Small size object type checker for js

check.js

var check = function(o) {
  return (o == null) ? null : o.constructor == Function ? o.name == "" ? "Callback" : "Class_" + o.name : o.constructor.name;
};

// how can be different check and typeof
// checkfor           check result    typeof result
check(null);          //null          "object"
check(undefined);     //null          "undefined"
check(1);             //"Number"      "number"
check([]);            //"Array"       "object"
check({});            //"Object"      "object"
check(true);          //"Boolean"     "boolean"
check("yello");       //"String"      "string"
check(function(){});  //"Callback"    "function"

function Test() {}
check(Test);          //"Class_Test"  "function"
check(new Test);      //"Test"        "object"

example

function myClass (value) { // var myClass = function() { # is invalid!
  this.value = value;
}

var myValue = 5;

function doSomethingWithMyValue(thing) {
  var value = check(thing);
  if(value == 'myClass')  value = thing.value;
  else if(value == 'Number') value = thing;
  return value * 2;
}

doSomethingWithMyValue(new myClass(5)); // 10
doSomethingWithMyValue(myValue); // 10