Advanced Type Processing

This guide covers advanced type processing capabilities in dtsx.

Complex Type Inference

Nested Types

interface User {
  profile: {
    name: string;
    address: {
      street: string;
      city: string;
      country: string;
    };
  };
  settings: {
    preferences: {
      theme: 'light' | 'dark';
      notifications: boolean;
    };
  };
}

Recursive Types

interface TreeNode {
  value: number;
  children: TreeNode[];
}

type JsonValue =
  | string
  | number
  | boolean
  | null
  | JsonValue[]
  | { [key: string]: JsonValue };

Type Relationship Tracking

Type Dependencies

// Type dependencies are tracked automatically
interface User {
  id: UserId;
  role: UserRole;
  permissions: Permission[];
}

type UserId = string;
type UserRole = 'admin' | 'user';
interface Permission {
  name: string;
  level: number;
}

Circular Dependencies

// Circular dependencies are handled correctly
interface Node {
  next: Node | null;
  value: number;
}

interface Parent {
  children: Child[];
}

interface Child {
  parent: Parent;
}

Custom Type Transformations

Type Mapping

// Transform types using type mapping
type ReadonlyDeep<T> = {
  readonly [P in keyof T]: T[P] extends object
    ? ReadonlyDeep<T[P]>
    : T[P];
};

type Nullable<T> = T | null;
type Optional<T> = T | undefined;

Type Composition

// Compose types using intersection types
type WithId<T> = T & { id: string };
type WithTimestamps<T> = T & {
  createdAt: Date;
  updatedAt: Date;
};

type UserWithMetadata = WithId<WithTimestamps<User>>;

Type Augmentation

Module Augmentation

// Augment existing modules
declare module './types' {
  interface User {
    metadata: {
      lastLogin: Date;
      preferences: UserPreferences;
    };
  }
}

Interface Merging

// Merge interfaces
interface User {
  name: string;
}

interface User {
  age: number;
}

// Result:
// interface User {
//   name: string;
//   age: number;
// }

Advanced Type Features

Conditional Types

type TypeName<T> =
  T extends string ? "string" :
  T extends number ? "number" :
  T extends boolean ? "boolean" :
  T extends undefined ? "undefined" :
  T extends Function ? "function" :
  "object";

Mapped Types with Constraints

type PickByType<T, U> = {
  [P in keyof T as T[P] extends U ? P : never]: T[P];
};

type StringProps<T> = PickByType<T, string>;

Best Practices

1. Use Type Composition

   • Combine types using intersection types
   • Create reusable type utilities
   • Keep type definitions modular

2. Handle Circular Dependencies

   • Use interface merging
   • Leverage type aliases
   • Consider type composition

3. Type Augmentation

   • Use module augmentation for extending types
   • Keep augmentations close to their usage
   • Document augmented types

4. Type Transformations

   • Create reusable type transformers
   • Use conditional types for complex logic
   • Leverage mapped types for property transformations