Unit Testing vs End To End Testing - Key Differences (2024)

Before designing a holistic testing strategy for an application, the first question that comes to mind is which testing strategy to use. The two most important and widely used testing strategies are Unit testing and end-to-end testing. To decide which one to use, it’s important to understand their key differences.

Unit testing is a testing strategy where we test individual units of the application code. In end-to-end testing, the tester tests the entire application from the user’s point of view. We can do this by simulating the common tasks performed by the user.

There are different types of bugs that users can encounter. Some might render some errors, they might return incorrect data, and others might incur functionality issues or browser issues. Depending on the type of bug, we should decide whether to use unit testing or end-to-end testing for your application.

In this article, let’s look at the unit testing VS end to end testing differences and when to use them.

Unit Testing vs End to End Testing – Overview

What is Unit Testing?

We write unit tests during the development phase of the code. It is a classic example of white-box testing. This is because to carry out any white-box testing, we need to know the internal mechanism of the code. However, the scope of a unit test case is limited. It can only test one code unit. It cannot test the interactions between different units of code.

Read here detailed guide on Unit Testing

What is End-to-End Testing?

End-to-end testing is the process of testing the functionality of the entire application from start to finish. It involves emulating the most commonly performed user actions and tests whether the output is desired. We perform end-to-end (E2E) testing after the unit and integration tests are over.

An E2E test involves testing all aspects of the application ranging from technical functionalities to the user interface. We can do automated as well as manual E2E tests. It involves finding out bugs and errors in the applications against the intended requirements.

E2E tests are black box tests, meaning that the tester does not know how the functionalities are programmed. If an end-to-end test is successful, it means that the user has validated the business requirements.

Read here detailed guide on – End to End Testing

Unit Testing vs End to End Testing – Differences

The Difference between Unit Testing and End-to-End Testing is that Unit testing ensures that the functions or calculations that generate data, a numerical value, a text string, are working as required. End-to-end testing ensures that buttons, forms, updates, links, and the complete workflows work properly.

Both unit tests and end-to-end tests serve their own purposes and have their own usage. However, let’s look at the key differences between them.

Scope of test

As discussed, unit tests focus mainly on individual code units, whereas end-to-end testing tests the application as a whole from a user’s perspective. It’s easy to run unit tests, and we can execute them in each build. Whereas, running a complete end-to-end test on an application takes time.

Type of testing

Unit tests are white-box tests. This is so because if you want to write a unit test to cover a specific function, you need to know the internal implementation of that function. On the other hand, end-to-end tests are black-box tests. These tests intend to simulate user actions on the application and test the functionality, which does not require any knowledge of the implementation.

Testing methodology

Unit tests are mostly automated tests that we can integrate with the CI/CD pipeline. As soon as we push a new code commit, the entire code is built, and the test cases are run to check for any bug because of the new change. Whereas end-to-end tests can be manual as well as automated. We can use several frameworks to make such tests automated or test them manually by simulating user actions.

Testing environment

The developers mostly create unit tests; hence, they run them on the developer machines. On the other hand, we do end-to-end tests in a dedicated testing environment after the subsystems are built.

Performers

Unit tests are performed by the developers who write them. Whereas, a team of testers or QAs execute end-to-end tests in a dedicated environment.

Parallel Run

We can run unit test cases in parallel because they focus on individual code units and are not dependent on each other. In comparison, end-to-end tests have to be performed sequentially.

Access to database and other resources

We usually mock the dependencies in unit tests; hence, they do not require access to any resources. End-to-end tests typically require more resources because the whole purpose of this test is to check whether the entire system works as intended.

Effort

We need less time and cost to create unit test cases as compared to end-to-end tests. Performance tests are complicated, and a lot of subsystems are involved.

Unit Testing vs End to End Testing? – Which is Better?

Unit tests and end-to-end tests complement each other. They are different testing strategies with different use cases. Based on the requirement, the correct testing strategy for your application can be determined. But it’s always better to keep both strategies in your bag.

We use unit test cases to find bugs during the early development phase and check whether the individual code units work as expected. We run these test suites every time a build is triggered in the CI pipeline. However, it does not mean you have to write unit tests for all your subsystems. For example, if your API lacks logic, it’s better to go with integration tests rather than writing unnecessary unit tests.

On the other hand, we use end-to-end tests to verify the business requirement of the application. We check whether the functionalities of the application are as expected before going into production. We use E2E testing for large and complicated applications which has multiple subsystems.

Therefore, to decide the best suitable approach for your project, you need to analyze your requirements, compare these strategies, and choose the best one.

Summary

To summarize, both unit and end-to-end tests serve their unique purposes. It’s always better to have them both in your testing strategies.

Writing, maintaining, and understanding unit test cases are very easy. But since it is a white-box testing technique, you must understand the code to properly create unit test cases. Also, we cannot cover all the functionalities of the software by unit test cases.

Hence, to verify whether each and every part of the application works as intended and well together, an end-to-end test just before the release is a must. Although they are very time-consuming to perform, they can help you find issues and performance impacts before a production release.

Also Know: Unit Testing Vs Integration Testing

Frequently Asked Questions

How do you write a test case in unit testing?

We usually write unit tests using a library of the same language as the codebase. There are two ways to write unit test cases. You can develop the code for the specific requirement first and then write test cases to validate the logic. Or you can write the test cases first by mocking the dependencies and then work on the code. We call this test-driven development. There are several libraries that allow you to create assertions to compare the actual vs. expected output.

Is End-to-End testing the same as UAT?

In a way, End-to-End testing is quite similar to user acceptance testing (UAT). This is so because in both cases, the testers replicate the user actions. Examples of user actions include making a transaction, filling out a form, clicking a button, etc. However, one major difference is that business users execute UAT. But an end-to-end test is generally performed by a team of testers or QAs.

Who writes unit tests?

Unit test cases are white-box tests that require an understanding of the code implementation. Hence, these tests are generally written by the developers themselves. Unit tests can be written either before starting the development of the feature or after developing the feature.

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