JS Fetch API → send POST and display response

check.php

<?php

$json = file_get_contents('php://input');
$obj = json_decode($json, true);

var_dump($obj);

index.html

<div class="grid-container">
<div id="main_window"></div>
<div class="radio">
  <fieldset>
    <input type="radio" name="ID" style="display:none" checked>
    <label><input type="radio" name="ID" onchange='sessionStorage.setItem("ID", "1");ajax();'>1</label>
    <br>
    <label><input type="radio" name="ID" onchange='sessionStorage.setItem("ID", "2");ajax();'>2</label>
    <br>
</fieldset>
  </div>

index.js

fetch("/wp-content/plugins/test/check.php", {
method: "POST",
body: JSON.stringify({
  "ID": sessionStorage.getItem('ID'),
  "time": time,
  "data": date,
})
})
.then((res) => {
 return res.text();
})
.then((data) => {
document.getElementById("main_window").innerHTML = data
})

style.css

.grid-container {
        display: grid;
        grid-template-columns: 800px 400px
        }