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

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?

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.

Firstly, true JSON is much stricter than JavaScript, and to be valid JSON, you have to have your keys quoted.

 { "a": 3 } 

Also, as you are using a bare XMLHttpRequest, which generally expects to receive an XML result unless MIME headers specify strictly otherwise.

You may however wish to make your own life easier by simply using a JavaScript framework such as jQuery which will abstract away all this problem for you and deal with all the nasty edge cases.

$.getJSON("data.json",{}, function( data ){ 
  /*  # do stuff here  */ 
});

Additionally, if you use both strict JSON and use a library to abstract it for you, when browsers start having native JSON parsers the library will be able to transparently make use of these and get a significant speed improvement.

( This is slated to happen sooner than later, and when it happens, your users will get a silent upgrade with no effort required! ).

This also occurs when Content-Type is completely empty (thereby circumventing the natural type-detection).

It should be {"a": 3} actually.

I found the same error message but from a very different cause. After a little time fruitlessly changing the JSON content, realized that I had accidentally restarted the page running off the local file system (file://Users/me/Sites/mypage.html) rather than the server (http://localhost/~me/Sites/mypage.html).