Null和Undefined
Null类型的值只有一个，就是null
-
null这个值专门用来表示一个为空的对象
-
代码
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta http-equiv="X-UA-Compatible" content="IE=edge">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>Document</title>
<script>
var a = null;
alert(a)
</script>
</head>
<body>
</body>
</html>
- 效果图：
==啥都没有就对了==
-
使用typeof检查一个null值是，会返回object
- 代码：
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta http-equiv="X-UA-Compatible" content="IE=edge">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>Document</title>
<script>
var a = null;
alert(typeof a)
</script>
</head>
<body>
</body>
</html>
- 效果图:
Undefined:
- Undefined（未定义）类型的值==只有一个==，就是undefined
当声明一个变量，但是并不给变量赋值时，它的值就是undefined
-
代码：
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta http-equiv="X-UA-Compatible" content="IE=edge">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>Document</title>
<script>
var b = undefined;
console.log(b)
</script>
</head>
<body>
</body>
</html>
-
效果图：
-
使用typeof检查一个undefined时也会返回undefined
①代码：
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta http-equiv="X-UA-Compatible" content="IE=edge">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>Document</title>
<script>
var b = undefined;
console.log(typeof b)
</script>
</head>
<body>
</body>
</html>
②效果图：
