"not well-formed" error in Firefox when loading JSON file with XMLHttpRequest

Question

I'm getting a "not well-formed" error in the error console of Firefox 3.0.7 when the JavaScript on my page loads a text file containing an object in JavaScript Object Notation format. If the file contains nothing but the JSON object, it produces the error. If I wrap the object in <document></document> tags it does not produce the error. The request succeeds either way, so I could just ignore it, but I don't want my error log filling up with these messages.

Here is some example code to illustrate the problem. First, the "not well-formed" file called "data.json":

{ a: 3 }

Now some code to load the file:

var req = new XMLHttpRequest();
req.open("GET", "data.json");
req.send(null);

Which produces the following error in the Firefox error console:

not well-formed
file://path/to/data.json Line: 1
{ a: 3 }
- ^

If data.json is modified to this:

<document>{ a: 3 }</document>

There is no error. I assumed that it is complaining because the plain JSON file is not a well formed XML document, so I tried overriding the MIME type before the "send" call to force it to load as plain text, but that didn't work.

var req = new XMLHttpRequest();
req.open("GET", "data.json");
req.overrideMimeType("text/plain");
req.send(null);
// Still produces an error!

I am going to continue with wrapping my JSON data in an XML document to get around whatever validation the XMLHttpRequest is performing, but I'd like to know if there is any way I can force it to just load plain text uncritically and not try to validate it. Alternatively, is there another method of loading data besides XMLHttpRequest that can be used with plain text?

Answer 1

Have you tried using the MIME type for JSON?

application/json

You could also configure your server to send this MIME type automatically for .json files.