一. 数据类型
JavaScript是一门弱类型的语言，和Java的强类型不同，js在声明一个变量的时候，不需要声明类型，JS无论声明什么类型的变量都统一使用var来声明，具体的类型需要给变量赋值后才能获得，且赋值可以动态转换
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string：字符串
-
number：数值
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boolean：布尔
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undefined：未定义的
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object：对象类型
下面针对上面的几种数据类型分别举例，string：字符串
<script type="text/javascript"> var a='s'; console.log(typeof a); //类型是String </script>
利用typeof可以输出a的类型，再看一下number：数值
<script type="text/javascript"> var a = 10; console.log(typeof a) //类型是number a = 2.5; console.log(typeof a) //类型是number </script>
boolean：布尔
<script type="text/javascript"> var a = true; console.log(typeof a) //类型是boolean </script>
undefined：未定义的
<script type="text/javascript"> var a; console.log(typeof a) //类型是undefined </script>
object：对象类型
<script type="text/javascript">
var a = null;
console.log(typeof a) //类型是Object
a = {}; //js对象
console.log(typeof a) //类型是Object
a = [];//js数组
console.log(typeof a) //类型是Object
</script>
js的number类型计算有时候会丢失精度：
<script type="text/javascript"> var a = 0.1; var b = 0.3; console.log(b + a); // 0.4 console.log(b - a); // 0.19999999999999998 </script>
字符串和数字的加法和减法规则是不同的，首先来看字符串和数字的加法：
<script type="text/javascript">
x = 1 + 2;
document.write(x); //3
document.write("<br />");
x = "1" + "2";
document.write(x); //12
document.write("<br />");
x = 1 + "2";
document.write(x); //12
document.write("<br />");
x = "1" + 2;
document.write(x); //12
document.write("<br />");
</script>
可以看到，字符串+数字，就是做了一个简单的拼接。再来看一下字符串和数字的减法：
<script type="text/javascript">
x = 6 - 1;
document.write(x); //5
document.write("<br />");
x = "6" - "1";
document.write(x); //5
document.write("<br />");
x = 6 - "1";
document.write(x); //5
document.write("<br />");
x = "6" - 1;
document.write(x); //5
document.write("<br />");
</script>
可以看到，无论被减数和减数是数字还是字符串，都会进行计算。
二. 类型转换
1. 其他类型转换为String
有两种方法，第一种：使用toString方法
<script type="text/javascript">
var a = 10;
a.toString();
document.write(typeof a + "——> a = " + a); //number——> a = 10
document.write("<br />");
a = a.toString();
document.write(typeof a + "——> a = " + a); //string——> a = 10
document.write("<br />");
a = true;
a = a.toString();
document.write(typeof a + "——> a = " + a); //string——> a = true
document.write("<br />");
a = false;
a = a.toString();
document.write(typeof a + "——> a = " + a); //string——> a = false
document.write("<br />");
/* a = undefined;
a = a.toString();
document.write(typeof a + "——> a = " + a); //报错
document.write("<br />");
a = null;
a = a.toString();
document.write(typeof a + "——> a = " + a); //报错
document.write("<br />");*/
</script>
第二种，使用String()方法
<script type="text/javascript">
var a = 10;
String(a);
document.write(typeof a + "——> a = " + a);; //number——> a = 10
document.write("<br />");
a = String(a);
document.write(typeof a + "——> a = " + a); //string——> a = 10
document.write("<br />");
a = true;
a = String(a);
document.write(typeof a + "——> a = " + a); //string——> a = true
document.write("<br />");
a = false;
a = String(a);
document.write(typeof a + "——> a = " + a); //string——> a = false
document.write("<br />");
a = undefined;
a = String(a);
document.write(typeof a + "——> a = " + a); //string——> a = undefined
document.write("<br />");
a = null;
a = String(a);
document.write(typeof a + "——> a = " + a); //string——> a = null
document.write("<br />");
</script>
可以看到，使用String方法后，undefined和null类型不再报错
2. 其他类型转换为number
<script type="text/javascript">
var a = "10";
Number(a);
document.write(typeof a + "——> a = " + a); //string——> a = 10
