GlobalEventHandlers.onsubmit

target.onsubmit = functionRef;

<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');

  // For this example, don't actually submit the form
  event.preventDefault();
}