Nozioni di base sulla Constraint Validation API

const input = document.getElementById('email');
const v = input.validity;

console.log(v.valid);         // true if all constraints pass
console.log(v.valueMissing);  // true if required but empty
console.log(v.typeMismatch);  // true if type doesn't match (e.g. bad email)
console.log(v.patternMismatch); // true if pattern fails
console.log(v.tooShort);      // true if shorter than minlength
console.log(v.tooLong);       // true if longer than maxlength
console.log(v.rangeUnderflow); // true if less than min
console.log(v.rangeOverflow);  // true if more than max

const form = document.getElementById('my-form');

// Check entire form:
if (!form.checkValidity()) {
  console.log('Form has errors');
}

// Check individual field:
const email = document.getElementById('email');
if (!email.checkValidity()) {
  console.log('Email is invalid');
}
// The 'invalid' event fires on invalid fields

const input = document.getElementById('username');

// Shows native browser error tooltip if invalid:
const valid = input.reportValidity();
console.log(valid);  // true or false

// On a form: validates all fields and shows first error:
document.getElementById('form').reportValidity();

const email = document.querySelector('input[type=email]');
email.value = 'not-an-email';

console.log(email.validationMessage);
// e.g. "Please include an '@' in the email address..."

console.log(email.validity.typeMismatch); // true

const username = document.getElementById('username');

// Check availability on blur:
username.addEventListener('blur', async () => {
  const res = await fetch(`/check-username?name=${username.value}`);
  const { taken } = await res.json();

  if (taken) {
    username.setCustomValidity('This username is already taken.');
  } else {
    username.setCustomValidity('');  // empty string = clear the error
  }
});

const input = document.getElementById('username');

input.addEventListener('input', () => {
  // Clear custom error when user starts typing again:
  input.setCustomValidity('');
});

input.addEventListener('blur', async () => {
  // Re-validate on blur:
  const taken = await checkUsername(input.value);
  input.setCustomValidity(taken ? 'Username taken' : '');
});

document.querySelectorAll('input').forEach(input => {
  input.addEventListener('invalid', (e) => {
    e.preventDefault();  // prevent default browser tooltip
    // Show your custom error UI instead:
    showError(input, input.validationMessage);
  });
});

document.querySelectorAll(':input').forEach(el => {
  if (el.willValidate) {
    console.log(el.name, 'will be validated');
  }
});
// Disabled, hidden, or output elements return willValidate = false

<form id="signup" novalidate>
  <input type="email" id="email" name="email" required>
  <span id="email-err" role="alert"></span>
  <button type="submit">Sign Up</button>
</form>

<script>
const form = document.getElementById('signup');
const email = document.getElementById('email');
const err = document.getElementById('email-err');

form.addEventListener('submit', (e) => {
  e.preventDefault();
  err.textContent = '';

  if (email.validity.valueMissing) {
    err.textContent = 'Email is required.';
    email.focus();
    return;
  }
  if (email.validity.typeMismatch) {
    err.textContent = 'Enter a valid email address.';
    email.focus();
    return;
  }
  // Submit...
});
</script>