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) {
                    console.log(cart.qty_to_order)
                }
            })
        }