Form.submit is not a function - How to create and POST form with (clean) Javascript in Firefox extension

Question

I am trying to create and submit a form with pure Javascript in a Firefox extension. However, even with the most basic form, the browser console continues to say:

TypeError: form.submit is not a function

I searched and read several posts on this subject, but they did not address this specific problem. A common mistake is to have an input named "submit", but this is not the case here.

Here is my code inspired by this wonderful JavaScript post request post on how to submit a form :

// Create a form
var form = document.createElement("form");
form.action = "http://example.com";
form.method = "POST"

var hiddenField = document.createElement("input");
hiddenField.setAttribute("type", "submit");
hiddenField.setAttribute("name", "sub");
hiddenField.setAttribute("value", "send");
form.appendChild(hiddenField);

// Create a document
var doc = document.implementation.createDocument ('http://www.w3.org/1999/xhtml', 'html', null);
var body = document.createElementNS('http://www.w3.org/1999/xhtml', 'body');
body.appendChild(form);
doc.documentElement.appendChild(body);

// Call the form submit method
form.submit();

I am new to Javascript, so it is likely that I am making a trivial mistake. Can someone let me know how to fix this?

Answer 1

"form", , xul. , body:

var form = document.createElementNS('http://www.w3.org/1999/xhtml', 'form');