我无法使用ajax从php文件中检索数据，我想在弹出窗口中使用从ajax返回的数据

Question

$(document).ready(function() {
  $('[data-toggle="popover"]').popover({
    title: fetchData,
    html: true,
    placement: 'right'
  });

  // here is the function for the ajax
  function fetchData() {
    var fetch_data = '';
    var element = $(this);
    var id = element.attr("id");

    $.ajax({
        url: "fetch.php",
        type: "GET",
        //  async:false,  
        data: {
          id: id
        },
        success: function(data) {
          fetch_data = data;
        });
      return fetch_data;
    }
  });