> strict, deterministic subset of TypeScript

I'll add that page, thanks. Today, almost all of idiomatic TS is supported including most of its utility classes. Dynamic JS-style code is not supported, for example adding a function or a field into an object, prototype-based class modifications etc. I'll compile a list, and include it along with the large docs cleanup planned before v1.

> Also, what's an "ambient surface" in this context?

The idea is that when JS gets transpiled into C# (or Rust, upcoming), JS globals and built-ins are invalid. The native "surface" is C#, meaning the string is .Net's string type and the methods that you expect on JS strings would be missing. But when you opt in to a surface, such as the "JS surface", the compiler applies surface defined translations such as substring becoming SubString, either directly or via a companion helper class. This allows you to write against standard JS and Node APIs, instead of relying on the stdlib/builtins of the target framework (currently CLR). And you get the JS "stdlib" - console, JSON, Date, Map, Set etc.

For example, all the projects you see under this use the JS surface: https://github.com/tsoniclang/proof-is-in-the-pudding/tree/m...