Unit Tests: These are the foundation of the pyramid and form the largest portion. Unit tests are small, fast, and isolated tests that verify the behavior of individual components or units of code in isolation, without dependencies on external systems or resources.
Integration Tests: This layer sits above the unit tests and focuses on testing the interactions between different components/modules of the system. Integration tests ensure that these components work together correctly. They are broader in scope than unit tests and may involve testing across boundaries such as APIs, databases, or external services.
End-to-End Tests (E2E): At the top of the pyramid are the end-to-end tests, also known as UI tests or acceptance tests. These tests simulate user interactions with the entire application, typically through its user interface. End-to-end tests verify that the entire system behaves as expected from the user’s perspective. However, they are slower and more brittle compared to unit and integration tests due to their reliance on the UI and external dependencies.