ajax 起步

写了个ajax调用的js，以备自己以后再用

var xmlHttp;    
function createXMLHttpRequest()...{    
    if (window.ActiveXObject)...{    
        var version = ["MSXML2.XMLHttp.5.0","MSXML2.XMLHttp.4.0",    
                       "MSXML.XMLHttp.3.0","MSXML.XMLHttp","Microsoft.XMLHttp"];    
        for(var i = 0; i < version.length; i++ )...{    
            try...{    
                var oXmlHttp =  new ActiveXObject(version[i]);       
                return oXmlHttp;    
            }catch(oError)...{}    
        }    
    }else if (window.XMLHttpRequest)...{      
        var oXmlHttp = new XMLhttpRequest();      
        return oXmlHttp;    
    }      
    throw new Error("不能创建XMLHttp对象，请确认您的电脑上已安装了XMLHttp对象!");    
}    
function sendRequest(url,param,callBack,sync) ...{    
    if(!xmlHttp) xmlHttp = createXMLHttpRequest();    
    xmlHttp.open("post",url,sync);    
    xmlHttp.setRequestHeader("Content-Type","application/x-www-form-urlencoded");    
    xmlHttp.onreadystatechange = function() ...{    
        if (xmlHttp.readyState == 4) ...{    
            if (xmlHttp.status == 200) ...{    
                 callBack(xmlHttp.responseXML);    
            }    
        }    
    }    
    xmlHttp.send(param);    
}    
function addParam(params,paramName,paramValue) ...{    
   var param = encodeURIComponent(paramName) + "=" + encodeURIComponent(paramValue);    
   params.push(param);    
}    
function getParams(params) ...{    
   var time = new Date().getTime();    
   params.push(time);    
   return params.join("&");    
}