Custom error extensions in GraphQL - Mini Project: Build & Apply

Custom Error Extensions in GraphQL

📖 Scenario: You are building a GraphQL API for a bookstore. You want to provide clear error messages with extra details when something goes wrong, so clients can understand and handle errors better.

🎯 Goal: Create a GraphQL schema with a query that can return custom errors including additional information in the extensions field.

📋 What You'll Learn

Define a simple GraphQL schema with a book query that takes an id argument.

Create a custom error class that adds an extensions field with extra error details.

Throw the custom error when a book with the given id is not found.

Ensure the error response includes the extensions field with the custom data.

💡 Why This Matters

🌍 Real World

Custom error extensions help API clients understand exactly what went wrong and how to handle errors gracefully.

💼 Career

Many companies use GraphQL APIs and expect developers to provide clear, structured error information for better client-side error handling.

1

Define the GraphQL schema with a book query

Create a GraphQL schema string called typeDefs with a Book type having fields id (ID!), title (String!), and a Query type with a book field that takes an id argument of type ID! and returns a Book.

GraphQL

# Your code here

Hint

Use backticks to create a multi-line string for the schema.

2

Create a custom error class with extensions

Define a class called BookNotFoundError that extends Error. In its constructor, accept a message and set this.extensions to an object with a code property set to BOOK_NOT_FOUND and a timestamp property set to the current ISO date string.

GraphQL

const typeDefs = `
  type Book {
    id: ID!
    title: String!
  }

  type Query {
    book(id: ID!): Book
  }
`;

// Your code here

Hint

Remember to call super(message) in the constructor.

3

Implement the resolver to throw the custom error

Create a resolvers object with a Query field. Inside Query, define a book function that takes parent, args, and context. Use a hardcoded array books with one book: { id: '1', title: '1984' }. Find the book by args.id. If not found, throw a BookNotFoundError with the message Book with id {args.id} not found. Otherwise, return the book.

GraphQL

const typeDefs = `
  type Book {
    id: ID!
    title: String!
  }

  type Query {
    book(id: ID!): Book
  }
`;

class BookNotFoundError extends Error {
  constructor(message) {
    super(message);
    this.extensions = {
      code: 'BOOK_NOT_FOUND',
      timestamp: new Date().toISOString()
    };
  }
}

// Your code here

Hint

Use Array.find to locate the book by id.

4

Export the schema and resolvers for server setup

Export the typeDefs and resolvers using module.exports as an object with keys typeDefs and resolvers.

GraphQL

const typeDefs = `
  type Book {
    id: ID!
    title: String!
  }

  type Query {
    book(id: ID!): Book
  }
`;

class BookNotFoundError extends Error {
  constructor(message) {
    super(message);
    this.extensions = {
      code: 'BOOK_NOT_FOUND',
      timestamp: new Date().toISOString()
    };
  }
}

const resolvers = {
  Query: {
    book(parent, args, context) {
      const books = [{ id: '1', title: '1984' }];
      const book = books.find(b => b.id === args.id);
      if (!book) {
        throw new BookNotFoundError(`Book with id ${args.id} not found`);
      }
      return book;
    }
  }
};

// Your code here

Hint

Use module.exports = { typeDefs, resolvers } to export both.