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HTML / CSS / JS

# ----- Union error pattern -----
type CreateUserSuccess {
  user: User!
}

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

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

union CreateUserResult =
  | CreateUserSuccess
  | ValidationError
  | DuplicateEmailError

type Mutation {
  createUser(input: CreateUserInput!): CreateUserResult!
}

# Resolver (Node.js)
# Mutation: {
#   createUser: async (_, { input }) => {
#     if (!isValidEmail(input.email)) {
#       return { __typename: 'ValidationError',
#                field: 'email', message: 'Invalid format' };
#     }
#     const exists = await User.findByEmail(input.email);
#     if (exists) {
#       return { __typename: 'DuplicateEmailError',
#                email: input.email, message: 'Already registered' };
#     }
#     const user = await User.create(input);
#     return { __typename: 'CreateUserSuccess', user };
#   }
# }

# Client query handles each case:
mutation Register($input: CreateUserInput!) {
  createUser(input: $input) {
    ... on CreateUserSuccess { user { id  email } }
    ... on ValidationError   { field  message }
    ... on DuplicateEmailError { email  message }
  }
}