Unit Tests vs End-To-End Tests: Who Wins?

As a software developer, you want to ensure that your code works.

After you’ve written it, you want to test it to ensure it’s doing what it’s supposed to do.

While you can do many kinds of testing, unit and end-to-end testing are the most popular. While both add value to the development process, they are different in many ways.

Unit testing verifies individual code units, ensuring correct functionality. End-to-end testing tests the software from the user’s perspective, validating buttons, forms, updates, links, and workflows. Unit testing focuses on code, while end-to-end testing assesses overall system functionality.

This article will show you what both types of tests are, how they are different, and how you can benefit from writing tests.

What is unit testing?

If you want to create reliable code and reduce manual testing, you should have automated testing. Unit testing is one of the essential aspects of test automation, as it tests individual units of source code and if those units work as expected. It focuses on individual units of work instead of whole applications. The unit of work can be single: 

  • function or method,
  • class,
  • module.

 The most crucial part about unit testing is it makes you write more reliable code.

Software developers usually write unit tests during the development phase. It is a white box testing technique. White box testing is a software testing method that involves examining the source code to test the internal workings of the software.

A single unit test case is limited in its scope. It only checks one unit of code and doesn’t check interactions between different code units.

To check how several units work together, you need an integration test. For example, when developing a finance application, a unit test would check the calculation of the price of a product. However, it would not check the interactions between the modules. Instead, you would use integration testing to check that the order processing and the checkout work correctly.

Unit tests are more detailed, smaller in scope, faster to run, and therefore more effective.

What is end-to-end testing?

End-to-end (E2E) testing is the process of functional testing an application from start to finish. The software testers do it by emulating the most common tasks a user would perform.

End-to-end testing typically takes place after unit and integration testing are complete. This testing may be automated or manual. It generally involves looking for errors in the product against the intended functionalities or requirements. A successful e2e test will verify that business requirements are met and validated by the end-user.

End-to-end testing is a black-box testing technique. Black-box means that the tester only knows the application’s expected functions but does not know how these functions are implemented.

Unit Tests vs End-To-End Tests: The Differences

There are many differences between unit and end-to-end tests:

  1. Scope – Unit tests are focused on testing the individual units of code. You can run them before the application is released. They are usually shorter than end-to-end tests. E2E testing covers the whole application.
  2. Testing Method – Unit tests are usually automated, while end-to-end tests can be manual or automated.
  3. Execution speed – Unit tests are usually faster than end-to-end tests.
  4. Testing Type – Unit tests are white-box, while end-to-end tests are black-box.
  5. Test Environment – Unit tests are run on the developer’s machine, while end-to-end tests are run on the test environment.
  6. Performed by – unit tests are performed by the developers, while the Quality Assurance team performs end-to-end tests.
  7. Run in parallel – unit tests can be run in parallel, while end-to-end tests are run in sequence. 
  8. Requires access to the database and other resources – End-to-end tests require access to databases and other external dependencies. These tests require more hardware, as well as more maintenance. You can run unit tests individually from the command line. This means you can even run just a few relevant to your work.
  9. The effort to write and maintain – Unit tests take minimal effort to write, especially if you’re following the TDD approach. End-to-end tests require a lot of time to write and maintain. The time taken depends on the complexity of the application and the number of test cases that you have to execute.

When to use unit testing?

Developers should use unit tests to check that their code works as expected. It is also possible to run the unit test suite every time there is a build in the continuous integration system. This can help you find bugs early in the development process. 

It’s not necessary to use unit testing for all applications. For example, if you have an API project that doesn’t have too much logic, the better approach would be to write integration tests. Integration API tests will check that the API works correctly when other applications or services call it.

When to use end-to-end testing?

E2E testing is usually used for large, complex applications with many functions. It helps check that the application performs all of its functionalities as specified. It’s essential to ensure that these functionalities work correctly and are consistent before releasing the application to production.

If you have automated end-to-end tests, you will run these tests after the application is deployed to the test environment before releasing the current build to production.

Which is better – unit testing or end to end testing?

Some people think that one testing type is better than the other. But the truth is that they complement each other as different tools in a toolbox and can be used in many ways. 

Moreover, you should never use a single testing approach for a complete testing strategy. Many companies use both unit and end-to-end tests. They have different roles in their testing strategy. Unit tests are used for catching bugs early in the development process. End-to-end tests check that the application performs all of its functionalities as specified before releasing it to production.

Finally, add to the mix integration testing and you get a testing pyramid.

To find the best approach for your project, you need to analyze which of these approaches is more appropriate for your requirements.

How to write unit tests?

Developers usually write unit tests in the same language as the main application is written. They don’t have to learn different languages to write test cases.

Another way to write unit tests is with the test-driven development (TDD) approach. This approach requires that you write the tests before the code. So, you write the unit test case first, then write the implementation. Then, you can use the tests to verify the correctness of your code.

How to write end-to-end tests?

QA team usually writes end-to-end tests using Selenium or a similar framework. They use a browser or a test automation tool to execute end-to-end tests. You should write end-to-end tests in a language that is easy for testers to use.

The problem with automated e2e tests is that they are slow and require a lot of resources to run. However, they can be a great asset to your testing strategy.


In summary, both unit and end-to-end tests have their benefits and drawbacks. But both are necessary, and there is no clear winner between the two.

Unit testing is easier to understand, write, and maintain. But it doesn’t cover all the functionalities of an application. End-to-end tests can be very time-consuming to create and maintain. But they can help you to ensure that the application performs all of its functionalities as specified before releasing it to production.

So use them both to achieve the best possible results.

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