With the introduction of technologies such as WebSockets and WebRTC, web applications have evolved beyond the simple request-response model. Tierless web languages such as Links and Ur/Web are uniform languages for writing code to be run across all tiers, instead of using different languages for client, server, and database code.

Armed with recent innovations such as WebSockets, which allow web applications to deviate from the classic request-response model, we provide the first full integration of session types and web programming. To handle the inevitable case of a user going offline during a session, we introduce the first implementation of exceptions in the presence of session types. Our implementation is grounded in a formal calculus with strong correctness properties.