Visual Studio autocompletes Javascript by executing the Javascript and using stub objects for anything that they don't actually want to run (e.g. window, XmlHttpRequest, etc).
Replacing Javascript with a better language has not been possible thus far. There have been many attempts, but getting every browser maker to agree on something is like herding cats.
So, when life gives you lemons, you make lemonade. We're stuck with Javascript. Personally, I think running the code while you're developing it is a brilliant way of dealing with the problem of how to do proper autocomplete on a dynamic language.
