proxy.js

/*  Ejemplo base  */

class subject{

  hello(){
    return 'hello';
  }

  goodbye(){
    return 'Goodbye people!';
  }

}

class Proxy(subject){

  constructor(subject){
    this.subject = subject;
  }

  hello(){
    return this.subject.hello()+' world!';
  }

  goodbye(){
    return this.subject.goodbye();
  }

}

let tmpsubj = new subject();
let proxy   = new Proxy(tmpsubj);

console.log(proxy.hello());
console.log(proxy.goodbye());

/*  Ejemplo Data Validation */

class calculator{

  sumCalc(a,b){
    return a+b;
  }

  difCalc(a,b){
    return a-b;
  }

}

class validatorCalc{

  constructor(subject){
    this.subject = subject;
  }

  sumar(numA,numB){

    if ((typeof numA=="number")&&(typeof numB=="number"))
      return this.subject.sumCalc(numA,numB);
    else
      throw "Parametros no numericos";

  }

  restar(numA,numB){

    if ((typeof numA=="number")&&(typeof numB=="number"))
      return this.subject.difCalc(numA,numB);
    else
      throw "Parametros no numericos";

  }

}

let calculator = new calculator();
let validator  = new validatorCalc(calculator);

console.log(validator.sumar(2,5));
console.log(validator.restar(80,5));


/*  Ejemplo log  */

function createLoggingWritable(writableOrig) {

 var proto = Object.getPrototypeOf(writableOrig);

 function LoggingWritable(subject){
  this.writableOrig = writableOrig;
 }

 LoggingWritable.prototype = Object.create(proto);

 LoggingWritable.prototype.write = function(chunk, encoding, callback) {

    if(!callback && typeof encoding === 'function') {
      callback = encoding;
      encoding = undefined;
    }

    console.log('Writing ', chunk);

    return this.writableOrig.write(chunk, encoding, function() {
      console.log('Finished writing ', chunk);
      callback && callback();
    });
 };

 LoggingWritable.prototype.on = function() {
  return this.writableOrig.on.apply(this.writableOrig, arguments);
 };

 LoggingWritable.prototype.end = function() {
  return this.writableOrig.end.apply(this.writableOrig, arguments);
 }

 return new LoggingWritable(this.writableOrig);

}

var fs = require('fs');
var writable = fs.createWriteStream('test.txt');
var writableProxy = createProxy(writable);
writableProxy.write('First chunk');
writableProxy.write('Second chunk');
writable.write('This is not logged');
writableProxy.end();