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

There is a very simple way to do this

function SharedPrivate(){
  var private = "secret";
  this.constructor.prototype.getP = function(){return private}
  this.constructor.prototype.setP = function(v){ private = v;}
}

var o1 = new SharedPrivate();
var o2 = new SharedPrivate();

console.log(o1.getP()); // secret
console.log(o2.getP()); // secret
o1.setP("Pentax Full Frame K1 is on sale..!");
console.log(o1.getP()); // Pentax Full Frame K1 is on sale..!
console.log(o2.getP()); // Pentax Full Frame K1 is on sale..!
o2.setP("And it's only for $1,795._");
console.log(o1.getP()); // And it's only for $1,795._

JavaScript prototypes are golden.

Answer 2

var getParams = function(_func) {
  res = _func.toString().split('function (')[1].split(')')[0].split(',')
  return res
}

function TestClass(){

  var private = {hidden: 'secret'}
  //clever magic accessor thing goes here
  if ( !(this instanceof arguments.callee) ) {
    for (var key in arguments) {
      if (typeof arguments[key] == 'function') {
        var keys = getParams(arguments[key])
        var params = []
        for (var i = 0; i <= keys.length; i++) {
          if (private[keys[i]] != undefined) {
            params.push(private[keys[i]])
          }
        }
        arguments[key].apply(null,params)
      }
    }
  }
}


TestClass.prototype.test = function(){
  var _hidden; //variable I want to get
  TestClass(function(hidden) {_hidden = hidden}) //invoke magic to get
};

new TestClass().test()

How's this? Using an private accessor. Only allows you to get the variables though not to set them, depends on the use case.

Answer 3

When I read this, it sounded like a tough challenge so I decided to figure out a way. What I came up with was CRAAAAZY but it totally works.

First, I tried defining the class in an immediate function so you'd have access to some of the private properties of that function. This works and allows you to get some private data, however, if you try to set the private data you'll soon find that all the objects will share the same value.

var SharedPrivateClass = (function(){ // use immediate function
    // our private data
    var private = "Default";

    // create the constructor
    function SharedPrivateClass () {}

    // add to the prototype
    SharedPrivateClass.prototype.getPrivate = function () {
        // It has access to private vars from the immediate function!
        return private;
    }

    SharedPrivateClass.prototype.setPrivate = function (value) {
        private = value;
    }

    return SharedPrivateClass;
})();

var a = new SharedPrivateClass();
console.log("a:", a.getPrivate()); // "a: Default"

var b = new SharedPrivateClass();
console.log("b:", b.getPrivate()); // "b: Default"

a.setPrivate("foo"); // a Sets private to 'foo'
console.log("a:", a.getPrivate()); // "a: foo"
console.log("b:", b.getPrivate()); // oh no, b.getPrivate() is 'foo'!

console.log(a.hasOwnProperty("getPrivate")); // false. belongs to the prototype
console.log(a.private); // undefined

There are plenty of cases where this would be adequate like if you wanted to have constant values like event names that get shared between instances. But essentially, they act like private static variables.

If you absolutely need access to variables in a private namespace from within your methods defined on the prototype, you can try this pattern.

var PrivateNamespaceClass = (function(){  // immediate function
    var instance = 0, // counts the number of instances
        defaultName = "Default Name",  
        p = []; // an array of private objects

    // careate the constructor
    function PrivateNamespaceClass () {
        // Increment the instance count and save it to the instance. 
        // This will become your key to your private space.
        this.i = instance++; 

        // Create a new object in the private space.
        p[this.i] = {};
        // Define properties or methods in the private space.
        p[this.i].name = defaultName;

        console.log("New instance " + this.i);        
    }
    PrivateNamespaceClass.prototype.getPrivateName = function () {
        // It has access to the private space and it's children!
        return p[this.i].name;
    }
    PrivateNamespaceClass.prototype.setPrivateName = function (value) {
        // Because you use the instance number assigned to the object (this.i)
        // as a key, the values set will not change in other instances.
        p[this.i].name = value;
        return "Set " + p[this.i].name;
    }

    return PrivateNamespaceClass;
})();

var a = new PrivateNamespaceClass();
console.log(a.getPrivateName()); // Default Name

var b = new PrivateNamespaceClass();
console.log(b.getPrivateName()); // Default Name

console.log(a.setPrivateName("A"));
console.log(b.setPrivateName("B"));
console.log(a.getPrivateName()); // A
console.log(b.getPrivateName()); // B

console.log(a.privateNamespace); // undefined

I'd love some feedback from anyone who sees an error with this way of doing it.

Answer 4

Here's what I came up with.

