Unit tests check the smallest pieces of code, such as a single calculation or button action, to ensure they work correctly in isolation.

Testing Individual Bricks

Integration testing verifies that different parts of a system, like a login form and a database, talk to each other correctly.

Connecting the Pieces

End-to-End testing simulates a real person using the entire app from start to finish to confirm every step works as intended.

The Full User Journey

This is a strategy that recommends having many small, fast tests at the bottom and only a few large, slow tests at the top.

The Testing Pyramid

Unit tests run in a fraction of a second, allowing developers to know immediately if a small change broke something.

Lightning Fast Feedback

To test one piece of code without starting the whole system, developers use 'mocks' or 'stubs,' which are simple placeholders for complex parts.

Using Fake Stand-ins

Integration tests focus on the 'seams' or boundaries where two different services meet to ensure data is passing through smoothly.

Checking the Seams

End-to-End tests ignore the code and focus on what a user sees, such as clicking menus and filling out forms.

Thinking Like a User

Small tests are cheap and easy to write, while large tests require more time, expensive servers, and complex setups to run.

The Cost of Testing

End-to-End tests are valuable because they catch issues that only happen when the entire system is running together in a real environment.

Real World Scenarios

Large tests can be 'flaky' or fragile, meaning they might break because of a tiny visual change even if the underlying code is fine.

Maintenance and Upkeep

This is an alternative strategy that suggests spending the most effort on integration tests because they provide the best balance of speed and reliability.

The Testing Trophy

Running small tests frequently during development prevents tiny mistakes from turning into massive, expensive problems later on.

Catching Errors Early

Instead of a human manually checking the app, these tests are written as scripts that run automatically every time the code is updated.

The Three Levels of Testing