为什么不是'当使用Google Distance Matrix Service API时，不会调用我的回调函数

除了callback函数未被调用外，以下代码工作正常。callback功能中没有显示任何警报，但同时显示Start和End警报。我已经检查了控制台，谷歌会返回你期望的数据，下面的例子。

我担心的是；

• 谷歌回应（/**/_xdc_._6ct5vb && _xdc_._6ct5vb(）之前的奇怪代码是什么

• 当包含Google javascript文件时，URL参数callback=initMap的作用是什么？

谷歌回应：

/**/_xdc_._6ct5vb && _xdc_._6ct5vb( {
   "destination_addresses" : [ "Church Street, Liverpool L36 9TJ, UK" ],
   "origin_addresses" : [ "Ashmount Rd, Liverpool L17 0BZ, UK" ],
   "rows" : [
      {
         "elements" : [
            {
               "distance" : {
                  "text" : "5.1 mi",
                  "value" : 8175
               },
               "duration" : {
                  "text" : "16 mins",
                  "value" : 959
               },
               "status" : "OK"
            }
         ]
      }
   ],
   "status" : "OK"
}
 )

Javascript/jQuery:

<script async defer src="https://maps.googleapis.com/maps/api/js?key=<?php echo GOOGLE_MAPS_API_KEY; ?>&callback=initMap" type="text/javascript"></script><!-- Google Maps -->
  <script>
$( document ).ready(function() {
// validate form
$( "#add_employee_mileage" ).validate({
errorClass: "error-class"
});
$( "#calculateMileage" ).click(function() {
    
alert('Start');
  
var origin = $( "#employee_mileage_start_postcode" ).val();
var destination = $( "#employee_mileage_end_postcode" ).val();
var service = new google.maps.DistanceMatrixService();
service.getDistanceMatrix(
    {
        origins: [origin],
        destinations: [destination],
        travelMode: google.maps.TravelMode.DRIVING,
        unitSystem: google.maps.UnitSystem.IMPERIAL,
        avoidHighways: false,
        avoidTolls: false
    }, 
    callback
);
  
function callback(response, status) {
    
    alert('Callback');
    if(status=="OK") {
        alert('Ok');
        alert(response.rows[0].elements[0].distance.value);
        $( "#employee_mileage_mileage" ).val(response.rows[0].elements[0].distance.value);
    } else {
        alert("Error: " + status);
    }
    
}
alert('End');
  
});
});
</script>

HTML:

  <form id="add_employee_mileage" name="add_employee_mileage" action="https://www.system-uk.com/admin-area/ajax/add_employee_mileage.php" method="post"><fieldset>
<legend>Mileage Details</legend>
<table class="nobord"><tr><td><table class="nobord"><tr>
    <td class="bold">Employee</td>
    <td class="bold">Vehicle</td>
    <td class="bold">Start Postcode</td>
    <td class="bold">End Postcode</td>
    <td class="bold">Mileage</td>
    <td class="bold">&nbsp;</td>
  </tr><tr>
    <td><select id="employee_mileage_employee_id" name="employee_mileage_employee_id" required></select></td>
    <td><select id="employee_mileage_vehicle_id" name="employee_mileage_vehicle_id" required></select></td><td><input type="text" id="employee_mileage_start_postcode" name="employee_mileage_start_postcode" maxlength="8" style="width:85px;" value="L17" required></td><td><input type="text" id="employee_mileage_end_postcode" name="employee_mileage_end_postcode" maxlength="8" style="width:85px;" value="L3" required></td><td><input type="number" id="employee_mileage_mileage" name="employee_mileage_mileage" min="0" step="any" required></td><td><input type="button" id="calculateMileage" value="Calculate Mileage">&nbsp;<input type="submit" value="Save"></td></tr></table></td></tr></table>
</fieldset></form></div>