for in

var x;
var person=[456,"qwe",890]; 
   for (x in person)
   {
   alert(x);           //依次显示 0  1  2 
    }

var x;
var person={fname:"Bill",lname:"Gates",age:56}; 
for (x in person)
{
alert(x);  //依次显示 fname lname age
alert(person[x]);   //  得到对象的属性值   不能写person.x 这是undefined
}