TypeScript如何进行类型检查？

let str: string = "Hello";
const num: number = 42;

function add(x: number, y: number): number {
  return x + y;
}

let str = "Hello"; // str的类型为string

let strLength: number = (<string>someValue).length;

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

let person: Person = {
  name: "Alice",
  age: 30,
};

type Point = { x: number, y: number };

function distance(p1: Point, p2: Point) {
  // ...
}