var request=new XMLHttpRequest();

request.open("method","url");

request.setRequestHeader(“Content-Type”,"text/plain");

   /*以post方式发送数据给服务器*/    
function postMessage(msg){
        var request=new XMLHttpRequest();
        request.open("post","handlerPost");
        request.setRequestHeader(‘Content-Type‘,‘text/plain;charset=UTF-8‘);
        request.send(msg);
    }

    //postMessage("This is post message!");
    //发送一个HTTP    GET请求以获得指定URL内容
    //当响应成功到达，验证它是否是纯文本
    //如果是，把它传递给指定回调函数
    function getText(url,callback){
        var request=new XMLHttpRequest();
        request.open("GET",url);
        request.onreadystatechange=function(){
            //如果请求完成，则它是成功的
            if(request.readyState===4 && request.status===200){
                var type=request.getResponseHeader("Content-Type");
                if(type.match(/^text/))  //确保相应是文本
                    callback(request.responseText);
            }
        }
    }

    //同步响应
    //发起同步的HTTP GET请求以获得指定URL的内容
    //返回响应文本，或如果请求不成功或响应不是文本就报错。
    function getTextSync(url){
        var request=new XMLHttpRequest();
        request.open("GET",url,false); //把fasle作为第三个参数传递给open()，那么send()方法将阻塞直到请求完成
        request.send(null);

        if(request.status!==200) throw new Error(request.statusText);

        var type=request.getResponseHeader(‘Content-Type‘);
        if(!type.match(/^text/)) throw new Error("Expected textual respomse;got:"+type);

        return request.responseText;
    }

 ///发送HTTP GET响应以获取指定URL内心哦那个
    //当响应到达时候，把它解析后的XML Document对象，解析后的JSON对象或字符串形式传递给回调
    function get(url,callback){
        var request=new XMLHttpRequest();
        request.open(‘GET‘,url);
        request.onreadystatechange=function(){
            if(request.readyState===4 && request.status===200){
                var type=request.getResponseHeader("Content-Type");
                if(type.indexOf("xml") !== -1 && request.responseXML){ 
                    callback(request.responseXML); //Document对象响应
                }else if(type=== ‘application/json‘){
                    callback(JSON.parse(request.responseText)) //JSON响应
                }else {
                    callback(request.responseText);
                }
            }
        };
        request.send(null);
    }

    //使用XMLHttpRequest下载Blob
    //以Blob的形式获取URL指定的内容，并将其传递给指定的回调函数
    function getBlob(url,callback){
        var xhr=new XMLHttpRequest(); //创建一个新的XHR对象
        xhr.open("GET",url); //指定要获取内容的URL
        xhr.responseType="blob"; //以BLOB形式
        xhr.onload=function(){ //onload比onstatechange更容易
            callback(xhr.response); //注意，这里是response不是.responseText
        };
        xhr.send(null);
    }

    //无论服务器端设置的Content-type是什么类型，豆浆把它当作文本类型来处理
    request.overrideMimeType("text/plain;charset=utf-8");

//将对象{find:"pizza",zipcode:01234,radius:"1km"}变成表单编码的格式 find=pizza&zipcode=01234$radius=1km 的形式
    function encodeFormData(data){
        if(!data) return "";
        var pairs=[];
        for(var name in data){
            if(!data.hasOwnProperty(name)) continue; //跳过继承方法
            if(typeof  data[name] === "function") continue;//跳过方法
            var value=data[name].toString();
            name=encodeURIComponent(name.replace("%20","+"));
            value=encodeURIComponent(value.replace("%20","+"));
            pairs.push(name+"="+value);
        }
        return pairs.join(‘&‘);
    }

//使用表单数据发起一个HTTP POST请求
function postData(url,data,callback){
        var request=new XMLHttpRequest();
        request.open("POST",url);
        request.onreadystatechange=function(){
            if(request.readyState===4 && callback)
                callback(request);
        };
        request.setRequestHeader("Content-Type","application/x-www-form-urlencoded");
        request.send(encodeFormData(data)); //发送表单编码数据
    }

