7.强制转换

7-1string—>number

<script>
        //强制转换
        //string boolean-->number
        //调用Number()函数
        //string-->number,只能识别纯数字的字符串
        var str="10";
        var s="hello world";
        var t="21ab";
        var num=Number(str);
        console.log(num);
        console.log(Number(s));//NaN
        console.log(Number(t));//NaN
    </script>

特殊情况
//parseInt()解析字符串中的有效整数
//parseFloat()解析字符串中的有效小数

        var str1="12px";
        var str2="12.1212";
        //parseInt()解析字符串中的有效整数
         //parseFloat()解析字符串中的有效小数
        console.log(parseInt(str1));
        console.log(parseFloat(str2));

7-2boolean—>number

 <script>
        //boolean-->number
        var bool1=true;
        var bool2=false;
        console.log(Number(bool1));//1
        console.log(Number(bool2));//0
    </script>

7-3number,string—>boolean

<script>
        //number-->boolean  0:false  #0true
        var a=0;
        var b=-10;
        var c=20;
        console.log(Boolean(a));//false
        console.log(Boolean(b));//true
        console.log(Boolean(c));//true
        //string-->boolean  字符串为""(空串)时false
        var str1="hello world";
        var str2="   ";
        var str3="";
        console.log(Boolean(str1));//true
        console.log(Boolean(str2));//true
        console.log(Boolean(str3));//false
    </script>