Send POST form data, serialized by FormData with fetch

fetch-send-post-data.js

const the_form = document.getElementById('the_form');
const formData = new FormData(the_form);
fetch('the-handler.php',{
  method: 'POST',
  body: formData
})
.then(response => response.json())
.then(data => {
  console.log('data is:', data);
});