Integration tests vs. end-to-end tests: How do they relate?

Most developers think software development is all about building and deploying applications, but there’s more to it. One important aspect that’s often overlooked is testing. In fact, testing is just as important as building the application itself.

Software testing gives you confidence that your application will perform well after deployment. It ensures your app works as expected and meets all the requirements and standards. There are different types of testing in software development, but in this article, we’ll focus on integration testing and end-to-end testing.

We’ll explore the relationship and differences between integration tests and end-to-end tests, including when to use each.

What is integration testing?

Integration testing is a type of software testing that checks how different components of software work together. Instead of testing each part in isolation (like you do with unit tests), integration testing focuses on the connections and data flow between services, how frontend components interact with each other and with backend services, and how your application works with third-party systems.

For example, imagine you’re building a social media application. You’ve already done unit tests to make sure individual features like login, profile, and messaging work on their own. With integration tests, you check if these features work well together as a system.

Suppose a user logs in—integration testing ensures they are authenticated, redirected to their profile where their information is displayed, and can immediately send or receive messages.

Importance of integration testing

Integration testing plays a vital role in ensuring that different parts of your app work together. Here’s why:

• Provides early detection of bugs: Testing how different components of your app interact helps you spot and fix bugs early. You don’t have to wait until things get out of hand. Early detection means early fixing. “In software testing, the earlier a bug is found, the cheaper it is to fix.” – Karen N. Johnson, independent software test consultant.
• Speeds up debugging: Finding bugs early enables you to address them immediately, making the entire development process faster and more efficient.
• Bridges the gap between unit tests and end-to-end tests: Integration tests fill the space between unit tests and end-to-end tests. They help you identify issues in the way components work together, so you can focus on areas that need attention as development progresses.

What is end-to-end testing?

As the name suggests, end-to-end (E2E) testing involves testing your application from start to finish. It lets you simulate real user behavior and interact with your system like an actual user would to verify that everything works as intended. E2E testing covers the complete user flow—logging in, interacting with the browser, performing key tasks, and logging out.

Imagine you’re building an e-commerce platform. E2E testing would require you to test the application as if you were a real user. You’d write test scripts that open the browser, log in, add items to the cart, and complete a payment. This process helps confirm that users can navigate through the entire system without any problems and that all subsystems, like authentication, shopping cart, and payment processing, work together as expected.

Importance of end-to-end tests

Here are some key reasons why E2E testing is important:

• Validates real user workflows: As mentioned earlier, the main purpose of E2E testing is to validate how real users interact with your app. This gives you a clear picture of what users will experience and ensures that all integrated features work together as expected.
• Provides high confidence for releases: When you thoroughly test your application and confirm that everything works well, you can deploy with confidence. You won’t have to worry about unexpected issues or broken features, because you’ve already verified everything.
• Improves user experience quality: When building software, always consider your users first. Think about how they will interact with each feature and what their experience will be like. E2E testing helps you answer these questions and lets you make better decisions. According to Michael Bolton, a consulting software tester and testing teacher, “The job of testing is not to find bugs but to find features that don’t meet the user’s needs.”

Key differences between integration testing and end-to-end testing

Integration tests and E2E tests are two different testing approaches in software development that ensure the release of quality applications. Here are the key differences between the two types of tests:

As discussed earlier, integration tests focus on validating that specific modules or components within the application communicate and function correctly. E2E tests, on the other hand, test the entire application from the user’s perspective.

Let’s use an e-commerce platform as an example. Integration tests verify that the payment module correctly updates the inventory system after a purchase. They make sure these two modules interact as expected. E2E tests go further by simulating a real user: adding items to the cart, completing payment, and receiving an order confirmation. This means E2E tests cover not just the payment and inventory modules, but also the user interface and all other involved subsystems.

Another example is user registration on a website:

• An integration test would check that the registration API stores the user data in the database and sends a verification email via an email service. It tests the interaction between the authentication service, database, and email module.
• An E2E test would simulate a user filling out the registration form on the UI, submitting it, receiving the verification email, clicking the verification link, and successfully logging in.

With these examples, you can see how integration and E2E tests work together. Using both testing approaches helps you build a more reliable and high-quality product.

Relationship between integration tests and end-to-end tests

As discussed earlier, integration tests focus on validating that specific modules or components within the application communicate and function correctly. E2E tests, on the other hand, test the entire application from the user’s perspective.

Let’s use an e-commerce platform as an example. Integration tests verify that the payment module correctly updates the inventory system after a purchase. They make sure these two modules interact as expected. E2E tests go further by simulating a real user: adding items to the cart, completing payment, and receiving an order confirmation. This means E2E tests cover not just the payment and inventory modules, but also the user interface and all other involved subsystems.

Another example is user registration on a website:

• An integration test would check that the registration API stores the user data in the database and sends a verification email via an email service. It tests the interaction between the authentication service, database, and email module.
• An E2E test would simulate a user filling out the registration form on the UI, submitting it, receiving the verification email, clicking the verification link, and successfully logging in.

With these examples, you can see how integration and E2E tests work together. Using both testing approaches helps you build a more reliable and high-quality product.

When to use integration tests vs. end-to-end tests

Now that you know the difference between integration and E2E tests, you probably have a good idea of when to use each. But let’s break it down clearly for reference.

Use integration tests when:

• You’re testing how two or more parts of your system work together. For example, verifying if the /submit-form API route correctly saves data to the database.
• You want fast and reliable feedback during development. Integration tests are quicker than E2E tests and are great for catching bugs early.
• You’re testing services or components without involving the UI. You don’t need to interact with the browser; you’re testing logic, APIs, or service layers directly.

Use E2E tests when:

• You’re testing a complete user flow. For example, a user signs in, lands on the dashboard, performs an action, and logs out.
• You want to validate the entire system from the user’s point of view. This includes the UI, backend, database, and even third-party services.
• You’re preparing for a production release. E2E tests help you confirm that everything works as expected before deploying.
• You want to identify UI or browser-related bugs. E2E tests run in real browsers, so they can catch visual or interaction issues.

In summary, use integration tests to check how the parts of your system work together. Use E2E tests to confirm how users experience your system as a whole.

Conclusion

In this post, we discussed the differences between integration and end-to-end tests, as well as how they work together in the software development process.

Testing your application is just as important as building it. You need to make sure everything works as expected before releasing it to users. By leveraging both integration and E2E tests, you can deliver higher quality and more reliable software.

Also, don’t forget to automate your tests when necessary. Remember the principle: “If you find yourself repeating a process more than three times, it’s probably time to automate.”

This post was written by Chosen Vincent. Chosen is a web developer and technical writer. He has proficient knowledge in JavaScript, ReactJS, NextJS, React Native, Node.js and databases. Aside from coding, Chosen loves playing chess and discussing tech-related topics with other developers.