Catching same origin exception in Javascript?

Question

I'm trying to create my own XMLHttpRequest framework to learn how this things work internally. A thing that puzzles me is that I cannot find how to catch a "Same origin" exception.

The idea behind this is that I try to load a URL, if I get a Same origin exception, I re-request the URL through a proxy script local for the script. The reason I do this is because I need to access production data from a development sandbox and I want it to be as transparent as possible for the script itself.

I know it's a bad practice but this is the least intrusive way of doing this at the moment :)

Just to clear things - I don't want to bypass same origin, I just want to catch the thrown exception so I can do something about it.

Here is the code I currently use for my xhr:

var net = function (url, cb, setts){
    this.url = url;
    this.cb = cb;

    var oThis = this;
    if (!this.xhr) {
        this.xhr = new XMLHttpRequest();
        this.xhr.onreadystatechange = function() {
            if (oThis.xhr.readyState == 4 && oThis.xhr.status == 200) {
                document.body.innerHTML += "RS: "+oThis.xhr.readyState+"; ST:"+oThis.xhr.status+"; RP:"+oThis.xhr.responseText+"<br>";
            }
            else {
                // do some other stuff :)
                document.body.innerHTML += "RS: "+oThis.xhr.readyState+"; ST:"+oThis.xhr.status+"; RP:"+oThis.xhr.responseText+"<br>";
            }
        }
    }
    this.xhr.open("GET", url,true);
    this.xhr.send();
} // It's WIP so don't be scared about the unused vars or hardcoded values :)

I've tried to try...catch around xhr.send(); but no avail, still can't catch the exceptions.

Any ideas or pointers would be greatly appreciated.

thejh · Accepted Answer · 2010-11-30 21:58:38Z

1

xhr.onreadystatechange = function() {
    if (xhr.readyState==4) {
        if (xhr.status==0) {
            alert("denied");
        } else {
            alert("allowed");
        }
    }
}

answered Nov 30, 2010 at 21:58

thejh

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1 Comment

bisko Over a year ago

This is the way I went too. Though in Chrome if you want to reuse the XHR after this, you must recreate it, because it doesn't respond to any requests after.

aditya · Accepted Answer · 2010-11-30 21:50:05Z

1

~~Are you sure it's actually supposed to throw an exception? I can't see anything in the specifications: http://www.w3.org/TR/XMLHttpRequest/#exceptions~~ Looks like it does. My bad.

In either case, you can always check the domain of the incoming string against the domain of the page the user is currently on.

FWIW, as you can see by this jsFiddle (open up Web Inspector), Chrome doesn't really throw an exception. It just says "Failed to load resource".

edited Nov 30, 2010 at 21:50

answered Nov 30, 2010 at 21:26

aditya

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1 Comment

Emmett Over a year ago

Section 3.6.1: "Throws a SECURITY_ERR exception if the origin of url does not match the XMLHttpRequest origin."

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