[cyber]

nerv_,

function is(str, obj) {
        
 var type = {}.toString.call(obj).toLowerCase();
     type = type.slice(7);
        

 return type.indexOf(str.toLowerCase()) != -1 && !(obj !== obj);
      
 }
      
      console.log(is("array", []));
      console.log(is("number", 3));
      console.log(is("number", "3"));
      console.log(is("object", undefined));

При желание можно сократить)

Date.isDate = function(v) {
    return is("Date", v);
};