Question

This is the error I get:

The line in question in the un-minified jQuery 1.7.2 javascript file:

 error: function( msg ) {
     throw new Error( msg );
 }, 

From this, there is no way to figure out where the error is (or at least it isn’t clear to me). I found that uncommenting this line from a jQuery plugin (http://shama.github.com/jmpress.js/#/docs) causes the error to stop:

$("#my_div").jmpress();

but I can’t get greater granularity than that. Any debugging tips to help me find out where this JSON parsing is happening?

=== EDIT: ===

using JSON.parse, I was able to track down this particular string that was attempting to be parsed as a string:

_FB_f3757ad5f032398xd_action=proxy_ready&data

but can’t nail down where it is coming into my code, but from the “FB” prefix it seems as though this string is part of the Facebook JS SDK (which I am using)

Answer 1

Ok, looking at the source code of jmpress it comes from the cross-origin iframe messaging. Basically jmpress does this:

window.addEventListener("message", function(event) {
    // We do not test orgin, because we want to accept messages
    // from all orgins
    try {
        var json = JSON.parse(event.data);
        switch(json.type) {
        case "select":
            // TODO SECURITY filter targetId
            $.each(eventData.settings.presentationMode.transferredValues, function(idx, name) {
                eventData.current[name] = json[name];
            });
            $(eventData.jmpress).jmpress("select", {step: "#"+json.targetId, substep: json.substep}, json.reason);
            break;
        case "listen":
            current.selectMessageListeners.push(event.source);
            break;
        case "ok":
            clearTimeout(current.presentationPopupTimeout);
            break;
        case "read":
            try {
                event.source.postMessage(JSON.stringify({type: "url", url: window.location.href, notesUrl: eventData.settings.presentationMode.notesUrl}), "*");
            } catch(e) {
                $.error("Cannot post message to source: " + e);
            }
            break;
        default:
            throw "Unknown message type: " + json.type;
        }
    } catch(e) {
        $.error("Recieved message is malformed: " + e);
    }
});

So all iframes that post to the main window, will go through this event handler and log the error when they pass invalid JSON. It looks like you have a facebook iframe on your website, that posts messages to the main window. Try removing all of those and see if the errors disappear.