<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>变量</title>
</head>
<body>
<script>
alert("变量练习");
/**
* document.write(num+"<br>");输出到页面上
*/
//定义number类型
var a = 123;
document.write(a + "<br>");
var b = 1.2;
document.write(b + "<br>");
var c = NaN;
document.write(c);
//定义string类型
var str = "abc";
var str2 = 'edf';
document.write(str + "<br>");
document.write(str2 + "<br>");
//定义boolean
var flag = true;
document.write(flag + "<br>");
// 定义null,undefined
var obj = null;
var obj2 = undefined;
var obj3;
document.write(obj + "<br>");
document.write(obj2 + "<br>");
document.write(obj3 + "<br>");
</script>
</body>
</html>