javascript XMLHttpRequest

xhr.js

function createXHR() {
  if(typeof XMLHttpRequest != undefined) {
    return new XMLHttpRequest();
  }else if(typeof ActiveXObject != undefined) {
    if(typeof arguments.callee.activeXString != 'string') {
      var versions = ['MSXML2.XMLHttp.6.0', 'MSXML2.XMLHttp.3.0', 'MSXML2.XMLHttp'],
          i, len;
      for(i = 0, len = versions.length; i < len; i++ ) {
        try {
          new ActiveXObject(versions[i]);
          arguments.callee.activeXString = versions[i];
          break;
        }catch(ex) {

        }
      }
    }
  }else {
    throw new Error("No XHR object available");
  }
}

// 同步请求
var xhr = createXHR();
// open不会真正的发送请求，而只是启动一个请求以备发送
// false表示请求是同步的，代码等待服务器响应之后再继续执行
xhr.open('GET', URL, false);
// send请求真正被分派到服务器
xhr.send(null);

if((xhr.status >= 200 && xhr.status < 300) || xhr.status == 304) {
  alert(xhr.responseText);
}else {
  alert("Request was unsuccessful: " + xhr.status);
}

// 异步请求
var xhr = createXHR();
xhr.onreadystatechange = function() {
  if (xhr.readyState == 4) {
    if((xhr.status >= 200 && xhr.status < 300) || xhr.status == 304) {
      alert(xhr.responseText);
    }else {
      alert("Request was unsuccessful:" + xhr.status);
    }
  }
};
xhr.open('GET', URL, true);
xhr.send(null);
// 取消异步请求
xhr.abort();