Utility Types

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

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

// Required<T> — all optional properties become required
type StrictUser = Required<UserUpdate>

// Readonly<T> — all properties become readonly
type FrozenUser = Readonly<User>

// Pick<T, Keys> — select a subset of properties
type UserPublic = Pick<User, "id" | "name" | "role">
// { id: number; name: string; role: "user" | "admin" }

// Omit<T, Keys> — exclude properties
type UserWithoutPassword = Omit<User, "password">

// Record<Keys, Type> — create an object type with specific keys
type RolePermissions = Record<"user" | "admin", string[]>
const perms: RolePermissions = {
  user:  ["read"],
  admin: ["read", "write", "delete"],
}

// Exclude<T, U> — remove members from union
type NotAdmin = Exclude<"user" | "admin" | "guest", "admin">
// "user" | "guest"

// Extract<T, U> — keep only members assignable to U
type StringOrNumber = Extract<string | number | boolean, string | number>
// string | number

// ReturnType<T> — extract return type of a function
function getUser() {
  return { id: 1, name: "Alice", email: "alice@example.com" }
}
type UserFromFn = ReturnType<typeof getUser>
// { id: number; name: string; email: string }

// Parameters<T> — extract parameter types as tuple
function createPost(title: string, body: string, authorId: number) {}
type CreatePostArgs = Parameters<typeof createPost>
// [string, string, number]

// Awaited<T> — unwrap Promise type
type ResolvedUser = Awaited<Promise<User>>   // User

// NonNullable<T> — remove null and undefined
type DefinitelyString = NonNullable<string | null | undefined>
// string

// Practical example — form handling pattern
type FormValues = Omit<User, "id" | "createdAt">
type FormErrors = Partial<Record<keyof FormValues, string>>

function validateUser(values: FormValues): FormErrors {
  const errors: FormErrors = {}
  if (!values.name)  errors.name  = "Name is required"
  if (!values.email.includes("@")) errors.email = "Invalid email"
  if (values.password.length < 8)  errors.password = "Min 8 characters"
  return errors
}