Client-side schema

You can optionally provide a client-side schema to Apollo Client that defines local-only types and fields. You can define completely new types, or extend types from your server's schema with new fields.

As with any GraphQL schema, your client-side schema must be written in Schema Definition Language.

The client-side schema is not used to validate operations like it is on the server (the graphql-js modules for schema validation would dramatically increase your bundle size). Instead, your client-side schema is used for introspection in the Apollo Client Devtools, where you can explore your schema in GraphiQL.

Setup

The following demonstrates how to define a client-side schema and provide it to the ApolloClient constructor:

import { ApolloClient, InMemoryCache, gql } from '@apollo/client';

const typeDefs = gql`
  extend type Query {
    isLoggedIn: Boolean!
    cartItems: [Launch]!
  }

  extend type Launch {
    isInCart: Boolean!
  }

  extend type Mutation {
    addOrRemoveFromCart(id: ID!): [Launch]
  }
`;

const client = new ApolloClient({
  cache: new InMemoryCache(),
  uri: 'http://localhost:4000/graphql',
  typeDefs,
});

If you open up the Apollo Client Devtools and click on the GraphiQL tab, you'll be able to explore your client schema in the "Docs" section. This example doesn't include a remote schema, but if it did, you would be able to see your local queries and mutations alongside your remote ones.