TypeScript的基本语法是什么？

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

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

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

class Student implements Person {
  name: string;
  age: number;
  constructor(name: string, age: number) {
    this.name = name;
    this.age = age;
  }
}

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

let output = identity<string>("Hello");

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