Typescript:如何正确组合不同的类型？

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

interface Employee {
  employeeId: number;
  department: string;
}

type EmployeePerson = Person & Employee;

const emp: EmployeePerson = {
  name: "John",
  age: 30,
  employeeId: 12345,
  department: "Engineering"
};

type StringOrNumber = string | number;

function padLeft(value: string, padding: StringOrNumber) {
  if (typeof padding === "number") {
    return Array(padding + 1).join(" ") + value;
  }
  if (typeof padding === "string") {
    return padding + value;
  }
  throw new Error(`Expected string or number, got '${padding}'.`);
}

type Name = string;
type Age = number;

interface Person {
  name: Name;
  age: Age;
}

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

let output = identity<string>("myString");
let output2 = identity("myString"); // 类型推断

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

type A = IsString<"hello">; // true
type B = IsString<42>; // false

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

interface Todo {
  title: string;
  description: string;
}

type ReadonlyTodo = Readonly<Todo>;