var numberObject=new Number(1333);numberObject.valueOf();
1333
var numberObject=new Number(1333);numberObject.toExponential(3);
"1.333e+3"
undefined
var numberObject=new Number(1333);numberObject.toFixed(2);
"1333.00"
var numberObject=new Number(1333);numberObject.toPrecision(2);
"1.3e+3"
var numberObject=new Number(1333);numberObject.toPrecision(6);
"1333.00"
instanceof 可以判断对象的 原型