js前台的值怎么传给后台

<form action="/submit" method="POST">
  <input type="text" name="username" id="username">
  <button type="submit">Submit</button>
</form>

document.getElementById('submitBtn').addEventListener('click', function() {
  const username = document.getElementById('username').value;

  fetch('/submit', {
    method: 'POST',
  })
  .then(response => response.json())
  .then(data => console.log('Success:', data))
  .catch((error) => console.error('Error:', error));
});

$('#submitBtn').click(function() {
  var username = $('#username').val();

  $.ajax({
    url: '/submit',
    type: 'POST',
    data: {username: username},
    success: function(response) {
      console.log('Success:', response);
    },
    error: function(xhr, status, error) {
      console.error('Error:', error);
    }
  });
});

const express = require('express');
const app = express();

app.use((req, res, next) => {
  res.header('Access-Control-Allow-Origin', '*');
  res.header('Access-Control-Allow-Headers', 'Origin, X-Requested-With, Content-Type, Accept');
  next();
});

app.post('/submit', (req, res) => {
  console.log(req.body);
  res.json({ message: 'Data received' });
});

app.listen(3000, () => console.log('Server running on port 3000'));