包装类

数字的对象参与运算就会变成原始数字, new Number newString new Boolean

//包装类
        var num=4;
        num.len=3;
        //new Number(4).len=3; delete
        
        //new Number(4).len
        console.log(num.len);

练习

var a=[1,2,3,4];
        a.length=2;
        console.log(a);//截取前两个
        
        //那么
        
        var str="abcd";
        str.length=2;//new String('abcd').lenth=2       delete
        console.log(str);

练习二

var str="abc";
        str+=1;
        var test=typeof str;
        if(test.length==6){
            test.sign="typeof的返回结果可能为String";
        }
        console.log(test.sign);