You might want to check out Managed JScript. It sounds like something you are looking for, but I don't really know much about it.
Edit: Sounds like Managed JScript is dead according to another post, so I would look into Google's V8.
You can also use Google's V8 Engine from .NET. Check out this other question, and the provided solution: Referencing Google's V8 engine from a .NET app. This example uses Managed C++.
