FormDataEvent.formData

// grab reference to form

const formElem = document.querySelector('form');

// submit handler

formElem.addEventListener('submit', (e) => {
  // on form submission, prevent default
  e.preventDefault();

  // construct a FormData object, which fires the formdata event
  new FormData(formElem);
});

// formdata handler to retrieve data

formElem.addEventListener('formdata', (e) => {
  console.log('formdata fired');

  // Get the form data from the event object
  let data = e.formData;
  for (var value of data.values()) {
    console.log(value);
  }

  // submit the data via XHR
  var request = new XMLHttpRequest();
  request.open("POST", "/formHandler");
  request.send(data);
});