user_serializer.js

class UserSerializer {

    constructor(user) {
        this.user = user;
    }
    
    serialize() {
      return (
          {
            name: this.user.name,
            age: this.user.age,
            username: this.user.username
           }
      );
    }

}

user = { name: 'Leandro', age: 33, username: 'leandromarques', position: 'software engineer' };

// você chamaria assim
new UserSerializer(user).serialize();