computed-effects-3.js

<price-details :order="order"
               @total-change="totalChange">
</price-details>

export default {
  // other properties which are not relevant in this example
  methods: {
    totalChange(grandTotal) {
      if (this.isSpecialCustomer) {
        this.order = {
          ...this.order,
          discount: this.order.discount + 0.1
        };
      }
    }
  }
};