true if the form should not be submitted, e.g. because the submit
button requires validation and the form is invalid. false if the
form will be submitted unless prevented by this or another callback.
The button that was pressed to submit the form. undefined when the form was submitted without
pressing a button, e.g. by pressing enter in a text field.
The name of the submitButton that was pressed to submit the form, ie. its name attribute.
undefined when submitButton is undefined.
The name of the button list ("Button-Leiste") that contains the submitButton that was pressed
to submit the form. undefined when submitButton is undefined.
The form that is about to be submitted.