调用XMLHttpRequest alert后为undefined

在其他地方调用，alert(MyXMLHttpRequest(myurl, mydata));后是undefined
//上面myurl,mydata,为传入的内容

function MyXMLHttpRequest(myurl, mydata) {  
    console.log("进入");  

    var xhr = new plus.net.XMLHttpRequest();  
    xhr.onreadystatechange = function() {  
        switch (xhr.readyState) {  
            case 0:  
                console.log("xhr请求已初始化");  
                break;  
            case 1:  
                console.log("xhr请求已打开");  
                break;  
            case 2:  
                console.log("xhr请求已发送");  
                break;  
            case 3:  
                console.log("xhr请求已响应");  
                break;  
            case 4:  
                if (xhr.status == 200) {  
                    console.log("xhr请求已成功"   xhr.responseText);  
                    var a = JSON.parse(xhr.responseText);  
                    console.log(a);  
                    return a;// 返回值  
                } else {  
                    plus.nativeUI.toast("发生了通讯错误！");  
                    console.log("xhr请求失败："   xhr.readyState);  
                }  
                break;  
        }  
    }  

    xhr.open("POST", myurl);  
    xhr.setRequestHeader("Content-Type", "application/x-www-form-urlencoded");  
    xhr.send(mydata);