Ruby's DelegateClass, implemented in ES6

delegate_class.js

// See http://ruby-doc.org/stdlib-2.3.1/libdoc/delegate/rdoc/Object.html for more information

// Some example classes...
class User {
  constructor(username) {
    this.username = username;
  }
}

class Server {
  constructor(id) {
    this.id = id;
  }
}

// This is where the magic happens
class DelegateClass {
  constructor(target) {
    function _generateDelegationHandler(_target) {
      return {
        get: function(__target, name) {
          return name in __target
            ? __target[name]
            : name in _target
              ? _target[name]
              : undefined;
        }
      };
    }
    
    this._delegate = new Proxy(this, _generateDelegationHandler(target));
  }
}

// Note that, unlike in Ruby, you don't need to specify the delegation target type.
class Member extends DelegateClass {
  constructor(user, server) {
    // Initialise the DelegateClass, like you would in Ruby
    super(user);
    
    // Do other initialisation steps
    this.user = user;
    this.server = server;
    
    // This is required
    return this._delegate;
  }
  
  get serverID() {
    return this.server.id;
  }
}

var izy = new User("izy521");
console.log(izy.username);

var discordAPI = new Server("81384788765712384");
var izyMember = new Member(izy, discordAPI);
console.log(izyMember.serverID);
console.log(izyMember.username);