将 Ajax 请求和响应封装

function createXMLHTTPObject() {
  var XMLHttpFactories = [
    function () {
      return new XMLHttpRequest();
    },
    function () {
      return new ActiveXObject('Msxml2.XMLHTTP');
    },
    function () {
      return new ActiveXObject('Msxml3.XMLHTTP');
    },
    function () {
      return new ActiveXObject('Microsoft.XMLHTTP');
    },
  ];
  var xmlhttp = false;
  for (var i = 0; i < XMLHttpFactories.length; i++) {
    try {
      xmlhttp = XMLHttpFactories[i]();
    } catch (e) {
      continue;
    }
    break;
  }
  return xmlhttp;
} // 封装异步请求函数
function request(url, callback, data) {
  var xmlHttp = createXMLHTTPObject();
  if (!xmlHttp) return;
  var method = data ? 'POST' : 'GET';
  xmlHttp.open(method, url, true);
  xmlHttp.setRequestHeader('User-Agent', 'XMLHTTP/1.0');
  if (data)
    xmlHttp.setRequestHeader(
      'Content-type',

      'application/x-www-form-urlencoded',
    );

  xmlHttp.onreadystatechange = function () {
    if (xmlHttp.readyState != 4) return;
    if (xmlHttp.status != 200 && xmlHttp.status != 304) {
      alert('HTTP 请求错误 ' + xmlHttp.status);
      return;
    }
    callback(xmlHttp);
  };
  if (xmlHttp.readyState == 4) return;
  xmlHttp.send(data);
}

function Dan() {
  var info = document.getElementById('info');
  if (xhr.readyState == 1) {
    info.innerHTML = "<img src='images/loading.gif' />连接中......";
  } else if (xhr.readyState == 2 || xhr.readyState == 3) {
    info.innerHTML = "<img src='images/loading.gif' />读数据......";
  } else if (xhr.readyState == 4) {
    if (xhr.status == 200) {
      info.innerHTML = '' + xhr.responseText + '</span>';
    } else alert(xhr.status);
  }
}