Move custom code out of theme's functions.php file

I have written code to use on a single page, but I’ve placed it in the theme’s functions.php and created a javascript file and placed it in the theme’s JS directory.

Recently, the theme was updated and functions.php was overwritten and JS file disappeared.

Where do I need to put this code so when the theme is updated, my code will not be lost?

functions.php:

add_action('wp_ajax_zip_search', 'zip_search');
add_action('wp_ajax_nopriv_zip_search', 'zip_search' );
function zip_search()
{
    global $wpdb;

    $zip = $_REQUEST["zip_code"];   


/**/

    $query = 'SELECT zone FROM Counties WHERE zip = %s';
    $zone = $wpdb->get_var( $wpdb->prepare($query, $zip) );

    $output = "<h1>".$zone."</h1>";
    $t_zone = $zone;
    $trimmed_zone = trim($t_zone);

    $query = 'SELECT * FROM Managers WHERE zone = %s;';
    $manager_info = $wpdb->get_row(  $wpdb->prepare($query, $trimmed_zone) );

    $output .= "<table style="width:100%">" . 
                    "<tr>".
                        "<th>Zone Manager</th>".
                        "<th>Phone</th>".
                        "<th>Email Address</th>".
                    "</tr>" .
                    "<tr>" . 
                        "<td>" . $manager_info->first_name . " " . $manager_info->last_name  . "</td>" . 
                        "<td>" . $manager_info->phone_number . "</td>" . 
                        "<td>" . $manager_info->email_address .  "</td>" . 
                    "</tr>" .
                "</table>";

    $query = 'SELECT Region_Number FROM Zones WHERE Zone = %s';
    $region = $wpdb->get_var( $wpdb->prepare($query, $zone) );

    $query = 'SELECT * FROM Agblist WHERE Region_Number = %s';
    $results = $wpdb->get_results( $wpdb->prepare($query, $region) );
    
    $output .= "<table style="width:100%">";
    
    $output .= "<tr>".
                "<th>Company Name</th>".
                "<th>Contact Info</th>".
                "<th>Channel</th>".
                "<th>Territory</th>".
               "</tr>";

    foreach( $results as $result ) 
    {
        $output .= "<tr>".
                        "<td>".$result->Company_Name."</td>".
                        "<td>".$result->Corp_Address_Line_1."</br>".$result->Corp_Address_Line_2."</br>".$result->Corp_City.", ".$result->Corp_State." ".$result->Corp_Zip_Code."</br>".$result->Office_Phone_1."</td>".
                        "<td>".$result->Channel."</td>".
                        "<td>".$result->Agent_Territory."</td>".
                    "</tr>";
    }
    //$output .= "<li>Result: ".$results."</li>";
    $output .= "</table>";
/**/
    //$output = "<p>here</p>";
    $response = array(
        'data' => $output,
    );

    wp_send_json_success($response);
}

add_action( 'wp_enqueue_scripts', 'my_load_scripts' );
function my_load_scripts() {

    // Enqueue javascript on the frontend.
    wp_enqueue_script(
        'zip_js',
        get_template_directory_uri() . '/js/zip_search.js',
        array('jquery')
    );

    // The wp_localize_script allows us to output the ajax_url path for our script to use.
    wp_localize_script(
        'zip_js',
        'myAjax',
        array( 'ajaxurl' => admin_url( 'admin-ajax.php' ) )
    );

}

/js/zip_search.js:

jQuery(document).ready( function() {
   console.log("Document loaded!");
   jQuery("#AgentSearchButton").click( function(e) {
      console.log("Search button clicked");
      e.preventDefault(); 
      var zipCode = document.getElementById("AgentInputField").value;
      console.log("Zip code entered: " + zipCode);
      jQuery.ajax({
         type : "post",
         dataType : "json",
         url : myAjax.ajaxurl,
         data : {
             'action': "zip_search",
             'zip_code' : zipCode
         },
         success: function(response) {
            console.log(response.data);
            if(response.success) {
                console.log("response.type == success");
                jQuery("#results").html(response.data.data);
            }
            else {
                console.log("response.type == else");
                console.log(response.data);
            }
         },
        error: function(errorThrown){
            console.log("In error, error thrown!");
            console.log(errorThrown);
        }
      })
   })
});