Types — Scalars, Enums & Interfaces

# Built-in scalars
# Int, Float, String, Boolean, ID

# Custom scalars (require resolver implementations)
scalar DateTime    # ISO 8601 date string
scalar Email       # Validated email address
scalar URL         # Valid URL string
scalar JSON        # Arbitrary JSON blob
scalar Upload      # File upload

type User {
  id:        ID!
  email:     Email!
  website:   URL
  createdAt: DateTime!
  metadata:  JSON
}

# Scalar resolver (Node.js)
const { GraphQLScalarType, Kind } = require("graphql");

const DateTimeScalar = new GraphQLScalarType({
  name: "DateTime",
  serialize:  (value) => new Date(value).toISOString(),
  parseValue: (value) => new Date(value),
  parseLiteral: (ast) => {
    if (ast.kind === Kind.STRING) return new Date(ast.value);
    return null;
  }
});

# Interface — shared contract
interface Node {
  id: ID!
}

interface Timestamped {
  createdAt: DateTime!
  updatedAt: DateTime!
}

type User implements Node & Timestamped {
  id:        ID!
  name:      String!
  createdAt: DateTime!
  updatedAt: DateTime!
}

type Post implements Node & Timestamped {
  id:        ID!
  title:     String!
  createdAt: DateTime!
  updatedAt: DateTime!
}

# Union — completely different types
union SearchResult = User | Post | Product

type Query {
  node(id: ID!): Node          # Returns any Node implementor
  search(q: String!): [SearchResult!]!
}

# __resolveType is required for interfaces and unions
const resolvers = {
  SearchResult: {
    __resolveType: (obj) => {
      if (obj.email)  return "User";
      if (obj.title)  return "Post";
      if (obj.price)  return "Product";
      return null;
    }
  }
};