cache.js
import { InMemoryCache, makeVar } from '@apollo/client';

const cache = new InMemoryCache({
  typePolicies: {
    Query: {
      fields: {
        readNumOfOutofStockItems: {
          read(cart, { readField }) {
            const { products } = readField('cart')?.data ?? [];
            const items = products.filter(product => 
              product.isOutOfStock);
            return items.length;
          }
        }
      }
    }
  }

const client = new ApolloClient({ cache });
components/outofstock.js
import React from 'react';
import { gql, useQuery } from '@apollo/client';

const GET_OOS_ITEMS_QUERY = gql`
  query GetOutOfStockItems {
    readNumOfOutofStockItems @client
  }
`;

const ViewOutOfStockItems = () => {
  const { loading, data, error } = useQuery(GET_OOS_ITEMS_QUERY);
  return (<div>{data?.products.map(product => 
    <p>{data.readNumOfOutofStockItems}</p>
  </div>);
}