References

Beginner-friendly references for web development, with live, editable examples.

The HTML onformdata event

Event All modern browsers Updated
Quick answer

The HTML onformdata attribute runs JavaScript when a FormData object representing the form is being constructed (on submit or via new FormData()). It is an inline handler for the formdata event; in modern code prefer addEventListener('formdata', …).

Overview

The onformdata event attribute runs JavaScript when form data is being gathered. In JavaScript the event itself is named formdata — drop the on prefix when you call addEventListener.

It is a form-related event, fired by form controls such as <input>, <select> and <textarea> (or the <form> itself) as the user interacts with them and as data is submitted or validated.

You can wire this up with the inline onformdata HTML attribute, but the modern, recommended approach is element.addEventListener('formdata', handler) in JavaScript. That keeps behavior out of your markup, lets you attach several handlers to the same event, and makes them easy to remove. The inline attribute is fine for quick demos.

Syntax

<element onformdata="handler()">…</element>

element.addEventListener('formdata', handler);

Best practices

  • Prefer element.addEventListener('formdata', handler) over the inline onformdata attribute — it separates behavior from markup and allows multiple handlers.
  • Use native form validation (required, type, pattern) alongside JavaScript, not instead of it.
  • Re-validate on the server too — client-side events can be bypassed.
  • Give every control a <label> so the interaction is accessible.

Frequently asked questions

What is the onformdata event?
It runs JavaScript when form data is being gathered. In JavaScript the event is named formdata.
What is the difference between oninput and onchange?
oninput fires on every keystroke as the value changes; onchange fires once the value is committed (often on blur).
Should I rely on form events for validation?
Use them for instant feedback, but always validate again on the server, since client-side checks can be bypassed.
Should I use the onformdata attribute or addEventListener?
Prefer addEventListener('formdata', …) in JavaScript. The inline onformdata attribute works but mixes behavior into the markup and allows only one handler per element.
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