What are the different types of tests and what is the difference between unit tests, integration tests, and end-to-end tests?

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Accepted Answer

There are several levels of tests, often represented by the test pyramid.

Unit tests form the base of the pyramid. They test an isolated unit of code, like a function or class, without external dependencies. They are very fast, very numerous, and inexpensive to maintain. For example, testing that a tax calculation function returns the correct amount.

Integration tests are in the middle of the pyramid. They verify that multiple components work correctly together. You test the interaction between code and a real database, an external API, or a file system. They are slower and more complex to set up. For example, testing that an API endpoint correctly creates a record in the database.

End-to-end tests, or e2e tests, are at the top of the pyramid. They test the complete application from the user's perspective, from the interface to the database. They are the slowest, most fragile, and most expensive to maintain. For example, simulating a user who signs up, logs in, and places an order.

The test pyramid recommends having many unit tests, a moderate number of integration tests, and few e2e tests. This strategy offers a good balance between coverage, execution speed, and maintenance cost.

In backend development, I focus primarily on unit tests for business logic and integration tests for API endpoints and database interactions.