TypeScript Utility Types

TypeScript Utility Types: Built-In Type Transformations

TypeScript provides built-in global utility types that perform common type transformations on object shapes, functions, and unions.

Common Utility Types Overview

• Partial<T>: Makes all properties in T optional.
• Required<T>: Makes all properties in T required.
• Readonly<T>: Makes all properties in T read-only.
• Record<K, T>: Constructs an object type with key type K and value type T.
• Pick<T, K>: Constructs a type by picking specific keys K from T.
• Omit<T, K>: Constructs a type by omitting specific keys K from T.
• ReturnType<T>: Extracts the return type of a function type T.

Practical Code Example

interface UserProfile {
  id: string;
  username: string;
  email: string;
  age: number;
  bio?: string;
}

// Partial<T>: Ideal for update DTOs
type UpdateUserProfileDto = Partial<UserProfile>;

function updateUser(id: string, updates: UpdateUserProfileDto): void {
  console.log(`Updating user ${id} with:`, updates);
}

updateUser("usr_100", { bio: "Full stack developer" });

// Pick<T, K> and Omit<T, K>
type PublicUserPreview = Pick<UserProfile, "id" | "username">;
type CreateUserPayload = Omit<UserProfile, "id">;

const newPayload: CreateUserPayload = {
  username: "charlie",
  email: "[email protected]",
  age: 28
};

// Record<K, T>: Dynamic dictionary map
type FeatureFlags = Record<"enableMetrics" | "enableDarkMode" | "betaAccess", boolean>;

const userFlags: FeatureFlags = {
  enableMetrics: true,
  enableDarkMode: false,
  betaAccess: true
};

console.log("Flags:", userFlags);

Best Practices & Gotchas

• Combine Utility Types: Compose utilities like Readonly<Partial<T>> for immutable optional updates.
• Use Omit to Remove Auto-Generated Database IDs: Ideal when separating database entities from creation DTOs.
• Use ReturnType<typeof func> for Inferred Return Types: Extract returntypes from external libraries without manually duplicating types.

Self-Check Challenge

Given an interface Todo { id: number; title: string; completed: boolean; }, create a type TodoPreview using Omit that excludes completed.