Support evolving the type with assignments for expando object

Copied from https://github.com/Microsoft/TypeScript/issues/12278#issuecomment-261000712
OP = @abgivant

I use a similar module pattern in an application I'm working on, and the lack of IntelliSense/definitions for those has been an issue for me as well.

Mine follow this basic pattern:

module.exports = function(x, y, z) {
  var exports = {};
  exports.method = function() {
    // Something that might use x, y, or z
  };
  return exports;
}

Would it be possible to do something like include an option in the jsconfig.json file to specify a property name or list of names to track when determining module exports?

Maybe something like this?

{
  "moduleOptions": {
    "exportProperty": ["exports", "this"]
  }
}

Start with the CommonJS module pattern and the proposed moduleOptions setting in jsconfig.json. Reproduce the missing IntelliSense and definitions behavior, then trace TypeScript’s handling of assignments to expando objects; done means the requested export properties evolve in type information and the configuration behavior is covered by tests.