var index = 0;//记录调用次数
var XHR = function(){
    var xhr = false;
    if (window.XMLHttpRequest) {
        xhr = new XMLHttpRequest();
    }
    else {
        try {
            xhr = new ActiveXObject("Msxml2.XMLHTTP");
        } 
        catch (e) {
            xhr = new ActiveXObject("Microsoft.XMLHTTP");
        }
    }
    return xhr ? xhr : "Your browser doesn't support XMLHttpRequest.";
}

var sendRequest = function(url, callback){
    var xhr = XHR();
    if (typeof xhr === "string") 
        throw xhr;
    xhr.open("GET", url, true);
    xhr.onreadystatechange = function(){
        if (xhr.readyState === 4) {
            callback(xhr);
            //设置一个0时间间隔的定时器
            setTimeout(function(){
                xhr.open("GET", url, true);
                xhr.send(null);
            }, 0);
        }
    }
    xhr.send(null);
}

var callback = function(xhr){
    index++;
    document.getElementById("div").innerHTML = xhr.responseText + index;
}
sendRequest("test.txt", callback);