I want my EJS to render again with new data after a date has been selected. Is onSelect running immediately? It keeps returning 'datum is undefined'

$("#txtDate").datepicker({ 
  dateFormat: 'yy-mm-dd',
  minDate: 0,
  onSelect: () => {
    let datum = $("#txtDate").datepicker('getDate').toLocaleDateString()
    let html = ejs.render('<%=datum%>', { datum: datum })
    $('#output').html(html)
  }
});

This is the HTML/EJS

<input type="text" autocomplete="off" name="datum" id="txtDate">
<p id="`output`"></p>