页面解析json数据:xmlHttp.responseText就是服务器端返回的json数据,
例如:[{"id":"1","name":"liu"},{"id":"2","name":"li"}{"id":"3","name":"wang"}]
var cityResult=eval("("+xmlHttp.responseText+")");
for(var i=0;i<cityResult.length;i++)
{
var city=cityResult[i];
alert(city.id);//获得id
alert( city.name); //获得name
}