WebPPL (pronounced ‘web people’), is a domain-specific open-source probabilistic functional programming language compiling to Javascript. This means that all mathematical and simulation operations are obtained by applying first class functions.

It is mainly developed by N. Goodman's Computation & Cognition Lab in Stanford. The language is intended by the designers to be simple to implement, fairly pleasant to write models in, and a good intermediate target for other languages. WebPPL-programs can be edited and run using only a browser. This makes WebPPL most suitable for prototyping demo models without data file access. WebPPL is still an experimental enterprise

The WebPPL doc can be found here and a tutorial eBook here. The browser-based IDE can be found here (screenshot below).