data() { return { qty_to_order: null, } }, methods: { showQty(product_id) { this.carts.filter(function(cart) { if (cart.product_id === product_id) { this.qty_to_order = cart.qty_to_order } }) } } When I call the method, I get an error. How to write cart.qty_to_order to this.qty_to_order (in data) ? This method works showQty(product_id) { this.carts.filter(function(cart) { if (cart.product_id === product_id) […]