 //使用表单编码数据发起get请求
    function getData(url,data,callback){
        var request=new XMLHttpRequest();
        request.open("GET",url+"?"+encodeFormData(data));
        request.onreadystatechange=function(){
            if(request.readyState===4 && callback) callback(request);
        };
        request.send(null);
    }

    //使用JSON编码主题发起HTTP POST请求
    function postJSON(url,data,callback){
        var request=new XMLHttpRequest();
        request.open("POST",url);
        request.onreadystatechange=function(){
            if(request.readyState===4&&callback)
                callback(request);
        };
        request.setRequestHeader("Content-Type","application/json"); 
        request.send(JSON.stringify(data));
    }

var req = new XMLHttpRequest();

req.addEventListener("progress", updateProgress, false);
req.addEventListener("load", transferComplete, false);
req.addEventListener("error", transferFailed, false);
req.addEventListener("abort", transferCanceled, false);

req.open();

...

// progress on transfers from the server to the client (downloads)
function updateProgress(evt) {
  if (evt.lengthComputable) {
    var percentComplete = evt.loaded / evt.total;
    ...
  } else {
    // Unable to compute progress information since the total size is unknown
  }
}

function transferComplete(evt) {
  alert("The transfer is complete.");
}

function transferFailed(evt) {
  alert("An error occurred while transferring the file.");
}

function transferCanceled(evt) {
  alert("The transfer has been canceled by the user.");
}

var req = new XMLHttpRequest();

req.upload.addEventListener("progress", updateProgress);
req.upload.addEventListener("load", transferComplete);
req.upload.addEventListener("error", transferFailed);
req.upload.addEventListener("abort", transferCanceled);

req.open();

    if("onprogress" in (new XMLHttpRequest())){
        //支持progress事件
    }

    request.onprogress=function(e){
        if(e.lengthComputable){
            progress.innerHTML=Math.round(100* e.loaded/ e.total)+"% Complete";
        }
    };

    ///实现超时
    //如果响应成功到达，传入responseText给回调函数
    //如果响应在timeout毫秒内没有到达，中止这个请求
    //浏览器可能在abort()后出发“readystatechange”
    //如果是部分请求结果到达，设置可能设置status属性
    //所以需要设置一个标记，当部分且超时的响应到达时不会调用回调函数
    ///如果使用load事件就没有这个风险
    function timedGetText(url,timeout,callback){
        var request=new XMLHttpRequest();
        var timedout=false;
        //启动计时器，在timeout毫秒之后将终止请求
        var timer=setTimeout(function(){
            timedout=true;
            request.abort();
        },
        timeout);
        
        request.open("GET",url);
        request.onreadystatechange = function(){ //定义事件处理程序
            if(request.readyState !== 4) return; //忽略未完成的请求
            if(timedout) return;                 //忽略终止请求
            clearTimeout(timer);                //取消等待的超时
            if(request.status === 200)          //如果请求成功
                callback(request.responseText); //把response传给回调
        };
        request.send(null);
    }

1 HTTP/1.1 200 OK
2 Set-Cookie: AO="o=1&s=1&dnt=1"; Version=1; Domain=yahoo.com; Max-Age=630720000; Expires=Sat, 18-Jun-2033 10:07:41 GMT; Path=/
3 Access-Control-Allow-Origin: *
4 Cache-Control: public, max-age=899
5 Content-Type: text/xml;charset=utf-8
6 Content-Encoding: gzip
7 Vary: Accept-Encoding
8 Date: Sun, 23 Jun 2013 10:07:40 GMT

function foo(data)
{
    // do stuff with JSON
}

var script = document.createElement(‘script‘);
script.src = ‘//example.com/path/to/jsonp?callback=foo‘

document.getElementsByTagName(‘head‘)[0].appendChild(script);