underscore.js mixin to create an arguments signature

underscore.signature.js

/** 
  * underscore.js mixin to create an arguments signature
  *
  * Usage:
  *   switch(_.signature(arguments)) {
  *     case "*":
  *       ...
  *     case "object":
  *     case "object *":
  *       ...
  *     case "function object":
  *       ...
  *     default:
  *       ...
  *   }
  */
(function(toString) {
  var NULL_TYPE = "*", ARGUMENTS_TYPE = toString.call(arguments);
  _.mixin({
    type: function(arg, type) {
      return arg == null ? NULL_TYPE : (type = toString.call(arg)) && type.substring(8, type.length - 1).toLowerCase();
    },
    signature: function(args) {
      return _(ARGUMENTS_TYPE === toString.call(args) ? args : arguments).map(_.type).join(" ") || NULL_TYPE;
    }
  });
}(Object.prototype.toString));