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:

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import { ApolloClient, InMemoryCache, gql } from '@apollo/client';
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const typeDefs = gql`
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  extend type Query {
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    isLoggedIn: Boolean!
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    cartItems: [Launch]!
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  }
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  extend type Launch {
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    isInCart: Boolean!
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  }
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  extend type Mutation {
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    addOrRemoveFromCart(id: ID!): [Launch]
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  }
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`;
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const client = new ApolloClient({
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  cache: new InMemoryCache(),
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  uri: 'http://localhost:4000/graphql',
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  typeDefs,
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});

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.