Starting with Firefox 56, if the
action attribute on a
<form> HTML element is empty or missing, the
action property on the corresponding
HTMLFormElement DOM object will return the actual form submission URL, which is the document’s URL, instead of an empty string.
This change was made to be compliant the latest HTML spec, and so far only 1 compatibility issue has been reported. Since other browsers are not following the spec yet, such issues may only happen in Firefox at the time of this writing.
It’s easy for Web developers to support both the old and new behaviour, like this:
// To get an empty string as before (backward compatibility) let action_url = form.action === document.URL ? "" : form.action; // To get the actual submission URL (forward compatibility) let actual_action_url = form.action || document.URL;