conclusion

To conclude, API and UI testing are two very important components of software quality assurance. API testing focuses on functionality at the back end and data exchange, while UI testing concerns the user interface at the front end and user experience.

A platform like Testsigma dramatically increases the efficiency of teams in terms of automated API and UI testing manifold and reduces potential risks due to defects while ensuring the delivery of higher-quality software.