store the json object to js variable work inside the call but not outside

Question

I want to get the json object via a ajax call. On using alert inside the call the alert show the expected result but when using the alert outside the block give 'undefined'. i want to use the same json object outside the block. if i using var result , its also give the same result which i get from event_array. I am not getting what is the problem

var event_array;
var results;
get_app_cal();
function get_app_cal() {
  jQuery.ajax({
    url:'<?php echo site_url();?>/xyzController/xyz_cal',
    type:'POST',
    dataType: "JSON",
    success:function(results) {
      //alert(results);
      //event_array = JSON.stringify(results)
      event_array = results;
      console.log(event_array);
      alert(event_array) // its show the required value (work perfectly)
    }
  });
}
alert(event_array) //  return undefined (Not working not showing required value)