Form Validation

form-validation.html

<form id="signUpForm">
  <input type="email" id="emailField" required>
  <button id="okButton" disabled>OK</button>
</form>

<script>
const signUpForm = document.getElementById('signUpForm');
const emailField = document.getElementById('emailField');
const okButton = document.getElementById('okButton');
  
emailField.addEventListener('keyup', function (event) {
  isValidEmail = emailField.checkValidity();
  
  if ( isValidEmail ) {
    okButton.disabled = false;
  } else {
    okButton.disabled = true;
  }
});
  
okButton.addEventListener('click', function (event) {
  signUpForm.submit();
});
</script>

On line 12, you are assigning value to an undeclared variable. Aren't you missing const or var keyword at the beginning of the line?