How to access method variables from within an anonymous function in JavaScript?

Question

I'm writing a small ajax class for personal use. In the class, I have a "post" method for sending post requests. The post method has a callback parameter. In the onreadystatechange propperty, I need to call the callback method.

Something like this:

this.requestObject.onreadystatechange = function() {
    callback(this.responseText); 
}

However, I can't access the callback variable from within the anonomous function. How can I bring the callback variable into the scope of the onreadystatechange anonomous function?

edit:

Here's the full code so far:

function request()
{
    this.initialize = function(errorHandeler)
    {
        try {
            try {
                this.requestObject = new XDomainRequest();
            } catch(e) {
                try {
                    this.requestObject = new XMLHttpRequest();
                } catch (e) {
                    try {
                        this.requestObject = new ActiveXObject("Msxml2.XMLHTTP"); //newer versions of IE5+
                    } catch (e) {
                        this.requestObject = new ActiveXObject("Microsoft.XMLHTTP"); //older versions of IE5+
                    }
                }
            } 
        } catch(e) {
            errorHandeler();
        }
    }

    this.post = function(url,data,callback)
    {
        var response;var escapedData = "";
        if (typeof data == 'object') {
            for (i in data) {
                escapedData += escape(i)+'='+escape(data[i])+'&';
            }
            escapedData = escapedData.substr(0,escapedData.length-1);
        } else {
            escapedData = escape(data);
        }
        this.requestObject.open('post',url,true);
        this.requestObject.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
        this.requestObject.setRequestHeader("Content-length", data.length);
        this.requestObject.setRequestHeader("Connection", "close");
        this.requestObject.onreadystatechange = function()
        {
            if (this.readyState == 4) {
                // call callback function
            }
        }
        this.requestObject.send(data);
    }
}

Answer 1

var that = this;

Then use that instead of this inside the anonymous function.

Answer 2

If callback is a variable in the containing function, it should be in scope. If it is not a variable, but is in scope in in the containing function, you may have to do something like

var cb = callback;
var xhrRequest = this;

then

cb(xhrRequest.responseText);

Answer 3

Just pass the callback function together with the rest of the arguments

this.post = function(url, data, callback) {
    ...
    this.requestObject.onreadystatechange = function() {
        if (this.readyState == 4) {
            callback(this.responseText);
        }
    };
    ...
}

And then

foo.post("foo.html", {foo:"bar"}, function(result){
    alert(result);
});

By the way, this is a better way to convert the data into a proper string

var q = [];
for (var key in data) {
    if (data.hasOwnProperty(key)) {
        q.push(key + "=" + encodeURIComponent(data[key]));
    }
}
data = q.join("&"); //data can now be passed to .send()

encodeURIComponent is the proper function to use here as encode will not escape data properly

If you want to get a ready made function for all of this you can take a look here http://github.com/oyvindkinsey/easyXDM/blob/master/src/easyXDM.js#L358