function identity<T>(value: T): T {
  return value;
}

const num = identity<number>(42); // Type: number
const str = identity<string>("hello"); // Type: string
const auto = identity(true); // Type inferred: boolean

interface HasLength {
  length: number;
}

function logLength<T extends HasLength>(item: T): T {
  console.log(item.length);
  return item;
}

logLength("hello"); // OK: string has length
logLength([1, 2, 3]); // OK: array has length
logLength({ length: 10 }); // OK: object has length
// logLength(42);             // Error: number has no length

function merge<T, U>(obj1: T, obj2: U): T & U {
  return { ...obj1, ...obj2 };
}

const merged = merge({ name: "John" }, { age: 30 });
// Type: { name: string } & { age: number }

type IsString<T> = T extends string ? true : false;

type A = IsString<string>; // true
type B = IsString<number>; // false

type ReturnType<T> = T extends (...args: any[]) => infer R ? R : never;

function getUser() {
  return { id: 1, name: "John" };
}

type User = ReturnType<typeof getUser>;
// Type: { id: number; name: string; }

type ToArray<T> = T extends any ? T[] : never;

type StrOrNumArray = ToArray<string | number>;
// Type: string[] | number[]

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

type T1 = TypeName<string>; // "string"
type T2 = TypeName<() => void>; // "function"

type Readonly<T> = {
  readonly [P in keyof T]: T[P];
};

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

type ReadonlyUser = Readonly<User>;
// Type: { readonly id: number; readonly name: string; }

type Partial<T> = {
  [P in keyof T]?: T[P];
};

type PartialUser = Partial<User>;
// Type: { id?: number; name?: string; }

type Getters<T> = {
  [K in keyof T as `get${Capitalize<string & K>}`]: () => T[K];
};

interface Person {
  name: string;
  age: number;
}

type PersonGetters = Getters<Person>;
// Type: { getName: () => string; getAge: () => number; }

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

interface Mixed {
  id: number;
  name: string;
  age: number;
  active: boolean;
}

type OnlyNumbers = PickByType<Mixed, number>;
// Type: { id: number; age: number; }

type EventName = "click" | "focus" | "blur";
type EventHandler = `on${Capitalize<EventName>}`;
// Type: "onClick" | "onFocus" | "onBlur"

type UppercaseGreeting = Uppercase<"hello">; // "HELLO"
type LowercaseGreeting = Lowercase<"HELLO">; // "hello"
type CapitalizedName = Capitalize<"john">; // "John"
type UncapitalizedName = Uncapitalize<"John">; // "john"

type Path<T> = T extends object
  ? {
      [K in keyof T]: K extends string ? `${K}` | `${K}.${Path<T[K]>}` : never;
    }[keyof T]
  : never;

interface Config {
  server: {
    host: string;
    port: number;
  };
  database: {
    url: string;
  };
}

type ConfigPath = Path<Config>;
// Type: "server" | "database" | "server.host" | "server.port" | "database.url"

// Partial<T> - Make all properties optional
type PartialUser = Partial<User>;

// Required<T> - Make all properties required
type RequiredUser = Required<PartialUser>;

// Readonly<T> - Make all properties readonly
type ReadonlyUser = Readonly<User>;

// Pick<T, K> - Select specific properties
type UserName = Pick<User, "name" | "email">;

// Omit<T, K> - Remove specific properties
type UserWithoutPassword = Omit<User, "password">;

// Exclude<T, U> - Exclude types from union
type T1 = Exclude<"a" | "b" | "c", "a">; // "b" | "c"

// Extract<T, U> - Extract types from union
type T2 = Extract<"a" | "b" | "c", "a" | "b">; // "a" | "b"

// NonNullable<T> - Exclude null and undefined
type T3 = NonNullable<string | null | undefined>; // string

// Record<K, T> - Create object type with keys K and values T
type PageInfo = Record<"home" | "about", { title: string }>;

// Type assertion tests
type AssertEqual<T, U> = [T] extends [U]
  ? [U] extends [T]
    ? true
    : false
  : false;

type Test1 = AssertEqual<string, string>; // true
type Test2 = AssertEqual<string, number>; // false
type Test3 = AssertEqual<string | number, string>; // false

// Expect error helper
type ExpectError<T extends never> = T;

// Example usage
type ShouldError = ExpectError<AssertEqual<string, number>>;