this is simple sending form data.

first variant for sending form

// This is function which send data
// In this variant, we need invoke preventDefault() function
// in first line code our function.
const sendData = (event) => {
  event.preventDefault(); // this line need for cancel default behavior and send via ajax
  // take our form
  const form = event.target;
  // create object for parsing to json
  const data = {};
  // add data, from form
  // use names out form fields for access inputs
  data.name = form.name.value;
  data.phone = form.phone.value;
  data.email = form.email.value;
  // and send data
fetch(form.action, {
  method: “POST”,
  headers: {
  "Content-Type": "application/json"
  },
  body: JSON.stringify(data)
  })
// check result
  .then(response => response.json())
  .then(response => console.log('Success:', JSON.stringify(response)))
  .catch(error => console.error('Error:', error))
};

first_form

<form onsubmit="return sendData(event);" action="https://my.example.com/save" method="post">
  <input type="text" name="name" >
  <input type="number" name="phone">
  <input type="email" name="email">
  <! — For supports all browsers we should use input element →
  <input type="submit" value="Send">
</form>

second_form

<form onsubmit=”return sendData(this);” action=”https://my.example.com/save” method=”post”>
  <input type=”text” name=”name” >
  <input type=”number” name=”phone”>
  <input type=”email” name=”email”>
  <! — For supports all browsers we should use input element →
  <input type=”submit” value=”Send”>
</form>

second_variant

// in first line code our function.
const sendData = (this) => {
// take our form
// create object for parsing to json
const data = {};
// add data, from form
// use names fields for access inputs
  data.name = this.name.value;
  data.phone = this.phone.value;
  data.email = this.email.value;
//and send data
fetch(form.action, {
  method: “POST”,
  headers: {
  "Content-Type": "application/json"
  },
  body: JSON.stringify(data)
});
// check result
  .then(response => response.json())
  .then(response => console.log('Success:', JSON.stringify(response)))
  .catch(error => console.error('Error:', error))
  return false; // it's line required for send data.
};