jQuery ajax call failing with undefined error

Question

My jQuery ajax call is failing with an undefined error. My js code looks like this:

$.ajax({
   type: "POST",
   url: "Data/RealTime.ashx",
   data: "{}",
   contentType: "application/json; charset=utf-8",
   dataType: "json",
   timeout: 15000,
   dataFilter: function(data, type) {
       alert("RAW DATA: " + data + ", TYPE: "+ type);
       return data;
   },
   error: function(xhr, textStatus, errorThrown) {
       alert("FAIL: " + xhr + " " + textStatus + " " + errorThrown);
   },
   success: function(data) {
       alert("SUCCESS");
   }
});

My ajax source is a generic ASP.NET handler:

[WebService(Namespace = "http://my.website.com")]
[WebServiceBinding(ConformsTo = WsiProfiles.BasicProfile1_1)]
public class RealTime : IHttpHandler
{
    public void ProcessRequest(HttpContext context)
    {
        context.Response.ContentType = "application/json";
        context.Response.Write("{ data: [1,2,3] }");
        context.Response.End();
    }

    public bool IsReusable
    { get { return false; } }
}

Now, if I return an empty object ("{ }") in my handler, the call will succeed. But when I return any other JSON object, the call fails.

The dataFilter handler shows that I am receiving a correct object. Firebug shows the response as expected, and the JSON tab shows that the object is parsed correctly.

So what could be the cause?

[Edit] I should have actually written "when I return any invalid JSON object, the call fails"! :D

What you have is not valid JSON, what do you get when you change it to this: context.Response.Write("{ \"data\": [1,2,3] }"); ? — Nick Craver
– Nick Craver, Commented Jun 5, 2010 at 10:42
Thanks a lot, that did it. Firebug is obviously a bit less strict on parsing it, so I didn't think about it at all. You could copy this into an answer so I can accept it. — vgru
– vgru, Commented Jun 5, 2010 at 11:03
Firebug isn't parsing JSON, it's parsing Javascript - there's a difference. If you had used Firebug to pass that string into a Javascript implementation of a strict JSON parser, you would have gotten errors. — Pointy
– Pointy, Commented Jun 5, 2010 at 11:06
@Pointy: Now it makes sense, but the Firebug tab displaying parsed object is actually named "JSON". — vgru
– vgru, Commented Jun 5, 2010 at 11:11

Nick Craver · Accepted Answer · 2010-06-05 11:19:55Z

2

You need valid JSON! :)

Change this line:

context.Response.Write("{ data: [1,2,3] }");

To this:

context.Response.Write("{ \"data\": [1,2,3] }");

jQuery 1.4+ doesn't tolerate invalid JSON like it used to (fails silently/in weird ways), so just add the double quotes and you're all set. For a handy tool to test JSON validity, checkout JSONLint: http://www.jsonlint.com/

answered Jun 5, 2010 at 11:19

Nick Craver

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2 Comments

vgru Over a year ago

Yes, last time I used this stuff was with jQuery 1.3.2, and it worked. It's funny it never occurred to me to try the old project and see what happens. :)

Nick Craver Over a year ago

@Groo - Ah yeah that's the most common problem with these, but as @Pointy says they're more strict...but for a reason, it's so if the native JSON.parse function is present, we can use the browser's much faster implementation to do the work, win for everybody once all browsers support it property, jQuery is just wrapping it until then :)

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