Odoo 15 javascript custom button at POS show TypeError is not a function

Question

I want to add custom button at POS product screen which confirm order and render the related QWEB and download it as pdf file into certain folder. However when i try to click the button an error message show "TypeError: order.has_changes_to_print is not a function onClick@https://167.172.87.114/web/assets/504-1c680be/point_of_sale.assets.min.js:5345:63". enter image description here Can anyone help me what does the error mean ? And how to solve it ? Thanks

Here is my code for the custom button.

odoo.define('pos_enhancement.ConfirmOrderButton', function(require) {
    'use strict';

    const PosComponent = require('point_of_sale.PosComponent');
    const ProductScreen = require('point_of_sale.ProductScreen');
    const { useListener } = require('web.custom_hooks');
    const Registries = require('point_of_sale.Registries');

    class ConfirmOrderButton extends PosComponent {
        constructor() {
            super(...arguments);
            useListener('click', this.onClick);
            this._currentOrder = this.env.pos.get_order();
            this._currentOrder.orderlines.on('change', this.render, this);
            this.env.pos.on('change:selectedOrder', this._updateCurrentOrder, this);
        }
        willUnmount() {
            this._currentOrder.orderlines.off('change', null, this);
            this.env.pos.off('change:selectedOrder', null, this);
        }
        async onClick() {
            const order = this.env.pos.get_order();
            if (order.has_changes_to_print()) {
                const isPrintSuccessful = await order.print_changes();
                if (isPrintSuccessful) {
                    order.saveChanges();
                } else {
                    await this.showPopup('ErrorPopup', {
                        title: this.env._t('Printing failed'),
                        body: this.env._t('Failed in printing the changes in the order'),
                    });
                }
            }
        }
        get addedClasses() {
            if (!this._currentOrder) return {};
            const changes = this._currentOrder.has_changes_to_print();
            return {
                highlight: changes,
            };
        }
        _updateCurrentOrder(pos, newSelectedOrder) {
            this._currentOrder.orderlines.off('change', null, this);
            if (newSelectedOrder) {
                this._currentOrder = newSelectedOrder;
                this._currentOrder.orderlines.on('change', this.render, this);
            }
        }
    }
    ConfirmOrderButton.template = 'ConfirmOrderButton';

    ProductScreen.addControlButton({
        component: ConfirmOrderButton,
        condition: function() {
            return this.env.pos.config.iface_confirmorder;
        },
    });

    Registries.Component.add(ConfirmOrderButton);

    return ConfirmOrderButton;
});

Here is my pos.js file where i define the function

odoo.define('pos_enhancement.pos', function(require) {
    "use strict";

    var models = require('point_of_sale.models');

    var QWeb = core.qweb;

    // var _super_order = models.Order.prototype;
    models.PosModel = models.PosModel.extend({
        get_table_url: async function () {
            var table_url = this.rpc({
                    model: 'restaurant.table',
                    method: 'get_table_url',
                    args: [this.table.id],
                });
            return table_url;
        }
    });

    var _super_order = models.Order.prototype;
    models.Order = models.Order.extend({
        export_for_printing: function() {
            var result = _super_order.export_for_printing.apply(this,arguments);
            const codeWriter = new window.ZXing.BrowserQRCodeSvgWriter();
            var url = this.qr_url || 'undefined';
            let table_qr_code_svg = new XMLSerializer().serializeToString(codeWriter.write(url, 150, 150));
            result.table_qr_code = "data:image/svg+xml;base64," + window.btoa(table_qr_code_svg);
            return result;
        },
        print_changes: async function() {
            let isPrintSuccessful = true;
            var changes = this.computeChanges(this.iface_available_categ_ids);
            if ( changes['new'].length > 0 || changes['cancelled'].length > 0){
                var receipt = QWeb.render('OrderChangeReceipt',{changes:changes, widget:this});
                // const result = await printers[i].print_receipt(receipt);
                // if (!result.successful) {
                //     isPrintSuccessful = false;
                // }
            }
            return isPrintSuccessful;
        },
        has_changes_to_print: function() {
            var changes = this.computeChanges(this.iface_available_categ_ids);
            if ( changes['new'].length > 0 || changes['cancelled'].length > 0){
                return true;
            }
            return false;
        },
    });
});

I tried the same method for submit button which is provided in odoo already, doing exactly the same way but with different function code. However the function that i added did not work.