GraphQL Codegen

Generate TypeScript types for resolvers and client operations from your GraphQL schema so resolver signatures stay in sync with the contract.

4 min read Level 3/5 #fastify#graphql#codegen

What you'll learn

Install @graphql-codegen/cli and resolver plugins
Configure codegen.yml or codegen.ts
Run codegen as part of the build

Hand-writing resolver types is error-prone — change the schema and your TS types silently drift. graphql-code-generator reads the SDL and emits matching TypeScript.

Install

npm install -D @graphql-codegen/cli \
  @graphql-codegen/typescript \
  @graphql-codegen/typescript-resolvers

Configure

// codegen.ts
import type { CodegenConfig } from '@graphql-codegen/cli'

const config: CodegenConfig = {
  schema: './schema.graphql',
  generates: {
    './src/generated/graphql.ts': {
      plugins: ['typescript', 'typescript-resolvers'],
      config: {
        contextType: 'FastifyRequest',
        useIndexSignature: true,
      },
    },
  },
}

export default config

Run npx graphql-codegen to produce src/generated/graphql.ts with Resolvers<Context> and per-field types.

Type Your Resolvers

import type { Resolvers } from './generated/graphql.js'

export const resolvers: Resolvers = {
  Query: {
    user: async (_parent, args, ctx) => {
      return ctx.server.db.user.findUnique({ where: { id: args.id } })
    },
  },
}

If you rename User.email to User.emailAddress in the schema, the compiler flags every resolver that returns the wrong shape.

Wire Into The Build

Add codegen to your scripts so it runs in CI before tsc:

{
  "scripts": {
    "codegen": "graphql-codegen",
    "build": "npm run codegen && tsc -p ."
  }
}

Commit the generated file or generate it in CI — either is fine, but commit only one source of truth.

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