TypeScript Lesson 10: Utility Types

TypeScript has built-in utility types that transform existing types. These save huge amounts of typing and are used constantly in real codebases.

Essential Utility Types

interface User {
  id: number;
  name: string;
  email: string;
  password: string;
  role: "user" | "admin";
  createdAt: Date;
}

// Partial: all properties optional
type UserUpdate = Partial<User>;
// { id?: number; name?: string; email?: string; ... }

// Required: all properties required (reverse of Partial)
type StrictConfig = Required<Config>;

// Readonly: nothing can be reassigned
type FrozenUser = Readonly<User>;
const u: FrozenUser = getUser();
u.name = "Bob"; // ❌ Error

// Pick: keep only specified properties
type UserPublic = Pick<User, "id" | "name" | "email">;
// Safe to send to frontend — no password!

// Omit: exclude specified properties
type UserWithoutPassword = Omit<User, "password">;

// Record: map keys to values
type Roles = Record<"user" | "admin" | "guest", string[]>;
// { user: string[]; admin: string[]; guest: string[] }

More Utility Types

type Status = "pending" | "active" | "done";

// Exclude: remove from union
type ActiveStatus = Exclude<Status, "done">;  // "pending" | "active"

// Extract: keep only matching
type DoneStatus = Extract<Status, "done" | "pending">; // "pending" | "done"

// ReturnType: get what a function returns
function createUser() { return { id: 1, name: "Alice" }; }
type UserObj = ReturnType<typeof createUser>; // { id: number; name: string }

// Parameters: get function params as tuple
type FnParams = Parameters<typeof createUser>;