Built-in Utility Types

interface User {
  id: number;
  name: string;
  email: string;
  password: string;
}

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

// Required — all properties required (reverses Partial)
type StrictUser = Required<UserUpdate>;

// Readonly — no property can be reassigned
type ImmutableUser = Readonly<User>;

// Pick — keep only selected properties
type PublicUser = Pick<User, "id" | "name">;
// { id: number; name: string }

// Omit — remove selected properties
type SafeUser = Omit<User, "password">;
// { id: number; name: string; email: string }

// Record — map keys to a value type
type RolePermissions = Record<"admin" | "student" | "mentor", string[]>;
const perms: RolePermissions = {
  admin: ["read", "write", "delete"],
  student: ["read"],
  mentor: ["read", "write"]
};

// ReturnType — extract a function's return type
function createSession() {
  return { token: "abc", expiresAt: new Date() };
}
type Session = ReturnType<typeof createSession>;
// { token: string; expiresAt: Date }

// Parameters — extract parameter types as a tuple
type AddParams = Parameters<(a: number, b: number) => number>;
// [a: number, b: number]

// Awaited — unwrap Promise types (TypeScript 4.5+)
type Resolved = Awaited<Promise<string>>; // string