Comparison of Unit, Integration, and End-to-End (E2E) Testing

Unit, Integration, and End-to-End (E2E) Test Pyramid

Feature	Unit Test	Integration Test	End-to-End (E2E) Test
Definition	Tests the smallest units of code (function, method, class) in isolation.	Tests the interaction between multiple modules or system components.	Tests whether all components of the system work together as expected in real user scenarios.
Purpose	Verifies that a single unit or function works correctly.	Verifies that multiple modules work together properly.	Simulates real user scenarios to test the entire system.
Scope	Very narrow (a single function, class, or module).	Medium (a few modules or services).	Broad (entire application).
Speed	Very fast (milliseconds).	Moderate (seconds).	Slow (several seconds to minutes).
Dependencies	Minimal (usually no external dependencies).	Medium (interaction with other modules or services).	Maximum (databases, APIs, user interfaces, etc.).
Tested Parts	The smallest units of code (e.g., methods or functions).	Multiple modules or services.	User scenarios and workflows.
Realism in Environment	Independent from real environments (mock or stub used).	Partially real (a mix of mock and real components).	Realistic (the entire system and environment are used).
Error Detection	Isolated small errors.	Communication errors between modules.	System-wide errors that affect user experience.
Difficulty to Write	Easy.	Moderate.	Hard (involves complex scenarios).
Maintenance	Easy.	Moderate.	Hard (often requires more updates).
Tools	JUnit, NUnit, pytest, Mocha.	Postman, Jest, NUnit + Mocking Frameworks.	Selenium, Cypress, Playwright.
Who Writes It?	Developers.	Developers and test engineers.	Test engineers or QA teams.
Example Scenario	A summing function returns the correct result.	User login service interacts correctly with session services.	User logs in, views the homepage, places an order.