FormDataEvent

// 

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

// submit 

formElem.addEventListener("submit", (e) => {
  // 
  e.preventDefault();

  console.log(form.querySelector('input[name="field1"]')); // FOO
  console.log(form.querySelector('input[name="field2"]')); // BAR

  // FormData  formdata 
  const formData = new FormData(formElem);
  //  formdata 
  console.log(formData.get("field1")); // foo
  console.log(formData.get("field2")); // bar
});

//  formdata 

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

  // 
  const formData = e.formData;
  formData.set("field1", formData.get("field1").toLowerCase());
  formData.set("field2", formData.get("field2").toLowerCase());
});