Kyle Huey created a reproducible test case that allows you to specify how long the server should wait before finishing the response. The longer the response takes to return, the more frequently the readystatechange event will not fire. Therefore, a request that receives a response in less than a second is far less likely to see this happen than a request that receives a response in ten seconds. It seemingly occurs randomly but it’s likelihood increases as the amount of time it takes for a response to be fully received increases. There is no JavaScript error to catch and no error condition to look for, and in fact, the response is received by the browser (as can be tested using the Firebug Net panel or Fiddler).įortunately, this doesn’t happen for all XHR communication. The primary symptom is that the readystatechange event on an XMLHttpRequest object doesn’t get fired past readyState 1, meaning that any script listening for readystatechange to test for readyState being equal to 4 will fail silently. The issue presents itself when using Firefox 3.5 with Firebug 1.4.x or 1.5.x. It’s now a week later and the issue hasn’t been resolved, so I’d like to share with everyone in the hopes of avoiding a lot of debugging by web developers around the world. Christopher pointed out to me that he believed the cause of the issue was actually Firebug rather than Firefox itself. He hadn’t, so I continued digging and eventually found two bugs, one in the Firefox queue via my co-worker Steve Carlson and one in the Firebug queue via Christopher Blum. I thought I had found a bug in Firefox 3.5.Īs I was debugging, I tweeted about this issue a handful of times and then pinged YUI Connection Manager creator Thomas Sha to see if he had heard of this issue. After checking my code, the rest of the page’s code, and debugging back into the YUI layer, I discovered that the source of the bug wasn’t JavaScript code at all – it was the browser.

The complaint was that our page wasn’t displaying an Ajax response even though Firebug clearly showed that a response had been received. Initially, they had neglected to mention their fast upgrade to the latest Firefox and I spent some time fruitlessly trying to reproduce the issue. Last Thursday I was debugging an issue at work that was reported by two colleagues using Firefox 3.5. Ajax, Firebug, Firefox, JavaScript, XMLHttpRequest

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