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()