A JavaScript construct for having private instance fields

PrivateFactory.js

function PrivateFactory(scope) {
  scope._ = function(fn) {
    return function() {
      return fn.apply(scope, arguments)
    }
  }
}

var Greeter = (function() { 

  function myPrivateMethod(greeting) {
    return greeting + ", " + this.name;
  }
  
  function Greeter(name) {
    PrivateFactory(this);
    this.name = name;
  } 

  Greeter.prototype.greet = function greet(greeting) {
    console.log(this._(myPrivateMethod)('Hello'));
  }

  return Greeter;
})();

var greeter = new Greeter('John');
greeter.greet('Hello'); // output: "Hello, John"