支持自定义回调函数的异步调用

    做一个最简单的POST方式异步调用的请求，支持自定义的回调函数，该回调函数获取异步请求返回的XMLDOM对象，代码如下：

function postRequest(url,parameters,callBack){
    var xmlHttp = getXmlHttp(); //create xmlHttpRequest
    if(xmlHttp !=null){
        xmlHttp.onreadystatechange = function(){
            if(xmlHttp.readyState == 4){
                if(xmlHttp.status == 200){
                    xmlDom = getXmlDom(xmlHttp.responseText);//create xmlDom
                    if(xmlDom != null){
                        eval(callBack(xmlDom));
                    }
                }
            }
        }
        xmlHttp.open('Post',url,true);
        xmlHttp.setRequestHeader("Content-Length",parameters.length);
        xmlHttp.setRequestHeader("Content-Type","application/x-www-form-urlencoded");
        xmlHttp.send(parameters);
    }
}