(function () {
    var staticVar = 0;
    var yrObj = function () {
        var private = {"a":1,"b":2};
        var MyObj = function () {
            private.a += staticVar;
            staticVar++;
        };
        MyObj.prototype = {
            "test" : function () {
                console.log(private.a);
            }
        };

        return new MyObj;
    };
    window.YrObj = yrObj;
}());

var obj1 = new YrObj;
var obj2 = new YrObj;
obj1.test(); // 1
obj2.test(); // 2

the main problem with this implementation is that it redefines the prototypes on every instanciation.

Answer 5

Was playing around with this today and this was the only solution I could find without using Symbols. Best thing about this is it can actually all be completely private.

The solution is based around a homegrown module loader which basically becomes the mediator for a private storage cache (using a weak map).

   const loader = (function() {
        function ModuleLoader() {}

    //Static, accessible only if truly needed through obj.constructor.modules
    //Can also be made completely private by removing the ModuleLoader prefix.
    ModuleLoader.modulesLoaded = 0;
    ModuleLoader.modules = {}

    ModuleLoader.prototype.define = function(moduleName, dModule) {
        if (moduleName in ModuleLoader.modules) throw new Error('Error, duplicate module');

        const module = ModuleLoader.modules[moduleName] = {}

        module.context = {
            __moduleName: moduleName,
            exports: {}
        }

        //Weak map with instance as the key, when the created instance is garbage collected or goes out of scope this will be cleaned up.
        module._private = {
            private_sections: new WeakMap(),
            instances: []
        };

        function private(action, instance) {
            switch (action) {
                case "create":
                    if (module._private.private_sections.has(instance)) throw new Error('Cannot create private store twice on the same instance! check calls to create.')
                    module._private.instances.push(instance);
                    module._private.private_sections.set(instance, {});
                    break;
                case "delete":
                    const index = module._private.instances.indexOf(instance);
                    if (index == -1) throw new Error('Invalid state');
                    module._private.instances.slice(index, 1);
                    return module._private.private_sections.delete(instance);
                    break;
                case "get":
                    return module._private.private_sections.get(instance);
                    break;
                default:
                    throw new Error('Invalid action');
                    break;
            }
        }

        dModule.call(module.context, private);
        ModuleLoader.modulesLoaded++;
    }

    ModuleLoader.prototype.remove = function(moduleName) {
        if (!moduleName in (ModuleLoader.modules)) return;

        /*
            Clean up as best we can.
        */
        const module = ModuleLoader.modules[moduleName];
        module.context.__moduleName = null;
        module.context.exports = null;
        module.cotext = null;
        module._private.instances.forEach(function(instance) { module._private.private_sections.delete(instance) });
        for (let i = 0; i < module._private.instances.length; i++) {
            module._private.instances[i] = undefined;
        }
        module._private.instances = undefined;
        module._private = null;
        delete ModuleLoader.modules[moduleName];
        ModuleLoader.modulesLoaded -= 1;
    }


    ModuleLoader.prototype.require = function(moduleName) {
        if (!(moduleName in ModuleLoader.modules)) throw new Error('Module does not exist');

        return ModuleLoader.modules[moduleName].context.exports;
    }



     return new ModuleLoader();
    })();

    loader.define('MyModule', function(private_store) {
        function MyClass() {
            //Creates the private storage facility. Called once in constructor.
            private_store("create", this);


            //Retrieve the private storage object from the storage facility.
            private_store("get", this).no = 1;
        }

        MyClass.prototype.incrementPrivateVar = function() {
            private_store("get", this).no += 1;
        }

        MyClass.prototype.getPrivateVar = function() {
            return private_store("get", this).no;
        }

        this.exports = MyClass;
    })

    //Get whatever is exported from MyModule
    const MyClass = loader.require('MyModule');

    //Create a new instance of `MyClass`
    const myClass = new MyClass();

    //Create another instance of `MyClass`
    const myClass2 = new MyClass();

    //print out current private vars
    console.log('pVar = ' + myClass.getPrivateVar())
    console.log('pVar2 = ' + myClass2.getPrivateVar())

    //Increment it
    myClass.incrementPrivateVar()

    //Print out to see if one affected the other or shared
    console.log('pVar after increment = ' + myClass.getPrivateVar())
    console.log('pVar after increment on other class = ' + myClass2.getPrivateVar())

    //Clean up.
    loader.remove('MyModule')

Answer 6

I suggest it would probably be a good idea to describe "having a prototype assignment in a constructor" as a Javascript anti-pattern. Think about it. It is way too risky.

What you're actually doing there on creation of the second object (i.e. b) is redefining that prototype function for all objects that use that prototype. This will effectively reset the value for object a in your example. It will work if you want a shared variable and if you happen to create all of the object instances up front, but it feels way too risky.

I found a bug in some Javascript I was working on recently that was due to this exact anti-pattern. It was trying to set a drag and drop handler on the particular object being created but was instead doing it for all instances. Not good.

Doug Crockford's solution is the best.