Reporting syntax errors in a JSON object with jQuery and/or Javascript

Answer 1

Call the URL or the XHR request directly in your browser's address bar, do a view-source and paste the result into and IDE that understands JavaScript. You'll probably spot a misplaced quote or something.

At least you'll be able to remove chunks of it until you find the offending syntax if nothing else.

Answer 2

You can bind a function to global ajaxerror events

$(document).ajaxError(function(event, request, ajaxOptions, thrownError){
  if ( 4==request.readyState) { // failed after data has received
    alert(ajaxOptions['url'] + " did not return valid json data");
  }
  else {
    alert("something else wnet wrong");
  }
});

or use $.ajax() instead of $.getJSON()

function foo() {
  // $.getJSON("http://localhost/test.txt", function(data){});
  $.ajax({
    type: "GET",
    url: "http://localhost/test.txt",
    success: function (data, textStatus) {
      alert("succuess");
    },
    error: function (XMLHttpRequest, textStatus, errorThrown) {
      alert("request failed: " + textStatus);
    },
    dataType: "json"
  });
}

edit: But you might want to keep in mind that both ajax(dataType:"json") and getJSON() (which simply invokes .ajax(dataType:"json") boil down to data = window["eval"]("(" + data + ")") ...this might not be what you want if you don't know what (arbitrary) data your script requests. And this could explain why firebug is catching a syntax error when you feed it an html document instead of json data.
In that case better request dataType:"string" und run the data through a fully fledged json parser lib.

Answer 3

Adding to Diodeus' answer, paste your potentially offending JSON into here this tool: http://jsonformatter.curiousconcept.com/

If you have firebug, you might even be able to hack together a way to use the site a service programmatically, though that may be frowned upon.

Answer 4

Thanks to all who answered. Because I was calling a JSON feed from an external domain, I wasn't able to just use jQuery's AJAX functionality and pull the feed as "text" instead of "json". jQuery only allows you to pull "json" and "script" from a remote source.

What I ended up doing instead was writing a PHP script to make the external call that lived on my own server. I use jQuery AJAX to call this, pass the requested source in the URL, and then grab that as "text." I then run my own check to ensure that it's properly formatted JSON, and then parse it with another library. jQuery doesn't have the ability to parse JSON at the moment; $.getJSON only works if you pass a URL to it.