Ajax基础创建XMLHttpRequest对象

var http_request = false;
function send_request(url) {
    //初始化,指定处理函数,发送请求的函数 
    http_request = false;
    //开始初始化XMLHttpRequest对象 
    if (window.XMLHttpRequest) {
        //Mozilla 浏览器 
        http_request = new XMLHttpRequest();
        if (http_request.overrideMimeType) {
            //设置MiME类别 
            http_request.overrideMimeType('text/xml');
        }
    }
    else if (window.ActiveXObject) {
        // IE浏览器 
        try {
            http_request = new ActiveXObject("Msxml2.XMLHTTP");
        }
        catch (e) {
            try {
                http_request = new ActiveXObject("Microsoft.XMLHTTP");
            }
            catch (e) { }
        }
    }
    if (!http_request) {
        // 异常,创建对象实例失败 
        window.alert("不能创建XMLHttpRequest对象实例.");
        return false;
    }
    http_request.onreadystatechange = processRequest;
    // 确定发送请求的方式和URL以及是否同步执行下段代码 
    http_request.open("GET", url, true);
    http_request.send(null);
}