apollo-client-patterns

# Apollo Client Patterns

Master Apollo Client for building efficient GraphQL applications with proper
query management, caching strategies, and state handling.

## Overview

Apollo Client is a comprehensive state management library for JavaScript that
enables you to manage both local and remote data with GraphQL. It integrates
seamlessly with React and provides powerful caching mechanisms.

## Installation and Setup

### Installing Apollo Client

```bash
# Install Apollo Client and dependencies
npm install @apollo/client graphql

# For React applications
npm install @apollo/client graphql react

# Additional packages
npm install graphql-tag @apollo/client/link/error
```

### Basic Configuration

```javascript
// src/apollo/client.js
import {
  ApolloClient,
  InMemoryCache,
  createHttpLink,
  from
} from '@apollo/client';
import { setContext } from '@apollo/client/link/context';
import { onError } from '@apollo/client/link/error';

const httpLink = createHttpLink({
  uri: process.env.REACT_APP_GRAPHQL_URI || 'http://localhost:4000/graphql',
});

const authLink = setContext((_, { headers }) => {
  const token = localStorage.getItem('authToken');
  return {
    headers: {
      ...headers,
      authorization: token ? `Bearer ${token}` : '',
    }
  };
});

const errorLink = onError(({ graphQLErrors, networkError }) => {
  if (graphQLErrors) {
    graphQLErrors.forEach(({ message, locations, path }) =>
      console.error(
        `[GraphQL error]: Message: ${message}, Location: ${locations}, Path: ${path}`
      )
    );
  }
  if (networkError) {
    console.error(`[Network error]: ${networkError}`);
  }
});

const client = new ApolloClient({
  link: from([errorLink, authLink, httpLink]),
  cache: new InMemoryCache({
    typePolicies: {
      Query: {
        fields: {
          posts: {
            merge(existing, incoming) {
              return incoming;
            }
          }
        }
      }
    }
  }),
  defaultOptions: {
    watchQuery: {
      fetchPoli