The oninvalid property of the GlobalEventHandlers mixin is an event handler that processes invalid events.
The invalid event fires when a submittable element has been checked and doesn't satisfy its constraints. The validity of submittable elements is checked before submitting their owner form, or after the checkValidity() method of the element or its owner form is called.
target.oninvalid = functionRef;
var functionRef = target.oninvalid;
functionRef is a function name or a function expression. The function receives a Event object as its sole argument.
This example demonstrates oninvalid and onsubmit event handlers on a form.
<form id="form">
<p id="error" hidden>Please fill out all fields.</p>
<label for="city">City</label>
<input type="text" id="city" required>
<button type="submit">Submit</button>
</form>
<p id="thanks" hidden>Your data has been received. Thanks!</p>
const form = document.getElementById('form');
const error = document.getElementById('error');
const city = document.getElementById('city');
const thanks = document.getElementById('thanks');
city.oninvalid = invalid;
form.onsubmit = submit;
function invalid(event) {
error.removeAttribute('hidden');
}
function submit(event) {
form.setAttribute('hidden', '');
thanks.removeAttribute('hidden');
event.preventDefault();
}