AJAX does not send POST

First of all, I have already read through many other posts and tried different things, but no solution approach has worked so far.

My problem: ajax does not send the POST data.

fillTable.js

function fillTableWithData(htmlID, tableSQL, colsSQL, orderSQL){
  var jsondata = new FormData();
  jsondata.append("action", htmlID);
  jsondata.append("table", tableSQL);
  jsondata.append("cols", colsSQL);
  jsondata.append("order", orderSQL);
  $.ajax({
      url:"table.php",
      method: "POST",
      dataType: 'json',
      contentType:false,
      processData:false,
      data: jsondata,
      success:function(result){
        //   $(htmlID).html(result);
        alert("test");
      }
  });
}

table.php

<?php
  require('db.php');
  echo (count($_POST)); // result is 0
  if(isset($_POST['action'])){ // not set
    if($_POST['action'] == "getDataAsRows"){
      getDataAsRows($_POST['table'], $_POST['cols'], $_POST['order']);
    }
  }  

  function getDataAsRows($table, $cols, $order) {
     ...
  }
?>

Both files are inside the same folder:

/scripts/fillTable.js

/scripts/table.php

The url should not be a problem, since the php code prints out the length of $_POST.