SOURCE

class Father {
    constructor(x, y) {
        this.x = x;
        this.y = y;
    }
    sum() {
        console.log(this.x + this.y);
    }
    say() {
        console.log('haha');
    }
}
class Son extends Father {
    constructor(x, y) {
            //使用super调用了父类中的构造函数
            super(x, y);
        }
        // say() {
        //     console.log('I am Son');
        // }
}

let son = new Son(10, 20);
let father = new Father()
son.sum();
son.say();