2.08 undefined和null类型
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Title</title>
<script type="text/javascript">
var age=3+4;
t=typeof age;
document.write(age,",",t,"<br>")
<!--typeof：检测数据类型，是运算符，不是函数-->
<!--undefined：表示空变量，变量被声明但没赋值-->
<!--null：空对象，表示什么也没有，连声明也没有-->
var wyz=null
var t1=typeof wyz;
document.write(wyz,",",t1,"<br>")
var wyzw
var t2=typeof wyzw;
document.write(wyzw,",",t2)
</script>
</head>
<body>
</body>
</html>
返回值：
7,number
null,object
undefined,undefined