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I'm building a web application, and am wondering how to handle errors with my AJAX calls. For instance, if the user enters some sort of data that is invalid (bad email address, user already exists) I want to be able to throw an error from PHP.

I've seen here http://php4every1.com/tutorials/jquery-ajax-tutorial/ that you could just use a JSON object and handle the error report from JQuery's Success function, but that doesn't seem like the right way to do it. It would make sense to me that jQuery's error function should be utilized when there is an error. I guess I'm a stickler for that kind of thing.

Here's how I'm doing it right now.

//In my PHP file called from JQuery
function error($msg) {
    header("HTTP/1.0 555 ".$msg);
    die();
}
//Then that error is handled accordingly from JQuery

So I'm creating an error code of 555—which is not defined as anything—and tacking on my own custom error message. Is that the right way to do this? Should I just use JSON? There's gotta be a standard way to send out error messages like this, right?

If you need to see more of my code to get a better idea, the whole project is up on github: https://github.com/josephwegner/fileDrop. The file in question is config/phpFuncts.php.

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    There's no reason to create a new status code for this, 400 would do fine if it's a user-input validation error. It can be considered a bad request (as in, their request body -- their user input -- is malformed). Commented Jun 6, 2012 at 18:54

3 Answers 3

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I'd just use a JSON object and an HTTP header of 200. There was nothing wrong with the request itself, and your server behaved as it was supposed to - the error was on a different layer of the abstraction.

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The 400 family of HTTP status codes indicate that there was a problem with the request. I would set the response code to 400 and include the list of error messages in the response body as JSON.

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Agreed. For errors the user can correct, use 400 Bad Request (as in, they have validation errors, etc.). For server exceptions like DB failures, fatal errors, etc use the 500 level status codes.
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I hate using HTTP status codes for indicating failures. The HTTP codes should be reserved for actual HTTP-level errors, and errors related to the AJAX request should be sent back via a JSON structure.

e.g.

$data = array()
if (some big ugly computation fails) {
   $data['errorcode'] = -123;
   $data['error'] = true;
   $data['success'] = false;
   $data['errormessage'] = 'some helpful error message';
} else {
   $data['success'] = true;
   $data['error'] = false;
   $data['response'] = 'whatever you wanted to send back....';
}
echo json_encode($data);

Then on the client-side

if (data.error) {
   alert('Request blew up: ' + data.errormessage);
}
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7 Comments

I disagree with the "HTTP codes should be reserved for actual HTTP-level errors". HTTP is a protocol, and the status codes are there reflect the service's response. 4xx response codes should be used for an issues that the client is responsible for, 5xx response codes should be used for issues that the server is responsible for
If I drive to the store and they don't have what I want, I don't call the trip a failure - the car worked, I parked properly, I got home - just came home empty handed. Errors with an AJAX service itself should be signaled in the service response text, not at the HTTP level. returning 404 on a "failed" ajax call should mean "hey, the service file is missing", not "sorry, that record in the database doesn't exist".
The HTTP status codes are meant to be used that way. a "400 Bad Request" doesn't mean HTTP failed. If HTTP failed, you wouldn't get an error response at all. HTTP isn't reporting on it's own performance.
A 404 is perfectly appropriate for saying "that content doesn't exist", just like you can ask for a file from a server and it returns a 404. With a regular 404, the server is still working, just like your AJAX call / HTTP is still working. I suggest reading Roy Fielding's dissertation on REST and the role HTTP should play in service architectures. AJAX is a communication method. If it failed you either get no response or you use a 5xx level response status if the server failed in dealing with the response.
After reading this very interesting debate and understanding and agreeing with both angles - I still don't have a clue which one is the 'best' method - because it can go both ways!!
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