Accessing private member variables from prototype-

Question

Is there any way to make “private” variables (those defined in the constructor), available to prototype-defined methods?

TestClass = function(){
    var privateField = "hello";
    this.nonProtoHello = function(){alert(privateField)};
};
TestClass.prototype.prototypeHello = function(){alert(privateField)};

This works:

t.nonProtoHello()

But this doesn’t:

t.prototypeHello()

I’m used to defining my methods inside the constructor, but am moving away from that for a couple reasons.

Answer 1

Yes, it's possible. PPF design pattern just solves this.

PPF stands for Private Prototype Functions. Basic PPF solves these issues:

1. Prototype functions get access to private instance data.
2. Prototype functions can be made private.

For the first, just:

1. Put all private instance variables you want to be accessible from prototype functions inside a separate data container, and
2. Pass a reference to the data container to all prototype functions as a parameter.

It's that simple. For example:

// Helper class to store private data.
function Data() {};

// Object constructor
function Point(x, y)
{
  // container for private vars: all private vars go here
  // we want x, y be changeable via methods only
  var data = new Data;
  data.x = x;
  data.y = y;

  ...
}

// Prototype functions now have access to private instance data
Point.prototype.getX = function(data)
{
  return data.x;
}

Point.prototype.getY = function(data)
{
  return data.y;
}

...

Read the full story here:

PPF Design Pattern

Answer 2

You can actually achieve this by using Accessor Verification:

(function(key, global) {
  // Creates a private data accessor function.
  function _(pData) {
    return function(aKey) {
      return aKey === key && pData;
    };
  }

  // Private data accessor verifier.  Verifies by making sure that the string
  // version of the function looks normal and that the toString function hasn't
  // been modified.  NOTE:  Verification can be duped if the rogue code replaces
  // Function.prototype.toString before this closure executes.
  function $(me) {
    if(me._ + '' == _asString && me._.toString === _toString) {
      return me._(key);
    }
  }
  var _asString = _({}) + '', _toString = _.toString;

  // Creates a Person class.
  var PersonPrototype = (global.Person = function(firstName, lastName) {
    this._ = _({
      firstName : firstName,
      lastName : lastName
    });
  }).prototype;
  PersonPrototype.getName = function() {
    var pData = $(this);
    return pData.firstName + ' ' + pData.lastName;
  };
  PersonPrototype.setFirstName = function(firstName) {
    var pData = $(this);
    pData.firstName = firstName;
    return this;
  };
  PersonPrototype.setLastName = function(lastName) {
    var pData = $(this);
    pData.lastName = lastName;
    return this;
  };
})({}, this);

var chris = new Person('Chris', 'West');
alert(chris.setFirstName('Christopher').setLastName('Webber').getName());

This example comes from my post about Prototypal Functions & Private Data and is explained in more detail there.

Answer 3

You can use a prototype assignment within the constructor definition.

The variable will be visible to the prototype added method but all the instances of the functions will access the same SHARED variable.

function A()
{
  var sharedVar = 0;
  this.local = "";

  A.prototype.increment = function(lval)
  {    
    if (lval) this.local = lval;    
    alert((++sharedVar) + " while this.p is still " + this.local);
  }
}

var a = new A();
var b = new A();    
a.increment("I belong to a");
b.increment("I belong to b");
a.increment();
b.increment();

I hope this can be usefull.

Answer 4

Update: With ES6, there is a better way:

Long story short, you can use the new Symbol to create private fields.
Here's a great description: https://curiosity-driven.org/private-properties-in-javascript

Example:

var Person = (function() {
    // Only Person can access nameSymbol
    var nameSymbol = Symbol('name');

    function Person(name) {
        this[nameSymbol] = name;
    }

    Person.prototype.getName = function() {
        return this[nameSymbol];
    };

    return Person;
}());

For all modern browsers with ES5:

You can use just Closures

The simplest way to construct objects is to avoid prototypal inheritance altogether. Just define the private variables and public functions within the closure, and all public methods will have private access to the variables.

Or you can use just Prototypes

In JavaScript, prototypal inheritance is primarily an optimization. It allows multiple instances to share prototype methods, rather than each instance having its own methods.
The drawback is that this is the only thing that's different each time a prototypal function is called.
Therefore, any private fields must be accessible through this, which means they're going to be public. So we just stick to naming conventions for _private fields.

Don't bother mixing Closures with Prototypes

I think you shouldn't mix closure variables with prototype methods. You should use one or the other.

When you use a closure to access a private variable, prototype methods cannot access the variable. So, you have to expose the closure onto this, which means that you're exposing it publicly one way or another. There's very little to gain with this approach.

Which do I choose?

For really simple objects, just use a plain object with closures.

If you need prototypal inheritance -- for inheritance, performance, etc. -- then stick with the "_private" naming convention, and don't bother with closures.

I don't understand why JS developers try SO hard to make fields truly private.

Answer 5

I'm late to the party, but I think I can contribute. Here, check this out:

// 1. Create closure
var SomeClass = function() {
  // 2. Create `key` inside a closure
  var key = {};
  // Function to create private storage
  var private = function() {
    var obj = {};
    // return Function to access private storage using `key`
    return function(testkey) {
      if(key === testkey) return obj;
      // If `key` is wrong, then storage cannot be accessed
      console.error('Cannot access private properties');
      return undefined;
    };
  };
  var SomeClass = function() {
    // 3. Create private storage
    this._ = private();
    // 4. Access private storage using the `key`
    this._(key).priv_prop = 200;
  };
  SomeClass.prototype.test = function() {
    console.log(this._(key).priv_prop); // Using property from prototype
  };
  return SomeClass;
}();

// Can access private property from within prototype
var instance = new SomeClass();
instance.test(); // `200` logged

// Cannot access private property from outside of the closure
var wrong_key = {};
instance._(wrong_key); // undefined; error logged

I call this method accessor pattern. The essential idea is that we have a closure, a key inside the closure, and we create a private object (in the constructor) that can only be accessed if you have the key.

If you are interested, you can read more about this in my article. Using this method, you can create per object properties that cannot be accessed outside of the closure. Therefore, you can use them in constructor or prototype, but not anywhere else. I haven't seen this method used anywhere, but I think it's really powerful.