document.write("<br />");
a = Number(a);
document.write(typeof a + "——> a = " + a); //number——> a = 10
document.write("<br />");
a = true;
a = Number(a);
document.write(typeof a + "——> a = " + a); //number——> a = 1
document.write("<br />");
a = false;
a = Number(a);
document.write(typeof a + "——> a = " + a); //number——> a = 0
document.write("<br />");
a = undefined;
a = Number(a);
document.write(typeof a + "——> a = " + a); //number——> a = NaN
document.write("<br />");
a = null;
a = Number(a);
document.write(typeof a + "——> a = " + a); //number——> a = 0
document.write("<br />");
document.write("------------------------------");
document.write("<br />");
a = "10.5a";
a = Number(a);
document.write(typeof a + "——> a = " + a); //number——> a = NaN
document.write("<br />");
document.write("------------------------------");
document.write("<br />");
a = "10";
a = parseInt(a);
document.write(typeof a + "——> a = " + a); //number——> a = 10
document.write("<br />");
a = "10.5";
a = parseInt(a);
document.write(typeof a + "——> a = " + a); //number——> a = 10
document.write("<br />");
a = "10a";
a = parseInt(a);
document.write(typeof a + "——> a = " + a); //number——> a = 10
document.write("<br />");
a = "10.5a";
a = parseInt(a);
document.write(typeof a + "——> a = " + a); //number——> a = 10
document.write("<br />");
document.write("------------------------------");
document.write("<br />");
a = "10";
a = parseFloat(a);
document.write(typeof a + "——> a = " + a); //number——> a = 10
document.write("<br />");
a = "10.5";
a = parseFloat(a);
document.write(typeof a + "——> a = " + a); //number——> a = 10.5
document.write("<br />");
a = "10a";
a = parseFloat(a);
document.write(typeof a + "——> a = " + a); //number——> a = 10
document.write("<br />");
a = "10.5a";
a = parseFloat(a);
document.write(typeof a + "——> a = " + a); //number——> a = 10.5
document.write("<br />");
a = "10.5.2";
a = parseFloat(a);
document.write(typeof a + "——> a = " + a); //number——> a = 10.5
document.write("<br />");
/* string——> a = 10
number——> a = 10
number——> a = 1
number——> a = 0
number——> a = NaN
number——> a = 0
------------------------------
number——> a = NaN
------------------------------
number——> a = 10
number——> a = 10
number——> a = 10
number——> a = 10
------------------------------
number——> a = 10
number——> a = 10.5
number——> a = 10
number——> a = 10.5
number——> a = 10.5*/
</script>
3. 其他类型转换为boolean
<script type="text/javascript">
var a = "111";
Boolean(a);
document.write(typeof a + "——> a = " + a); //string——> a = 111
document.write("<br />");
a = "111";
a = Boolean(a);
document.write(typeof a + "——> a = " + a); //boolean——> a = true
document.write("<br />");
a = -1.1; //只要是非0的数字都是true，比如1和-1, -1.1和1.1都是true
a = Boolean(a);
document.write(typeof a + "——> a = " + a); //boolean——> a = true
document.write("<br />");
document.write("------------------------------");
document.write("<br />");
a = 0;
a = Boolean(a);
document.write(typeof a + "——> a = " + a); //boolean——> a = false
document.write("<br />");
a = "";
a = Boolean(a);
document.write(typeof a + "——> a = " + a); //boolean——> a = false
document.write("<br />");
a = undefined;
a = Boolean(a);
document.write(typeof a + "——> a = " + a); //boolean——> a = false
document.write("<br />");
a = null;
a = Boolean(a);
document.write(typeof a + "——> a = " + a); //boolean——> a = false
document.write("<br />");
a = NaN; //NaN本身是number类型
a = Boolean(a);
document.write(typeof a + "——> a = " + a); //boolean——> a = false
document.write("<br />");
</script>
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