1-2 转换

1.string—>number

只能识别纯数字的字符串，其它字符输出NaN
  示例：var str ="3";
        var num = Number(str);
        console.log(typeof num)
        console.log(Number("3.0"))
        console.log(Number("3.00"))
        console.log(Number("3px"))//Nan 不是一个数字的意思

2.boolean—>number

Number() 
只有两种输出方式：true--1;false--0;
  示例：var b = true;
        var f = false;
        console.log(Number(b));
        console.log(Number(f));

3.string—>boolean

Boolean()
“”为false(也就是“”之间不加任何东西),其他都为true
  示例：var str = "good";
        var s = "";
        var t = "";
        console.log(Boolean(str));
        colnsole.log(Boolean(s));
        console.log(Boolean(t));

4.number—>boolean

只有0为false,其他情况都为true
  示例：var num = 0;
        var n = -1;
        var m = 1;
        cosole.log(Boolean(num));
        consol.log(Boolean(n));
        console.log(Boolean(m))

5.any—>string

number,boolean--string
toString()
  示例：var num = 10;
        var t =true;
        console.log(String(num))
        console.log(t.toString)

6.any—>number

string-->number
    1.能够识别纯数字的字符串
    2.""," "，识别成0
    3.NaN 
boolean--number
       true  1 
       false 0