promise POST request empty

Hi,
I'm having problems getting the body content of my promise post using CI4.
No matter how I configure the await fetch() call, file_get_contenys('php://input) is empty.
Any help with this would be greatly appreciated?


My route

PHP Code:

$routes->post('s_pay_form', 'Signup\Payments'); 


My Javascript

Code:

<script>
// The items the customer wants to buy
const items = [{ id: "xl-tshirt" }];
initialize();

// Fetches a payment intent and captures the client secret
async function initialize() {
    const { clientSecret } = await fetch("s_pay_form", {
    method: "POST",
    //headers: { "Content-Type": "application/json" },
    body: JSON.stringify({ items }),
  }).then(
      data => { alert(JSON.stringify(data)) },
  error => { alert(error) }
);
</script>

Controller

PHP Code:

<?php namespace App\Controllers\Signup;

use CodeIgniter\Controller;
use CodeIgniter\Database\Query;
use App\Controllers\BaseController;

class Payments extends BaseController
{

    public function index()
    {
        function calculateOrderAmount(array $items): int {

            return 1400;
        }

        header('Content-Type: application/json');

        try {


        // retrieve JSON from POST body
        $jsonStr = file_get_contents('php://input');

        return json_encode($jsonStr);

        } catch (Error $e) {
            http_response_code(500);
            return json_encode(['error' => $e->getMessage()]);
        }
    }
}    


Result = ""

PHP Code:

// it return string
$jsonStr = file_get_contents('php://input'); 

// it does not make sense
return json_encode($jsonStr);