Autocomplete is not tied to statically types languages. For instance, in the Python command line prompt you have autocomplete directly on the live objects.
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.
Even in Java and C#, arguably the best languages re: deep IDE integration, there are edge cases where the IDE can't know everything (reflection, for instance), but it still does the trick most of the time.