Card Shop Engineering — Study & Prep Portal

import React, { useState, useTransition, useMemo } from 'react';

// Custom Hook for Card Filter with Non-Blocking Render
export function CardSelectionContainer({ cardList }) {
  const [filter, setFilter] = useState('');
  const [deferredFilter, setDeferredFilter] = useState('');
  const [isPending, startTransition] = useTransition();

  const handleFilterChange = (e) => {
    const nextVal = e.target.value;
    setFilter(nextVal); // Immediate input update
    startTransition(() => {
      setDeferredFilter(nextVal); // Low-priority list filter
    });
  };

  const filteredCards = useMemo(() => {
    return cardList.filter(c => c.name.toLowerCase().includes(deferredFilter.toLowerCase()));
  }, [cardList, deferredFilter]);

  return (
    <div role="region" aria-label="Credit Card Selection">
      <label htmlFor="card-search" className="sr-only">Search Cards</label>
      <input
        id="card-search"
        type="search"
        value={filter}
        onChange={handleFilterChange}
        aria-describedby="search-status"
        placeholder="Search Amex Cards..."
      />
      <span id="search-status" role="status" aria-live="polite" className="sr-only">
        {isPending ? 'Filtering card list...' : `${filteredCards.length} cards available`}
      </span>

      <ul className="card-grid" aria-busy={isPending}>
        {filteredCards.map(card => (
          <li key={card.id} tabIndex={0}>
            <h4>{card.name}</h4>
            <p>Annual Fee: ${card.fee}</p>
          </li>
        ))}
      </ul>
    </div>
  );
}

const DataLoader = require('dataloader');
const { fetchRewardsByCardIds } = require('./services/rewardService');

// Batch function to load rewards for multiple cards in 1 batch query
const rewardBatchLoader = new DataLoader(async (cardIds) => {
  const rewards = await fetchRewardsByCardIds(cardIds);
  const rewardMap = {};
  rewards.forEach(r => { rewardMap[r.cardId] = r; });
  return cardIds.map(id => rewardMap[id] || null);
});

const resolvers = {
  Query: {
    getCardOffers: async (_, { category }, context) => {
      return await context.db.cards.find({ category, active: true });
    }
  },
  CardOffer: {
    rewardsProgram: (parent, _, context) => {
      // High-performance batched call via DataLoader
      return rewardBatchLoader.load(parent.id);
    }
  }
};

module.exports = { resolvers };

import { render, screen, fireEvent } from '@testing-library/react';
import { CardSelectionContainer } from './CardSelectionContainer';

const mockCards = [
  { id: '1', name: 'Amex Platinum', fee: 695 },
  { id: '2', name: 'Amex Gold', fee: 250 }
];

describe('CardSelectionContainer', () => {
  it('renders all cards initially and filters dynamically on user search', async () => {
    render(<CardSelectionContainer cardList={mockCards} />);

    expect(screen.getByText('Amex Platinum')).toBeInTheDocument();
    expect(screen.getByText('Amex Gold')).toBeInTheDocument();

    const searchInput = screen.getByPlaceholderText(/search amex cards/i);
    fireEvent.change(searchInput, { target: { value: 'Gold' } });

    expect(await screen.findByText('Amex Gold')).toBeInTheDocument();
    expect(screen.queryByText('Amex Platinum')).not.toBeInTheDocument();
  });
});