Write integration tests to verify that different parts of your application work together as intended.
While unit tests are essential for verifying that individual components work in isolation, they don't guarantee that those components will work correctly when combined. Integration testing is the phase in software testing in which individual software modules are combined and tested as a group. It occurs after unit testing and before system testing. The purpose of integration testing is to expose faults in the interaction between integrated units. For example, you might have a unit test for a React form component and a separate unit test for the function that sends the form data to an API. An integration test would test this entire flow together: it would simulate a user filling out the form, clicking submit, and then verify that the correct API request was made and that the UI updated correctly based on the API's response. Integration tests are generally more complex and slower to run than unit tests because they involve more parts of the system. They might require a running database or a mock server to simulate the backend API. Because of this, you typically have fewer integration tests than unit tests. They are a crucial part of the testing strategy, as they catch bugs that can occur at the boundaries between different modules, which unit tests would miss.