Error Handling di GraphQL

GraphQL selalu return HTTP 200. Errors ada di response body field errors. Ini berbeda dari REST yang pakai status codes.

Default Error Format

{
  "data": { "user": null },
  "errors": [{
    "message": "User not found",
    "locations": [{ "line": 2, "column": 3 }],
    "path": ["user"],
    "extensions": {
      "code": "NOT_FOUND",
      "http": { "status": 404 }
    }
  }]
}

Throwing Errors in Resolvers

import { GraphQLError } from "graphql";

const resolvers = {
  Query: {
    user: async (_, { id }, context) => {
      if (!context.user) {
        throw new GraphQLError("Authentication required", {
          extensions: { code: "UNAUTHENTICATED" },
        });
      }

      const user = await context.db.users.findById(id);
      if (!user) {
        throw new GraphQLError("User not found", {
          extensions: { code: "NOT_FOUND" },
        });
      }

      return user;
    },
  },
};

Union-based Errors (Recommended)

# Schema — error as data
union CreateUserResult = User | ValidationError | DuplicateEmailError

type ValidationError {
  field: String!
  message: String!
}

type DuplicateEmailError {
  email: String!
  message: String!
}

# Resolver
createUser: async (_, { input }) => {
  if (!input.email.includes("@")) {
    return { __typename: "ValidationError", field: "email", message: "Email tidak valid" };
  }
  const existing = await db.users.findByEmail(input.email);
  if (existing) {
    return { __typename: "DuplicateEmailError", email: input.email, message: "Email sudah terdaftar" };
  }
  const user = await db.users.create({ data: input });
  return { __typename: "User", ...user };
}

# Client query
mutation {
  createUser(input: { name: "Budi", email: "[email protected]" }) {
    ... on User { id name }
    ... on ValidationError { field message }
    ... on DuplicateEmailError { email message }
  }
}

Partial Errors

GraphQL bisa return data DAN errors sekaligus — partial success. Field yang error jadi null, sisanya tetap ada.