1、变量的数据类型简介:
变量是用来存储值的所在处,它们有名字和数据类型。变量的数据类型决定了如何将代表这些值的位存储到计算机的
内存中。JavaScript 是一种弱类型或者说动态语言。这意味着不用提前声明变量的类型,在程序运行过程中,类型会
被自动确定。
数据类型Demo:
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<meta http-equiv="X-UA-Compatible" content="ie=edge">
<title>Document</title>
<script>
var num = 10;
var str = 'echo';
var x = 10;
x = 'pink';
</script>
</head>
<body>
</body>
</html>
1.1 数据类型的分类:
简单数据类型 (Number,String,Boolean,Undefined,Null)
复杂数据类型 (object)

1.2 数字型 Number
数字类型既可以用来保存整数值,也可以保存小数(浮点数)。
数字型三个特殊值:
Infinity ,代表无穷大,大于任何数值
-Infinity ,代表无穷小,小于任何数值
NaN ,Not a number,代表一个非数值
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<meta http-equiv="X-UA-Compatible" content="ie=edge">
<title>Document</title>
<script>
var num = 10;
var PI = 3.14
var num1 = 010;
console.log(num1);
var num2 = 012;
console.log(num2);
var num3 = 0x9;
console.log(num3);
var num4 = 0xa;
console.log(num4);
console.log(Number.MAX_VALUE);
console.log(Number.MIN_VALUE);
console.log(Number.MAX_VALUE * 2);
console.log(-Number.MAX_VALUE * 2);
console.log('Echo' - 100);
</script>
</head>
<body>
</body>
</html>
1.2.1 isNaN():用来判断一个变量是否为非数字的类型,返回 true 或者 false
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<meta http-equiv="X-UA-Compatible" content="ie=edge">
<title>Document</title>
<script>
console.log(isNaN(12));
console.log(isNaN('pink老师'));
</script>
</head>
<body>
</body>
</html>
1.3 字符串型 String
字符串型可以是引号中的任意文本,其语法为 双引号 "" 和 单引号''
JS 可以用单引号嵌套双引号 ,或者用双引号嵌套单引号
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<meta http-equiv="X-UA-Compatible" content="ie=edge">
<title>Document</title>
<script>
var str = 'Echo';
console.log(str);
var str1 = "E\ncho";
console.log(str1);
</script>
</head>
<body>
</body>
</html>
1.3.1 字符串转义符

1.4 布尔型 Boolean
布尔类型有两个值:true 和 false ,其中 true 表示真(对),而 false 表示假(错)。
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<meta http-equiv="X-UA-Compatible" content="ie=edge">
<title>Document</title>
<script>
var flag = true;
var flag1 = false;
console.log(flag + 1);
console.log(flag1 + 1);
var str;
console.log(str);
var variable = undefined;
console.log(variable + 'Echo');
console.log(variable + 1);
var space = null;
console.log(space + 'Echo');
console.log(space + 1);
</script>
</head>
<body>
</body>
</html>
1.5 Undefined
一个声明后没有被赋值的变量会有一个默认值 undefined
<!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>
</head>
<script>
var variable;
console.log(variable);
console.log('你好' + variable);
console.log(11 + variable);
console.log(true + variable);
</script>
<body>
</body>
</html>
1.6 Null
<!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>
</head>
<script>
var vari = null;
console.log('你好' + vari);
console.log(11 + vari);
console.log(true + vari);
</script>
<body>
</body>
</html>