js var

var x= new Number();
		alert(x)//0 
		x=new Object();
		alert(JSON.stringify(x)) //{}
		x=new Boolean();
		alert(x)//false
		x=new RegExp();
		alert(x)//  /(?:)/
		x=new String();
		alert(x=="")// true

<script> var x; alert(x)//undefined alert(x===undefined)//true //聲明var 沒有給類型或者值 還是undefined ！！！！ </script>