练习

1-1

    <script>
        /* number,string,boolean */
        /* 强制转换  
        string,boolean  -- number
        Number(value);
         */
         var str = "123";
         var num  = Number(str);
         var t = true;
         var f = false;
         console.log(str)
         console.log(num)
         console.log(typeof num)
         /* 
         true -->1
         false -->0
          */
         console.log(Number(t));
         console.log(Number(f))
    </script>

1-2

    <script>
        /* tips:Number方法针对string类型,只能识别纯数字的字符串,""," " */
        /* string,boolean-->number */
        var str = "12px";
        console.log(Number("12px"))  //NaN   不是一个数字
        /* ""," "  -->0 */
        var s="";
        console.log(Number(s));
        console.log(Number(" "))
        console.log(Number(" 13"))
    </script>

1-3

    <script>
        /* 
        string,number--boolean
        Boolean();
         */
        /* tips:string-->boolean  只有""为false,其他都为true */
        var str = "hello world";
        var num = "12";
        console.log(Boolean(str))
        console.log(Boolean(num))
        console.log(Boolean(""))
        console.log(Boolean(" "))
    </script>

1-4

    <script>
        /* number-->boolean Boolean() */
        /* tips:number转换成boolean只有0为false */
        var a = 10;
        var b = 0;
        var c = -1;
        console.log(Boolean(a))
        console.log(Boolean(b))
        console.log(Boolean(c))
    </script>

1-5

    <script>
        /* number,boolean string
        String()
        toString()
         */
        var num  = 10;
        var t = true;
        console.log(String(num))
        console.log(String(t))
        console.log(num.toString())
    </script>

1-6

    <script>
        var str = "12px";
        var s = "12.34px";
        /* 
           Number();
           parseInt
           parseFloat
           将string-->number有以上三个方法
         */
        console.log(parseInt(str))
        console.log(parseInt(s))
        console.log(parseFloat(s))
        console.log(Number("12.343"))
    </script>

1-7

    <script>
        /* 
        String();
        toString();
         */
        var a =10;
        var str = a +"";
        console.log(typeof str);
    </script>

1-8

    <script>
        /* 自动转换一般发生在运算中 */
        /* 
        1、算术运算  +,-,*,/,%
        在算术运算中,先使用Nubmer方法将两边的值转为number,再计算
        tips: 特殊情况+,只要有一边为字符串,那么结果一定是字符串，+起拼接符的作用
         */
        var a = 10;
        var b = "20";
        var c=true;
        // console.log(Number(b)-10)
        console.log(b-a);
        // Number(b)-Number(c);
        console.log(b-c);  
        console.log(a+b); 
    </script>

1-9

<script>
        var a = "10";
        var b = true;
        var c = 1;
        console.log(a+c-b);  
        console.log(a-c+b);  
    </script>

1-10

    <script>
        var a ="21";
        var b =  4;
        console.log(a+b-true+"3"); 
        console.log(a-b+true-"false"); 
        /* 
        18-"false"
        18-Number("false")
        18-NaN  
        Tips:NaN和任何值计算,结果一定是NaN
         */
        var c = "false";
        console.log(a-b+Boolean(c));  
    </script>

1-11

 <script>
        var a = "10";
        var b = true;
        var c = 1;
        console.log(a+b+c);  
        /* "10true1" */
        console.log(a+c-b);
        /* 100 */
    </script>

1-12

    <script>
        console.log("9"-8+"false") 
        console.log("9"-true-"false");
        console.log("10"-true+"hello");
        console.log(""-"9"+true-12);
        var a = 10;
        var b = "true";
        var c = "5";
        var sum = a++ -c+b;
        console.log(sum);
        console.log(++a-c-b);
        console.log(1-10*"5"+true);
        console.log((2-"6")*3-false+true); 
    </script>

1-13

    <script>
        var b="12"-Boolean('')+true+1;
        console.log(b);
        var c="10"+1-Boolean("good")+"";
        var d="1"+true-false;
        console.log(d);
        var f=10-""+Boolean("good")-Boolean("world");
        console.log(f);
        var g="10"-Boolean(" ")+"1"-true;    ---(" ")----true
        console.log(g);
    </script>