Trying to GET data with ajax from database and show in fullcalendar

There were a couple security issues and error checking that I did see. Try these:

<?php 
add_action('wp_ajax_get_timeslot_data', 'get_timeslot_data');
add_action('wp_ajax_nopriv_get_timeslot_data', 'get_timeslot_data');

function get_timeslot_data() {
    if (!isset($_POST['activityId']) || !ctype_digit($_POST['activityId'])) {
        wp_send_json_error("Invalid activity ID");
    }

    global $wpdb;
    $activity_id = intval($_POST['activityId']);
    $table_name = $wpdb->prefix . 'booking_seasons'; // Support custom table prefixes

    $result = $wpdb->get_row($wpdb->prepare("SELECT timeslot_dates FROM $table_name WHERE id = %d", $activity_id));

    if (!$result || empty($result->timeslot_dates)) {
        wp_send_json_error("No time slots found");
    }

    $time_slots = maybe_unserialize($result->timeslot_dates);

    if (!is_array($time_slots)) {
        wp_send_json_error("Invalid time slot data format");
    }

    $events = [];
    foreach ($time_slots as $slot => $dates) {
        $times = explode(" - ", $slot);
        if (count($times) !== 2) {
            continue; // Skip invalid time slot format
        }

        foreach ($dates as $date) {
            $events[] = [
                'title' => sanitize_text_field($slot),
                'start' => esc_html($date . "T" . trim($times[0])),
                'end' => esc_html($date . "T" . trim($times[1]))
            ];
        }
    }

    wp_send_json_success($events);
}
?>

• Supports dynamic table prefix ($wpdb->prefix)

• Ensures activity ID is numeric (ctype_digit())

• Uses maybe_unserialize() for safer data handling

• Handles invalid time slot formats Uses sanitize_text_field()

• esc_html() to prevent security issues

   jQuery(document).ready(function() {
           if (typeof activityData !== 'undefined' && activityData.activityId) {
       var activityId = activityData.activityId;
       } else {
           console.error("activityData is undefined or missing activityId.");
       return;
       }
  
   console.log(activityId);
   console.log("jQuery is working");
  
   var calendarEl = jQuery('#calendar-' + activityId);
  
   if (!calendarEl.length) {
       console.error("Calendar element not found for activity ID:", activityId);
       return;
   }
  
   var calendar = new FullCalendar.Calendar(calendarEl[0], {
       initialView: 'timeGridWeek',
       slotDuration: '00:15:00',
       selectable: true,
       allDaySlot: false,
       events: function(fetchInfo, successCallback, failureCallback) {
           console.log('Activity ID inside events:', activityId);
  
           if (typeof my_ajax_object === 'undefined' || !my_ajax_object.ajaxurl) {
               console.error("my_ajax_object.ajaxurl is undefined.");
               failureCallback("AJAX URL missing");
               return;
           }
  
           jQuery.ajax({
               url: my_ajax_object.ajaxurl,
               type: 'POST',
               data: {
                   action: 'get_timeslot_data',
                   activityId: activityId,
               },
               success: function(response) {
                   if (response.success) {
                       successCallback(response.data);
                   } else {
                       console.error("Error fetching events:", response.data);
                       failureCallback("Error fetching events");
                   }
               },
               error: function(xhr, status, error) {
                   console.error("AJAX Error:", error);
                   failureCallback("Failed to fetch events: " + error);
               }
           });
       },
       headerToolbar: {
           left: 'prev,next today',
           center: 'title',
           right: 'timeGridDay,timeGridWeek,dayGridMonth'
       },
   });
  
   calendar.render();
   });
  

• Prevents errors if activityData or calendarEl is missing

• Ensures my_ajax_object.ajaxurl exists before making the AJAX request

• Improved error handling for failureCallback

Without knowing your error, I’m not checking with actual usage, those are what I can see.