Functional Testing

Between Unit Tests and E2E Tests

Functional tests occupy the middle layer of the test pyramid. While unit tests guarantee correctness at the function level and E2E tests verify the behavior of the entire system, functional tests validate "the result of a single user action."

For example, a functional test for "creating an employee" runs through the entire flow from invoking the server action, through validation, saving to the DB, and returning the response. However, launching a browser is not required; it is executed via HTTP request simulation or direct server action calls.

The Boundaries of Functional Tests

The scope of "a single feature" can easily become ambiguous, but the criterion is simple. One server action call or one API endpoint call corresponds to one functional test. Chains of multiple actions — such as "create department → create job type → create employee" — are classified as integration tests.

Differences from Acceptance Tests

Functional tests verify from a developer's perspective whether "the implementation conforms to the specification." Acceptance tests verify from a business perspective whether "the requirements are met." Even when validating the same operation, the viewpoint of verification and the level of granularity in description differ.