GraphQL Errors

In this lesson you will learn errors in GraphQL, why it matters within error handling, and how to use it correctly with clear, copy-ready examples.

Errors Overview

Errors lets you structure GraphQL work so it stays readable, testable, and easy to scale. Instead of ad-hoc code, you follow a clear pattern that other developers can recognise immediately.

The key is to keep errors focused and predictable. Start from the minimal example here, then layer in only the complexity your feature actually needs.

import { GraphQLError } from 'graphql';

if (!input.email.includes('@')) {
  throw new GraphQLError('Invalid email', {
    extensions: { code: 'BAD_USER_INPUT', field: 'email' },
  });
}

Throw GraphQLError with an extensions code so clients can handle failures precisely.

Errors Example

const typeDefs = gql`
  type Query { hello: String! }
`;
const resolvers = { Query: { hello: () => 'world' } };
const server = new ApolloServer({ typeDefs, resolvers });

Start from a minimal Errors example and grow it only as needed.
Keep configuration explicit so Errors behaves the same in every environment.
Name things clearly so teammates understand your Errors at a glance.
Add tests around Errors early to lock in expected behaviour.

GraphQL Cheatsheet

Quick GraphQL reference related to errors.

Concept	Example	Purpose
Schema	`type Query { user(id: ID!): User }`	Define the API shape
Resolver	`Query: { user: (_, { id }) => ... }`	Provide field data
Query	`query { user(id: 1) { name } }`	Read exactly what you need
Mutation	`mutation { createUser(input) { id } }`	Change data
Subscription	`subscription { postAdded { id } }`	Real-time updates
Context	`context: ({ req }) => ({ user })`	Auth and shared state
DataLoader	`loader.load(id)`	Batch to avoid N+1

How Errors Works in GraphQL

Errors fits into GraphQL's model of a single typed schema that clients query for exactly the data they need. The server resolves each requested field through resolver functions.

Throw GraphQLError with an extensions code so clients can handle failures precisely.

The schema is the contract between client and server.
Resolvers fetch data field by field, including nested types.
Clients request only the fields they use, avoiding over-fetching.
Context carries auth and shared services into every resolver.

Practical Guidance for Errors

In production, errors should be efficient and secure. Batch data access with DataLoader, guard resolvers with authorization, and limit query depth and complexity.

Concern	Recommendation
N+1 queries	Batch with DataLoader
Security	Auth in context, depth/complexity limits
Errors	Typed GraphQLError with extension codes
Performance	Cache and paginate large lists

Common Mistakes

Copying errors snippets without understanding what each line does.
Skipping error handling and edge cases when wiring up errors.
Leaving errors untested, so regressions slip into production.
Over-engineering errors before you actually need the extra flexibility.

Key Takeaways

Errors is a core part of working effectively with GraphQL.
Start small and keep errors focused on a single responsibility.
Apply consistent patterns so errors scales across your project.
Test and document errors to keep it maintainable over time.

Pro Tip

When you get stuck on errors, reduce it to the smallest reproducible example first — most GraphQL issues become obvious once the noise is gone.