Javascript 对象和 this 关键字

我知道这指的是对象所有者。但是我很难尝试使类工作，同时试图确定这指的是什么。

猜最好只显示代码：

function Ajax_Class(params) {
// Public Properties
this.Response = null;
// Private Properties
var RequestObj = null;
// Prototype Methods
this.OnReset    = function() { };
this.OnOpenConn = function() { };
this.OnDataSent = function() { };
this.OnLoading  = function() { };
this.OnSuccess  = function() { alert("default onSuccess method."); };
this.OnFailure  = function() { alert("default onFailure method."); };
// Public Methods
this.Close = function() {   // Abort current Request
    if (RequestObj) {
        RequestObj.abort();
        RequestObj = null;
        return true;
    } else return false;
};
// Private Methods
var Instance = function() {     // To create instance of Http Request
    try { return new XMLHttpRequest(); }
    catch (error) {}
    try { return new ActiveXObject("Msxml2.XMLHTTP"); }
    catch (error) {}
    try { return new ActiveXObject("Microsoft.XMLHTTP"); }
    catch (error) {}
    // throw new Error("Could not create HTTP request object.");
    return false;
};
var ReadyStateHandler = function() {
    // Switch through possible results
    switch(RequestObj.readyState) {
        case 0:
            this.OnReset();
        break;
        case 1:
            this.OnOpenConn();
        break;
        case 2:
            this.OnDataSent();
        break;
        case 3:
            this.OnLoading();
        break;
        case 4:
            // Check Status
            if (RequestObj.status == 200)  {
                // Handle Response
                Response = HandleResponse();
                // Call On Success
                this.OnSuccess();
                // Hide Loading Div
                LoadingDiv(true);
            } else {
                this.OnFailure();
            }
        break;
    } // End Switch
};
var Open = function() {
    // In case it's XML, Override the Mime Type
    if ((params["ResponseType"] == "XML") && (RequestObj.overrideMimeType)) 
        RequestObj.overrideMimeType('text/xml');
    // 
    if ((params["User"]) && (params["Password"]))
        RequestObj.open(params["Method"], params["URL"],  params["Async"], params["User"], params["Password"]);
    else if (params["User"])
        RequestObj.open(params["Method"], params["URL"],  params["Async"], params["User"]);
    else
        RequestObj.open(params["Method"], params["URL"],  params["Async"]);
    // Set Request Header ?
    if (params["method"] == "POST") 
        //this.SetRequestHeader("Content-Type", "application/x-www-form-urlencoded; charset=UTF-8");
        RequestObj.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
};
var Send = function() {
    if (params["Data"])     RequestObj.send(params["Data"]);
    else                    RequestObj.send(null);
};
var HandleResponse = function() {
    if (params["ResponseType"] == "JSON")
        return ParseJSON(RequestObj.responseText);
    else if (params["ResponseType"] == "XML")
        return RequestObj.responseXML;
    else 
        return RequestObj.responseText;
};
// Method ParseJSON
var ParseJSON = function(obj) {
    if (obj)
        return JSON.parse(obj);
}; // End ParseJSON
// Method LoadingDiv -> Either Shows or Hides the Loading Div
var LoadingDiv = function(hide) {
    // Hide the Modal Window
    if (hide) { document.getElementById("Async").style.display = "none"; return false; }
    // Show Modal Window
    document.getElementById("Async").style.display = "block";
    // Reset the Position of the Modal_Content to 0, x and y
    document.getElementById("Async_Container").style.left = "0px";
    document.getElementById("Async_Container").style.top = "0px";
    // Center the Modal Area, no matter what the content
    var Screen_Width, Screen_Height;
        // Get screen data
        if (typeof(window.innerWidth) == "number") { Screen_Width = window.innerWidth; Screen_Height = window.innerHeight; }            //Non-IE
        else if (document.documentElement && (document.documentElement.clientWidth || document.documentElement.clientHeight)) {         //IE 6+ in 'standards compliant mode'
            Screen_Width = document.documentElement.clientWidth;
            Screen_Height = document.documentElement.clientHeight;
        } else if (document.body && (document.body.clientWidth || document.body.clientHeight)) {                                        //IE 4 compatible
            Screen_Width = document.body.clientWidth;
            Screen_Height = document.body.clientHeight;
        }
        // Set Modal_Content Max Height to allow overflow
        document.getElementById("Async_Container").style.maxHeight = Screen_Height - 200 + "px";
        // Get Modal_Container data
        var El_Width = document.getElementById("Async_Container").offsetWidth;
        var El_Height = document.getElementById("Async_Container").offsetHeight;

    // Set the Position of the Modal_Content
    document.getElementById("Async_Container").style.left = ((Screen_Width/2) - (El_Width/2)) + "px";
    document.getElementById("Async_Container").style.top = ((Screen_Height/2) - (El_Height/2)) + "px";
};

// Constructor
// Check the Params
// Required Params
if (!params["URL"]) return false;
// Default Params
params["Method"]        = (!params["Method"]) ? "GET" : params["Method"];   // GET, POST, PUT, PROPFIND
params["Async"]         = (params["Async"] === false) ? false : true;
params["ResponseType"]  = (!params["ResponseType"]) ? "JSON" : params["ResponseType"];  // JSON, TXT, XML
params["Loading"]       = (params["Loading"] === false) ? false : true;
// Optional Params
// params["User"])
// params["Password"]
// Create Instance of Http Request Object
RequestObj = Instance();
// Handle Ajax according to Async
if (params["Async"]) {
    // Handle what should be done in case readyState changes
    if (params["Loading"]) LoadingDiv();
    // State Handler || Response is Handled within the code
    RequestObj.onreadystatechange = ReadyStateHandler;
    // Open & Send
    Open();
    Send();
} else {
    // Handle the Loading Div
    if (params["Loading"]) LoadingDiv();
    // Open & Send
    Open();
    Send();
    // Handle Response
    this.Response = HandleResponse();
    // No State Handler, Application has been waiting for modifications.
    //this.OnSuccess(Response);
    // Handle the Loading Div
    LoadingDiv(true);
}
// no problems?
return true;
} // End Ajax

然后，在页面内：

window.onload = function() {
update_states = function() {
    this.OnSuccess = function() {
        alert("overriden onSuccess Method");
    };
};
    update_states.prototype = new Ajax_Class({URL:"ajax/states.php?id=5&seed="+Math.random()});
    run_update_states = new update_states;      
} // Window Onload

我正在尝试做的是有一个默认的成功(等等(方法，如果需要，我可以用子类方法覆盖默认方法，但它仍然会在 HttpRequest 状态更改时自动调用。

如果有人能为我指出正确的方向，我将不胜感激，如果我能理解为什么在这种情况下不起作用以及如何使其引用正确的对象，我会感到惊讶。

感谢您的任何帮助。