练习

                 var str ="12px";
        console.log(Number("12px"));//NaN不是一个数字
        var s ="";
        console.log(Number(s));
        console.log(Number(""));
        console.log(Number(13));

 var b =20;
 var c =30 + ++b + ++b;
  console.log(c);
  console.log(b);

 var a =1;
 var c =4 + a++ + ++a;
 console.log(c);
 console.log(a);

 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);

 var a ="21";
var b =4;
console.log(a+b-true+"3");
console.log(a-b+true-"false");
 var c ="false";
  console.log(a-b+Boolean(c));

  console.log("9"-8+false);//1false
   console.log("9"-true-"false");//NaN
   console.log("10"-true+"hellow");//9hellow
   console.log(""-"9"+true-12);

                 var str ="12px";
        var s = "12.34px";
        console.log(parseInt(str));
        console.log(parseInt(s));
        console.log(parseFloat(str));
        console.log(Number("12.343"))

number类型的转换
             var a =10;
       var b =0;
       var c =-1;
       console.log(Boolean(a))
       console.log(Boolean(b))
       console.log(Boolean(c))

    var b = "12"-Boolean("")+true+1
        console.log(b);//14
        var c ="10"+1-Boolean("good")+"";
        console.log(c);//100
        var b ="1"+true-false;
        console.log(b);//NaN