Cool! Sucks that it has to be so stringly typed, but I can't see another alternative. One concern: "Each pattern will be tried in order until a match is found." As far as I know, ECMAScript doesn't specify any enumerations on objects that obey an order... browsers do syntactic order, but it's by no means a guarantee. (e.g. http://stackoverflow.com/a/280861 )
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;