WunderGraph is an open-source BFF (Backend-for-Frontends) framework that enables developers to build scalable and maintainable APIs. The framework provides a type-safe testing library, making it easier to write tests for the data sources used in the BFF system. This library allows developers to create test servers with full typesafe access to their data, making it possible to isolate and verify the correctness of the actual code being tested. WunderGraph also supports mocking, which enables developers to control the behavior of dependencies and external functions during testing, without affecting the actual datasources. The framework is fully compatible with PlayWright for E2E testing. By using WunderGraph's built-in testing library, developers can write better tests more easily, mock responses, and test their app and BFF implementation without calling the actual data sources during development.