Question

I would like to handle all the errors in a single place.
Is it possible to attach an error handler to the XMLHttpRequest object (or somthing like that), in one place that will handle all the requests?

Thanks

Answer 1

If you had a single single XMLHttpRequest object, you would listen to the readystatechange event to detect, when the operation is complete, and then read the status property to detect an error.

Now, if you want to do this with all objects, you need to listen to all occurring readystatechange events, i.e. register to the respective event, if a XMLHttpRequest is created or initialized. This can be done, by overriding the open-method of the XMLHttpRequest prototype, so that it registers your event listener and then calls the old open.

I have used the following code for that (in Firefox):

 (function(open) {
   XMLHttpRequest.prototype.open = function(method, url, async, user, pass) {
     this.addEventListener("readystatechange", function() {
       if (this.readyState==4 && (this.status != 200 && this.status != 301 && this.status != 302))  {
         alert("Error: "+this.statusText)
       }
     }, false);
     open.call(this, method, url, async, user, pass);
   };
 })(XMLHttpRequest.prototype.open);

(this assumes that open is called only once per XMLHttpRequest object or your event is registered multiple times, if you want to be safe, you could probably reset the open-function by adding this.open = open; )