References

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

The HTML onbeforeinput event

Event All modern browsers Updated
Quick answer

The HTML onbeforeinput attribute runs JavaScript when the value of an editable element is about to change. It is an inline handler for the beforeinput event; in modern code prefer addEventListener('beforeinput', …).

Overview

The onbeforeinput event attribute runs JavaScript just before editable content changes. In JavaScript the event itself is named beforeinput — 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 onbeforeinput HTML attribute, but the modern, recommended approach is element.addEventListener('beforeinput', 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 onbeforeinput="handler()">…</element>

element.addEventListener('beforeinput', handler);

Example

Live example 
<input onbeforeinput="if(/[0-9]/.test(event.data)) event.preventDefault()" placeholder="Letters only" style="padding:8px;">

Best practices

  • Prefer element.addEventListener('beforeinput', handler) over the inline onbeforeinput 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 onbeforeinput event?
It runs JavaScript just before editable content changes. In JavaScript the event is named beforeinput.
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 onbeforeinput attribute or addEventListener?
Prefer addEventListener('beforeinput', …) in JavaScript. The inline onbeforeinput attribute works but mixes behavior into the markup and allows only one handler per element.
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