class Person {
                constructor(name, age) {
                    // 构造器中的 this 是类的实例对象
                    this.name = name
                    this.age = age
                }

                speak() {
                    // speak 方法放在了类的原型对象上，供实例使用
                    // 通过 Person 实例调用 speak 时，speak中的 this 就是 Person 实例
                    console.log(`my name is ${this.name}, my age is ${this.age}`)
                }
            }
            /*
            const p1 = new Person('tony', 20)
            const p2 = new Person('jack', 18)

            console.log(p1)
            console.log(p2) 

            p1.speak()
            p2.speak()
            */
            
            class Student extends Person {
                constructor(name, age, grade) {
                    super(name, age)
                    this.grade = grade
                }

                speak() {
                    console.log(`my name is ${this.name}, my age is ${this.age}, my grade is ${this.grade}`)
                }

                study() {
                    console.log('I love study')
                }
    
            }

            const s1 = new Student('bob', 16, 'three')
            console.log(s1)
            s1.speak()
            s1.study()