We present a novel technique, type regression testing, to automatically determine whether an update of a library implementation affects the types of its public interface, according to how the library is being used by other npm packages. By leveraging available test suites of clients, type regression testing uses a dynamic analysis to learn models of the library interface. Comparing the models before and after an update effectively amplifies the existing tests by revealing changes that may affect the clients.
Experimental results on widely used libraries show that the technique can identify type-related breaking changes with high accuracy. It fully automatically classifies at least 89% of the updates correctly as either major or as minor or patch, and it detects 25 breaking changes among the minor and patch updates.