Microsoft's JScript is in a poor shape and is being deprecated. They'll probably build another interpreter on top of the DLR.

I don't know how good or complete the alternatives are though (probably not as mature as Rhino) ... http://github.com/fholm/IronJS ; http://jint.codeplex.com/

Thanks for the comments. It looks like Node.net should port to any of those Javascript implementations. The basic requirements are the ability to use functions as delegates for callbacks from the host platform and thread sync primitives to serialize access to the event dispatch queue.

It's cool that you're trying for basic API compatibility with Node, and it would be neat if you could make use of some of the libraries that have been written for Node. The only problem with that is that some code written for Node is already using features of Javascript which exist in V8 but not in JScript. Ryan has written about which ES5 features are implemented in V8 and available in Nodejs here:

http://wiki.github.com/ry/node/ecma-5mozilla-features-implem...

You could just write a mixed-mode assembly to bridge V8 to the .NET thread pool and libraries...

My understanding is that IE9 does have a new JS runtime, but it doesn't use the CLR/DLR at all.