Simulate a JSONP response with JavaScript URLs

EDIT: So, after a few days battling with this I'll post my final solution that might help others trying to communicate with Pardot using JSONP. It's a three part problem:

1. Send a JSONP request to a Pardot Form Handler
2. Redirect Pardot Form Handler success/error to your own server
3. Return JSONP from your server

Send a JSONP request to Pardot From Handler

To send form data to the Form Handler you need to URI encode the field names and values.

(Example using JQuery. The '?callback=' is added by ajax() when specifying the dataType: 'jsonp'):

var data = { 'field1' = 'value1', 'field' = 'value2' };
var uriEncodedParams = $.param(data);
var targetUrl = <Pardot Form Handler URL> + '?' + uriEncodedParams;

$.ajax({
    url: targetUrl,
    dataType: 'jsonp',
    jsonpCallback: 'callback'
});

window.callback = function (data) {...}

Redirect Pardot From Handler success/error to your own server

See @nickjag's answer:

Set the Success Location and Error Location to endpoints on your backend.

As pardot will not forward any of the GET parameters you passed in you'll have to use some defaults on i.e. the callback function name (hence specifying jsonpCallback and not having a success in my request).

Return JSONP from your server

I was having problems with console errors (Uncaught SyntaxError: Unexpected token :) when using: return "{ 'result' : 'success' }" as it is a JSON-object and JSONP expects a file with JavaScript. So the format to return should be: return "callback({ 'result' : 'success' })"

And as again Pardot don't forward the GET params, the generated callback function name from JQuery didn't propagate and I couldn't return the correct JavaScript code. Defaulted to use the function name "callback" if none was provided.

Guide for returning JSONP from .NET MVC backend

What Pardot is suggesting is to create 2 static URLs on your own server that return a simple JSON response.

So for example:

1. mysite.com/pardot-success

Returns: {"result":"success"}

2. mysite.com/pardot-error

Returns: {"result":"error"}

You'll then use those two URLs as your success and error redirect URLs for your Pardot form handler settings.

An AJAX request can then be made to your Pardot form handler using JSONP, which will wrap and return the JSON response from one of those URLs (depending on the result).

Your AJAX response data would include your JSON result (success or error).