简单地说说TypeScript中的let

class Car {
  private _make: string;
  private _model: string;
  private _year: number;

  constructor(make: string, model: string, year: number) {
    this._make = make;
    this._model = model;
    this._year = year;
  }

  get make(): string {
    return this._make;
  }

  set make(make: string) {
    this._make = make;
  }

  get model(): string {
    return this._model;
  }

  set model(model: string) {
    this._model = model;
  }

  get year(): number {
    return this._year;
  }

  set year(year: number) {
    this._year = year;
  }

  drive(): void {
    console.log(`Driving a ${this._year} ${this._make} ${this._model}`);
  }
}

class SportsCar extends Car {
  private _topSpeed: number;

  constructor(make: string, model: string, year: number, topSpeed: number) {
    super(make, model, year);
    this._topSpeed = topSpeed;
  }

  get topSpeed(): number {
    return this._topSpeed;
  }

  set topSpeed(topSpeed: number) {
    this._topSpeed = topSpeed;
  }

  drive(): void {
    console.log(`Driving a ${this.year} ${this.make} ${this.model} at top speed ${this._topSpeed}`);
  }
}

let myCar: Car = new Car("Ford", "Mustang", 2021);
myCar.drive();

let mySportsCar: SportsCar = new SportsCar("Lamborghini", "Aventador", 2021, 350);
mySportsCar.drive();

Driving a 2021 Ford Mustang
Driving a 2021 Lamborghini Aventador at top speed 350

console.log(a); // undefined
var a = 10;

console.log(func()); // "Hello world!"
function func() {
  return "Hello world!";
}