下面是typeof运算符对各个类型的返回结果：
1.undefined:undefined
2.null:object
3.string:string
4.number:number
5.boolean:boolean
6.function:function
7.object:object

<script type="text/javascript">
<!--
alert(typeof(NaN));//输出number
var a=undefined;//等价于var a;
alert(typeof (a));//输出undefined
alert(null == undefined);//输出true
alert(null==null);//输出true
alert(NaN == NaN);//输出false
alert(typeof (m));//输出undefined,一个未定义的对象
alert(typeof(document.xx));//输出undefined,一个不存在的对象属性
//-->
</script>

<script>
<!--
var t1 = null;
var t2;//或var t2 = undefined;
if(t1 === undefined){
	alert('t1 undefined!');
}
if(t2 === undefined){
	alert('t2 undefined!');
}
if(typeof(t2) == 'undefined'){
	alert('t2 undefined use typeof!');
}
//同时判断是否为undefined和null的方法，如下:
if(!t1){
	alert('t1 is not set value!');
}
if(!t2){
	alert('t2 is not set value!');
}
//-->
</script>

<script>
<!--
function f1(){
	document.write('test function <br/>');
}
function f2(){
	document.write('test function2 <br/>');
	return;
}
alert(f1());//等价于：alert(typeof f1());输出undefined
alert(f2());//等价于：alert(typeof f2());输出undefined
//-->
</script>