# Error handling

A flexible way of error handling is essential for a framework like Vue Storefront. As composables are hearth of our app - we decided to put there whole error handling mechanism.

Each composable returns error - computed property. It is an object which has names of async functions from composable as keys and Error instance or null as a value.

  <button @click="addToCart(product)">Add to cart</button>
  <div v-if="error.addToCart">{{ error.addToCart.message }}</dIv>

export default {
  setup () {
    const { addToCart, error } = useCart()

    return {

There is a dedicated interface for each composable. Take a look at this one from useCart:

export interface UseCartErrors {
  addItem?: Error;
  removeItem?: Error;
  updateItemQty?: Error;
  load?: Error;
  clear?: Error;
  applyCoupon: Error;
  removeCoupon?: Error;
Where does the error come from?

Inside each async method we are clearing the current error before integration's method call and setting it in catch block.

const addItem = async ({ product, quantity, customQuery }) => {
  Logger.debug('useCart.addItem', { product, quantity });

  try {
    loading.value = true;
    error.value.addItem = null; // Clearing the current error
    const updatedCart = await factoryParams.addItem(
        currentCart: cart.value,
    cart.value = updatedCart;
  } catch (err) {
    error.value.addItem = err; // Setting a new error
    Logger.error('useCart/addItem', err);
  } finally {
    loading.value = false;
Where can I find interface of the error property from a certain composable?

When you are writing a code inside a script part of the Vue's component, your IDE should give you hints dedicated for each type of composable. That's why you probably do not need to check these interfaces in the core's code.

However, if somewhy you still want to do that, you could find them inside packages/core/core/src/types.ts (opens new window).

# How to listen for errors?

Let's imagine you have some global components for error notifications. You want to send information about each new error to this component. But how to know when new error appears? You can observe error object with a simple watcher!

const { cart, error } = useCart()

watch(error, (error, prevError) => {
  if (error.value.addItem && error.value.addItem !== prevError.value.addItem) sendInAppNotification('error', error.value.addItem.message)
  if (error.value.removeItem && error.value.removeItem !== prevError.value.removeItem) sendInAppNotification('error', error.value.removeItem.message)