Complicated clear code

main.js

function myFunction(name, telephone, email) {
  const requiredFields = name && telephone;
  
  if (!requiredFields) {
    const modalContainer = document.getElementById('modal-container');
    const modal = document.createElement('div');
    const basicString = 'Encountered a problem submitting the form. Missing '
    if (!name && !telephone) {
      modal.textContent = basicString + 'name & telephone fields.';
    } else if (!name) {
      modal.textContent = basicString + 'name field.';
    } else {
      modal.textContent = basicString + 'telephone field.';
    }
    modalContainer.appendChild(modal);
    return;
  }
  
  window.fetch('https://myapi.com', {
    method: 'POST',
    body: JSON.stringify({
      name,
      telephone,
      email
    })
  });
}