1 var url = "/data.php"; 2     var param = [ 3         'id=9898', 4         'limit=20' 5     ]; 6  7     var req = new XMLHttpRequest(); 8  9     req.onreadystatechange = function(){10         if(req.readyState == 4){11             var responseHeaders = req.getAllResponseHeaders();    // 获取响应头信息12             var data = req.responseText;    // 获取数据13             // 数据处理14 15         }16     };17 18     req.open("GET", url + "?" + param.join("&"), true); // open 三个参数： 请求方式/ url/ 是否异步19     req.setRequestHeader("X-Request-With", "XMLHttpRequest"); // 设置请求头信息20     req.send(null);    // 发送一个请求 send() 方法接受一个参数：即作为请求主体发送的数据21 22 23     // 动态脚本注入24     var scriptElement = document.createElement("script");25     scriptElement.src = "http://g.pptv.com/javascript/user.js";26     document.getElementsByTagName("head")[0].appendChild("scriptElement");27     function jsoncallback(jsonString){28         var data = eval('(' + jsonString + ')');29         // 处理数据30     }31 32     // 本地数据存储33     var localCache = {};34     function xhrRequest(url, callback){35 36         // 检查此 URL 的本地缓存37         if(localCache[url]){38             callback.success(localCache[url]);39             return;40         }41 42         // 此 url 对应的缓存没找到，则发送请求43         var req = new XMLHttpRequest();44         req.onerror = function(){45             callback.error();46         };47 48         req.onreadystatechange = function(){49             if(req.readyState == 4){50                 if(req.responseText === "" || req.status == "404"){51                     callback.error();52                     return;53                 }54 55                 // 存储响应文本到本地缓存56                 localCache[url] = req.responseText;57                 callback.success(req.responseText);58             }59         };60 61         req.open("GET", url, true);62         req.send(null);63     }