Override a javascript function in Odoo 11? - odoo

Question

I want to inherit function add_product of models.Order.
Here is the original function
/point_of_sale/static/src/js/models.js
add_product: function(product, options){
if(this._printed){
this.destroy();
return this.pos.get_order().add_product(product, options);
}
this.assert_editable();
options = options || {};
var attr = JSON.parse(JSON.stringify(product));
attr.pos = this.pos;
attr.order = this;
var line = new exports.Orderline({}, {pos: this.pos, order: this, product: product});
if(options.quantity !== undefined){
line.set_quantity(options.quantity);
}
if(options.price !== undefined){
line.set_unit_price(options.price);
}
//To substract from the unit price the included taxes mapped by the fiscal position
this.fix_tax_included_price(line);
if(options.discount !== undefined){
line.set_discount(options.discount);
}
if(options.extras !== undefined){
for (var prop in options.extras) {
line[prop] = options.extras[prop];
}
}
var to_merge_orderline;
for (var i = 0; i < this.orderlines.length; i++) {
if(this.orderlines.at(i).can_be_merged_with(line) && options.merge !== false){
to_merge_orderline = this.orderlines.at(i);
}
}
if (to_merge_orderline){
to_merge_orderline.merge(line);
} else {
this.orderlines.add(line);
}
this.select_orderline(this.get_last_orderline());
if(line.has_product_lot){
this.display_lot_popup();
}
},
I need to add one more condidtion like,
if(options.is_promo !== undefined){
line.set_promo(options.is_promo);
}
},
So in my custom module, I tried this,
var _super_order = models.Order.prototype;
models.Order = models.Order.extend({
add_product: function(product, options){
if(options.is_promo !== undefined){
line.set_promo(options.is_promo);
}
_super_order.add_product.apply(this,arguments);
},
But it throws an error, line is not defined.
How can i do it?