构建Ajax浏览器兼容

if(window.XMLHttpRequest){
  http_REQUEST = new XMLHttpRequest() //非IE
if (http_request.overrideMimeType) {
          http_request.overrideMimeType('text/xml'); //发XML时
}
else if (window.ActivexObject){ //IE
try {
          http_request = new ActiveXObject("Msxml2.XMLHTTP");
        } catch (e) {
          try {
            http_request = new ActiveXObject("Microsoft.XMLHTTP");
          } catch (e) {}
        }
    }

if (http_request.readyState == 4) {
  // 要做的事情
} else {
  //要做的事情
}