a.value="5644445";
//                b.onclick=function(){
//        
//                if(isNaN(a.value))
//                {
//                   alert(a.value)           
//                   alert("1");
//                }
//                else
//                {
//                    alert(a.value)
//                    alert(2);                     //正常会进入 else
//                }
//            }
//                
//                
                
                b.onclick=function(){
                    var c=a.value;
                    if(isNaN(c))
                    {
                        alert(Number(c))          //即使输入为 "5644445";  就会转为NaN为什么？
                        alert(1);
                    }
                    else
                    {
                        alert(Number(c))
                        alert(2)
                    }
                }