Most software teams face a common issue: they build features that work perfectly in isolation but fail when integrated with the rest of the system. This results in broken user experiences, last-minute hotfixes, and extended testing cycles.

According to the World Quality Report 2023–24 by Capgemini, QA leaders reported integration-related defects as a significant challenge during the later stages of testing.

To prevent these issues, it’s crucial to understand the difference between unit testing and integration testing, and how both should be used together throughout the app or website development cycle.

What is Unit Testing?

Unit testing is the process of testing individual units or components of a software application in isolation. Its main objective is to validate that the internal logic of a component performs correctly for a variety of input values and edge cases.

In test automation, selecting the right approach is essential. There are several types of automation frameworks, each designed to support different testing needs, which handle unit-level validations to end-to-end system checks.

How It Works in an App or Website

Unit testing checks individual functions or components of a software product. In a website or app, this means testing the smallest blocks of code, such as a form validator or a cart price calculator, in isolation without connecting to a database, API, or UI elements.

1) How to Work in a Mobile App

Suppose you are building a fitness app, and you have a function that calculates calories burned based on time and activity type. A unit test will directly test the calculateCalories() function by providing different inputs and checking if the result is correct.

2) How to Work in a Web App

Suppose you have a web store checkout page. Unit tests would test the calculateTotalPrice(cartItems) function to ensure it returns the correct total for various cart combinations. It’s not involved in checking how the total appears on the screen; the focus is only on whether the function returns the correct result based on the input.

What is Integration Testing?

Integration testing checks how two or more components of a software application interact with each other. It is conducted after unit testing and focuses on verifying data flow and behavior across modules.

The primary objective of integration testing is to verify that individual units function as expected and to identify interface-related bugs that may not be apparent in isolated unit tests.

1) How to Work in a Mobile App

As we discussed earlier about building a fitness app, after unit testing the calculateCalories() function, integration testing will verify whether selecting an activity on the UI sends the correct data to the backend. Then, the backend processes it correctly and checks whether the result is accurately displayed on the screen.

2) How to Work in a Web App

If you have a web-based e-commerce store, integration testing ensures that adding an item to the cart updates the total cost and accurately reflects the correct number of items in the cart icon. It involves UI, backend, and data storage together.

Common Mistakes and Misconceptions

Misunderstanding how and when to apply unit and integration testing often leads to poor coverage and hidden bugs. Let’s explore more:

1) Thinking Unit Testing Is Enough on Its Own

Many developers believe that if all unit tests pass, the application will work flawlessly. But unit tests only confirm that individual functions work in isolation. They don’t validate how those functions interact with other parts of the system, which is precisely what integration testing is meant for.

2) Skipping Integration Testing in Projects

In app or website development teams often skip integration testing to save time, especially in an agile environment. This can lead to issues where different modules fail to communicate effectively, such as UI form data that the backend cannot interpret correctly.

3) No Clear Boundary Between Tests

Without a clear understanding of what unit and interaction tests are supposed to validate, teams might write mixed tests that don’t test one thing. This led to confusion and flaky test results.

4) Delaying Testing Until the End

Testing should not be left until the final stage of app or web development, and even fixing issues becomes more expensive. Unit tests should be written during development, and integration tests should be added as modules are connected.

5) Confusing the Purpose of Each Test Type

Some teams mistakenly perform integration testing when unit testing is needed or vice versa. For example, testing a login form’s button click in isolation is a UI behavior check, often part of integration testing, not a unit test of the backend logic.

Conclusion

One of the biggest mistakes in app and website development is assuming that testing individual components is enough to guarantee a fully functional product. Unit testing ensures that each small piece of code works as intended; it doesn’t tell you whether those pieces work well together. This is where integration testing becomes essential.