ajax同步请求和异步请求

ajax同步和异步的差异， 先看2段代码：

 代码一：

Synchronize = function(url,param) {
    function createXhrObject() {
        var http;
        var activeX = [ "MSXML2.XMLHTTP.3.0", "MSXML2.XMLHTTP", "Microsoft.XMLHTTP" ];
        try {
            http = new XMLHttpRequest;
        } catch (e) {
            for (var i = 0; i < activeX.length; ++i) {
                try {
                    http = new ActiveXObject(activeX[i]);
                    break;
                } catch (e) {}
            }
        } finally {
            return http;
        }
    }
    var conn = createXhrObject();
    conn.open("POST", url, false);//ajax同步
    conn.send(param);
    var strReturn = conn.responseText;
    alert("1");
    if (strReturn != "") {
        return Ext.decode(conn.responseText);
    } else {
        return null;
    }
    alert("2");
};

代码二：

Ajax 同步请求方式:


Synchronize = function(url,param) {
    function createXhrObject() {
        var http;
        var activeX = [ "MSXML2.XMLHTTP.3.0", "MSXML2.XMLHTTP", "Microsoft.XMLHTTP" ];
        try {
            http = new XMLHttpRequest;
        } catch (e) {
            for (var i = 0; i < activeX.length; ++i) {
                try {
                    http = new ActiveXObject(activeX[i]);
                    break;
                } catch (e) {}
            }
        } finally {
            return http;
        }
    }
    var conn = createXhrObject();
    conn.open("POST", url, true);//ajax异步
    conn.send(param);
    var strReturn = conn.responseText;
    alert("1");
    if (strReturn != "") {
        return Ext.decode(conn.responseText);
    } else {
        return null;
    }
    alert("2");
};

同步和异步的差异如下：

conn.open('POST',Url,true);         // ajax异步

conn.open('POST',Url,false);        // ajax同步

对于代码二，为异步的ajax请求，执行结果为：先执行alert(2)再执行alert(1), 异步的意思就是说一旦conn.open请求一发出，前端不去等待它的响应便执行后面的代码，所以alert(2)先执行了，然后当响应response到达以后才执行alert(1);

对于代码一，为同步的ajax请求，执行结果为：先执行alert(1)再执行alert(2), 同步的意思就是说一旦conn.open请求一发出，前端就去等待它的响应，响应完成以后，alert(1)先执行了，然后alert(2);