Customizing Netlify form submission

/// define the form, don't let it send user to another page
const baseForm = document.getElementById('base-form')
if (baseForm) {
    baseForm.addEventListener('submit', e => {
        e.preventDefault();
        handleSubmit(baseForm);
    })
}

const handleSubmit = form => {
    const data = new FormData(form)
    data.append('form-name', 'baseform');
    fetch("/", {
      method: "POST",
      body: data,
    })
      .then(() => {
          // these are for the success message, customizing below 
          var messageDiv = document.getElementById('form-success');
          displayMessage(messageDiv);
      })
      .catch((error) => {
          var messageDiv = document.getElementById('form-warning');
          displayMessage(messageDiv);
          console.log(error);
      }
      );

      function displayMessage(messageDiv){
          messageDiv.classList.remove('hidden');
          // role=alert so the message is received by screenreaders
          messageDiv.setAttribute('role', 'alert');
  
          setTimeout(function() {
              // opacity is unnecessary, but for a smoother transition add/remove it before applying the hidden class
              messageDiv.style.opacity = 1;
              }, 1000)
  
          setTimeout(function() {
              messageDiv.style.opacity = 0;
  
              setTimeout(function() {
                  messageDiv.classList.add('hidden');
                  messageDiv.removeAttribute('role');
              }, 1000)
          }, 6000);
      };
  };