method overloading based on defined params

overload.js

function addMethod(obj, name, fn) {
  var old = obj[name]
  obj[name] = function() {
    if (fn.length === arguments.length)
      return fn.apply(this, arguments)
    else if (typeof old == 'function')
      return old.apply(this, arguments)
  }
}

var myObj = {}
addMethod(myObj, 'doIt', function() {})
addMethod(myObj, 'doIt', function(a) {})
addMethod(myObj, 'doIt', function(a, b) {})