To modify javascript in Odoo-9 follow the below steps
Go to point of sale module ->static file->src->js->screens.js
For example you can use the below code in screens.js file.

function openerp_pos_screens(instance, module){ //module is instance.point_of_sale
    var QWeb = instance.web.qweb;

    module.ScreenSelector = instance.web.Class.extend({
        init: function(options){
            this.pos = options.pos;

            this.screen_set = options.screen_set || {};

            this.popup_set = options.popup_set || {};

            this.default_client_screen = options.default_client_screen;
            this.default_cashier_screen = options.default_cashier_screen;

            this.current_popup = null;

            this.current_mode = options.default_mode || 'client';

            this.current_screen = null; 

            for(screen_name in this.screen_set){
                this.screen_set[screen_name].hide();
            }

            for(popup_name in this.popup_set){
                this.popup_set[popup_name].hide();
            }

            this.selected_order = this.pos.get('selectedOrder');
            this.selected_order.set_screen_data({
                client_screen: this.default_client_screen,
                cashier_screen: this.default_cashier_screen,
            });

            this.pos.bind('change:selectedOrder', this.load_saved_screen, this);
        },
        add_screen: function(screen_name, screen){
            screen.hide();
            this.screen_set[screen_name] = screen;
            return this;
        },
        show_popup: function(name){
            if(this.current_popup){
                this.close_popup();
            }
            this.current_popup = this.popup_set[name];
            this.current_popup.show();
        },
        close_popup: function(){
            if(this.current_popup){
                this.current_popup.close();
                this.current_popup.hide();
                this.current_popup = null;
            }
        },
        load_saved_screen:  function(){
            this.close_popup();

            var selectedOrder = this.pos.get('selectedOrder');

            if(this.current_mode === 'client'){
                this.set_current_screen(selectedOrder.get_screen_data('client_screen') || this.default_client_screen,null,'refresh');
            }else if(this.current_mode === 'cashier'){
                this.set_current_screen(selectedOrder.get_screen_data('cashier_screen') || this.default_cashier_screen,null,'refresh');
            }
            this.selected_order = selectedOrder;
        },
        set_user_mode: function(user_mode){
            if(user_mode !== this.current_mode){
                this.close_popup();
                this.current_mode = user_mode;
                this.load_saved_screen();
            }
        },
        get_user_mode: function(){
            return this.current_mode;
        },
        set_current_screen: function(screen_name,params,refresh){
            var screen = this.screen_set[screen_name];
            if(!screen){
                console.error("ERROR: set_current_screen("+screen_name+") : screen not found");
            }

            this.close_popup();
            var selectedOrder = this.pos.get('selectedOrder');
            if(this.current_mode === 'client'){
                selectedOrder.set_screen_data('client_screen',screen_name);
                if(params){ 
                    selectedOrder.set_screen_data('client_screen_params',params); 
                }
            }else{
                selectedOrder.set_screen_data('cashier_screen',screen_name);
                if(params){
                    selectedOrder.set_screen_data('cashier_screen_params',params);
                }
            }

            if(screen && (refresh || screen !== this.current_screen)){
                if(this.current_screen){
                    this.current_screen.close();
                    this.current_screen.hide();
                }
                this.current_screen = screen;
                this.current_screen.show();
            }
        },
        get_current_screen_param: function(param){
            var selected_order = this.pos.get('selectedOrder');
            if(this.current_mode === 'client'){
                var params = selected_order.get_screen_data('client_screen_params');
            }else{
                var params = selected_order.get_screen_data('cashier_screen_params');
            }
            if(params){
                return params[param];
            }else{
                return undefined;
            }
        },
        set_default_screen: function(){
            this.set_current_screen(this.current_mode === 'client' ? this.default_client_screen : this.default_cashier_screen);
        },
    });

    module.ScreenWidget = module.PosBaseWidget.extend({

        show_numpad:     true,  
        show_leftpane:   true,

        init: function(parent,options){
            this._super(parent,options);
            this.hidden = false;
        },

        help_button_action: function(){
            this.pos_widget.screen_selector.show_popup('help');
        },

        barcode_product_screen:         'products',     //if defined, this screen will be loaded when a product is scanned
        barcode_product_error_popup:    'error-product',    //if defined, this popup will be loaded when there's an error in the popup

        // what happens when a product is scanned : 
        // it will add the product to the order and go to barcode_product_screen. Or show barcode_product_error_popup if 
        // there's an error.
        barcode_product_action: function(ean){
            var self = this;
            if(self.pos.scan_product(ean)){
                self.pos.proxy.scan_item_success(ean);
                if(self.barcode_product_screen){ 
                    self.pos_widget.screen_selector.set_current_screen(self.barcode_product_screen);
                }
            }else{
                self.pos.proxy.scan_item_error_unrecognized(ean);
                if(self.barcode_product_error_popup && self.pos_widget.screen_selector.get_user_mode() !== 'cashier'){
                    self.pos_widget.screen_selector.show_popup(self.barcode_product_error_popup);
                }
            }
        },

        // what happens when a cashier id barcode is scanned.
        // the default behavior is the following : 
        // - if there's a user with a matching ean, put it as the active 'cashier', go to cashier mode, and return true
        // - else : do nothing and return false. You probably want to extend this to show and appropriate error popup... 
        barcode_cashier_action: function(ean){
            var users = this.pos.get('user_list');
            for(var i = 0, len = users.length; i < len; i++){
                if(users[i].ean13 === ean.ean){
                    this.pos.set('cashier',users[i]);
                    this.pos_widget.username.refresh();
                    this.pos.proxy.cashier_mode_activated();
                    this.pos_widget.screen_selector.set_user_mode('cashier');
                    return true;
                }
            }
            this.pos.proxy.scan_item_error_unrecognized(ean);
            return false;
        },

        // what happens when a client id barcode is scanned.
        // the default behavior is the following : 
        // - if there's a user with a matching ean, put it as the active 'client' and return true
        // - else : return false. 
        barcode_client_action: function(ean){
            var partners = this.pos.get('partner_list');
            for(var i = 0, len = partners.length; i < len; i++){
                if(partners[i].ean13 === ean.ean){
                    this.pos.get('selectedOrder').set_client(partners[i]);
                    this.pos_widget.username.refresh();
                    this.pos.proxy.scan_item_success(ean);
                    return true;
                }
            }
            this.pos.proxy.scan_item_error_unrecognized(ean);
            return false;