javascript中的undefined

<script type="text/javascript">
var obj = "aaa";
var nullobj;

 if (obj == null || obj == undefined || (!obj) ){
  alert("obj is null");
 }

 if (nullobj == null){
  alert("nullobj is null");
 }

 if (typeof nullobj == 'undefined'){
  alert("typeof nullobj == 'undefined'");
 }
 
 if (nullobj == undefined) {
  alert("nullobj == undefined");
 }

 if ( !nullobj ){
  alert("! nullobj ");
 }
</script>