var xmlhttp;
if (window.XMLHttpRequest) {
 // code for IE7+, Firefox, Chrome, Opera, Safari
 xmlhttp = new XMLHttpRequest();
} else {
 // code for IE6, IE5 
 xmlhttp = new ActiveXObject("Microsoft.XMLHTTP");
}
xmlhttp.onreadystatechange = function() {
 if (xmlhttp.readyState == 4 && xmlhttp.status == 200){
  // to do...
 }
}
xmlhttp.open("GET","test1.txt",true);
xmlhttp.send();

var = new XMLHttpRequest();

var = new ActiveXObject("Microsoft.XMLHTTP");

 ajax({  
  url: "./test.php",
  type: "POST",      
  data: { name: "abc", age: 18 },   
  dataType: "json",  
  success: function (response, xml) {   
   // 执行成功回调  
  },  
  fail: function (status) {   
   // 执行失败回调  
  }
 });
  
  function ajax(options) { 
   options = options || {}; 
   options.type = (options.type || "GET").toUpperCase();  
   options.dataType = options.dataType || "json";  
   var params = formatParams(options.data);    
   
   // 创建对象  
   if (window.XMLHttpRequest) {   
    var xhr = new XMLHttpRequest(); 
   } else {   
    var xhr = new ActiveXObject('Microsoft.XMLHTTP');  
   } 
   
   //接收 - 第三步
   xhr.onreadystatechange = function () {   
    if (xhr.readyState == 4) {    
     var status = xhr.status;    
     if (status >= 200 && status < 300) {     
      options.success && options.success(xhr.responseText,xhr.responseXML);    
     } else {     
      options.fail && options.fail(status);    
     }   
    }
   }  
    
   //连接 和 发送 - 第二步 
   if (options.type == "GET") {   
    xhr.open("GET", options.url + "?" + params, true);   
    xhr.send(null);  
   } else if (options.type == "POST") {   
    xhr.open("POST", options.url, true);   
    //设置表单提交时的内容类型   
    xhr.setRequestHeader("Content-Type", "application/x-www-form-urlencoded");
    xhr.send(params);  
   }  
   
   //格式化参数 
   function formatParams(data) { 
    var arr = [];  
    for (var name in data) {   
     arr.push(encodeURIComponent(name) + "=" + encodeURIComponent(data[name])); 
    }  
    arr.push(("v=" + Math.random()).replace(".",""));  
    return arr.join("&"); 
   }