Sucks that it has to be so stringly typed, but I can't see another alternative.

Multimethods would be an alternative to this type of ad-hoc polymorphism. I've encoded immutable multimethod environments in my new library, bilby.js:

https://github.com/pufuwozu/bilby.js

Lets you write things like this:

    var env = λ.environment()
        .method('length', λ.isArray, function(a) {
            return a.length;
        })
        .method('length', λ.isString, function(s) {
            return s.length;
        })
        .property('empty', function(o) {
            return !this.length(o);
        });

    env.empty([]) == true;
    env.empty([1, 2, 3]) == false;

Where isArray and isString are any functions that return true/false based on the input arguments. The environment then dispatches whichever method that has a predicate return true first.