类的创建方法：记得一点要用constructor

class ClassName {
  constructor() { ... }
}

　　实例

class Car {
  constructor(name, year) {
    this.name = name;
    this.year = year;
  }
}

　　多个方法：

class ClassName {
  constructor() { ... }
  method_1() { ... }
  method_2() { ... }
  method_3() { ... }
}

　　实例：

class Car {
  constructor(name, year) {
    this.name = name;
    this.year = year;
  }
  age() {
    let date = new Date();
    return date.getFullYear() - this.year;
  }
}

let myCar = new Car("Ford", 2022);
document.getElementById("demo").innerHTML = "My car is " + myCar.age() + " years old.";