FormData.append("key", "value") is not working

New in Chrome 50+ and Firefox 39+ (resp. 44+):

• formdata.entries() (combine with Array.from() for debugability)
• formdata.get(key)
• and more very useful methods

Original answer:

What I usually do to 'debug' a FormData object, is just send it (anywhere!) and check the browser logs (eg. Chrome devtools' Network tab).

You don't need a/the same Ajax framework. You don't need any details. Just send it:

var xhr = new XMLHttpRequest;
xhr.open('POST', '/', true);
xhr.send(data);

Easy.