Which callback for XMLHttpRequest status 0? - google-app-engine

Question

I'm occasionally seeing a failed XMLHttpRequest.send(). From Chrome's network panel I'm seeing a status of 0 - see below from the .har file. The code that runs the send() succeeds >99% of the time, but very occasionally (~ 1/300) it returns 0.
My question is: how do I catch this? Is there are callback that will catch it?
I'm currently using onload and onerror:
var xhr = new XMLHttpRequest();
xhr.onload = function(){};
xhr.onerror = function(){};
Neither of those are being called, so it's failing silently.
A few other things to note:
There is no evidence of the request in the server logs
There are no error messages in Chrome's console
The server is google app engine
Here's the .har file output. ##=redacted.
{
"startedDateTime": "2013-03-30T23:52:20.972Z",
"time": 97,
"request": {
"method": "GET",
"url": "https://www.#######.com/project/auth_upload?to_sign=PUT%0A%0A%0A%0Ax-amz-date%3ASat%2C%2030%20Mar%202013%2023%3A52%3A20%20GMT%0A/s3.#####.com/######.mp4%3FpartNumber%3D50%26uploadId%3D#################.w--&asset_id=#############&project_id=###########",
"httpVersion": "HTTP/1.1",
"headers": [
{
"name": "Referer",
"value": "https://www.######.com/console/###########"
},
{
"name": "User-Agent",
"value": "Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/537.22 (KHTML, like Gecko) Chrome/25.0.1364.152 Safari/537.22"
}
],
"queryString": [
{
"name": "to_sign",
"value": "PUT%0A%0A%0A%0Ax-amz-date%3ASat%2C%2030%20Mar%202013%2023%3A52%3A20%20GMT%0A/s3.#######.com/############.mp4%3FpartNumber%3D50%26uploadId%3D##############"
},
{
"name": "asset_id",
"value": "###########"
},
{
"name": "project_id",
"value": "###############"
}
],
"cookies": [],
"headersSize": 595,
"bodySize": 0
},
"response": {
"status": 0,
"statusText": "",
"httpVersion": "HTTP/1.1",
"headers": [],
"cookies": [],
"content": {
"size": 0,
"compression": 0
},
"redirectURL": "",
"headersSize": 13,
"bodySize": 0
},
"cache": {},
"timings": {
"blocked": 0,
"dns": -1,
"connect": -1,
"send": -1,
"wait": -1,
"receive": null,
"ssl": -1
}
}
thanks,
tom

Answer 1

It seems that statusCode 0 indicates that response is empty even headers are not sent, that is something hard to figure out and you need to find it out yourself
but you said that the response doesn't arrive in your functions is because you are using load and error events for listening for the response, and actually you shouldn't entirely depend on those two errors, consider as case where timeout has occured before your request gets completed then 'timeout' event will be fired not 'error'
Instead you should use readystatechange event that will be called on every state change of request and you can also keep track of timeouts or errors related to no response received
httpRequest.onreadystatechange = stateChangeHandler;
stateChangeHandler = function() {
// The readyState can be 4 values:
// 0 - uninitialized
// 1 - loading
// 2 - loaded
// 3 - interactive
// 4 - complete
//
// readyState 0 - 3 can be completely ignored by us, as they are only updates
// about the current progress. Only on readyState 4, should we continue and
// start checking for the response status.
if (xmlHttpRequest.readyState != 4) {
return;
}
// Check HTTP Response code
if (xmlHttpRequest.status != 200) {
// response is ok process it
} else {
// there was some error
}
}
Reference:
https://developer.mozilla.org/en-US/docs/HTTP#HTTP_Response_Codes
http://www.w3.org/TR/XMLHttpRequest/#event-xhr-readystatechange
http://msdn.microsoft.com/en-us//library/ms534361%28en-us,VS.85%29.aspx
http://www.w3.org/TR/XMLHttpRequest/#the-status-attribute