How to fix an error when connecting captcha from google?

Question

JavaScript

Connected captcha to the site, but the visitor can submit data from the form even when the verification is not passed. How to fix?

let form = document.querySelector('#form');

form.addEventListener('submit', function(evt) {
  evt.preventDefault();

    let formData = {
      name: document.querySelector('input[name="name"]').value,
      phone: document.querySelector('input[name="phone"]').value,
      email: document.querySelector('input[name="email"]').value,
      textarea: document.querySelector('textarea[name="textarea"]').value,
    };

    var request = new XMLHttpRequest();

    request.addEventListener('load', function() {
      alert('Ваша заявка успешно отправлена!');
      form.innerHTML = '<h2 class="form__height">Спасибо за заявку!</h2>';
    });

    request.open('POST', './mail.php', true);
    request.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded; charset=UTF-8');
    request.send('name=' + encodeURIComponent(formData.name) + '&phone=' + encodeURIComponent(formData.phone) + '&email=' + encodeURIComponent(formData.email) + '&textarea=' + encodeURIComponent(formData.textarea) );
  });

 let onloadCallback = function() {
    alert("grecaptcha is ready!");
  };