HTMLFormElement: formdata event

Baseline

Widely available

This feature is well established and works across many devices and browser versions. It’s been available across browsers since September 2021.

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The formdata event fires 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 event is not cancelable and does not bubble.

Syntax

Use the event name in addEventListener():

addEventListener("formdata", (event) => { })

Alternatively, assign to the event handler property of the interface:

onformdata = (event) => { }

Event type

A FormDataEvent. Inherits from Event.

Examples

// grab reference to form

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

// submit handler

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

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

  // construct a FormData object, which fires the formdata event
  const formData = new FormData(formElem);
  // formdata gets modified by the formdata event
  console.log(formData.get("field1")); // foo
  console.log(formData.get("field2")); // bar
});

// formdata handler to retrieve data

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

  // modifies the form data
  const formData = e.formData;
  // formdata gets modified by the formdata event
  formData.set("field1", formData.get("field1").toLowerCase());
  formData.set("field2", formData.get("field2").toLowerCase());
});

The onformdata version would look like this (but note that this replaces other event listeners added using onformdata, if any):

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

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