overload.js

var __slice = [].slice;
var __noop = function(){};
 
function typeOf(x) {
  return {}.toString.call(x).slice(8,-1);
}
 
function overload(fs) {
  return function() {
    var types = __slice.call(arguments).map(typeOf);
    var f = fs[types.join('_')] || __noop;
    return f.apply(this, arguments);
  };
}
 
var fn = overload({
  String: function greet(s) {
    return 'Hello '+ s;
  },
  Number: function square(x) {
    return x * x;
  },
  Array_Function: function map(xs, f) {
    return xs.map(f);
  },
  RegExp_String_Number: function replace(re, s, x) {
    return s.replace(re, x);
  }
});
 
fn('Joe'); //=> "Hello Joe"
fn(3); //=> 9
fn([1,2,3], function(x){ return x+1 }); //=> [2,3,4]
fn(/a/g, 'wawa', 2); //=> "w2w2"