ajax passing parameter to php file not working

UPDATE: the php and javascript was updated/changed, based on the intial user comments:

This javascript does not get the response... what to change? How to make the javascript get the success or failed response from the php?

       jQuery('.btndel').on('click', function(e){
          e.preventDefault();
          alert("click");
          jQuery.ajax({
            url: '/file.php',
            type:'POST',
            dataType: 'json',
            data: { p: 'test' },
            success: function( data ) {
                response = JSON.parse(data);
                if (response['success']){
                     alert("success");
                } else {
                     alert("fail");
                }
            },
        });
    });
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file.php

   <?php

    if ($_SERVER['REQUEST_METHOD'] !== 'POST') {
    http_response_code(404);
    error_log( '404 Not Found');
    exit;
   }

    header('Content-type: application/json');
    error_log("=====".$_POST['p']);
    if (!isset($_POST['p'])) {
       http_response_code(400);
       error_log( "NO PARAM");
       exit;
    }

   $response = array();
   if ($update_success === false) {
       $response['status'] = 'error';
   } else {
       $response['status'] = 'success';
   }

    exit(json_encode($response));
    ?>
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what is wrong or missing?