The FormDataEvent interface represents a formdata event — such an event is fired on an HTMLFormElement object after the entry list representing the form's data is constructed. This happens when the form is submitted, but can also be triggered by the invocation of a FormData() constructor.
This allows a FormData object to be quickly obtained in response to a formdata event firing, rather than needing to put it together yourself when you wish to submit form data via a method like XMLHttpRequest (see Using FormData objects).