let sayHiMixin = {
  sayHi() {
    alert(`Hello ${this.name}`);
  }
}


let sayByeMixin = {
  sayBye() {
    alert(`Bye ${this.name}`);
  }
}


class User {
  constructor(name) {
    this.name = name;
  }
}

// copy the methods
Object.assign(User.prototype, sayHiMixin);
Object.assign(User.prototype, sayByeMixin);


const user = new User("Dude")

user.sayHi()
user.sayBye()