how to have Javascript Object creation pattern with Reusable methods and private properties?

Question

Considering object creation patterns with private properties, one way to do is :

function MyStack (){
    var list = [],
        index = 0;

    this.push =  function(val){ 
        return list[index++] = val;
    };
    this.pop = function(){// ...}
}

var stack1 = new MyStack();       stack1.push(5);
var stack2 = new MyStack();       stack2.push(11);

Problem with this: Every instance of Stack has it's own copy of methods 'push' and 'pop'.

Another way for implementing constructor method is:

function MyStack(){ 
    this.list = []; 
    this.index = 0;
}
MyStack.prototype = {
    insert: function(val){
            return this.list[this.index++] = val;
        },
    pop:function(){//...}
}

Problem here: We lose the privacy of list and index.

Is there a way, such that we can have both methods reuse among instances and privacy of properties ?

I understand that we can have this for methods that don't operate on any state of the object, but I am talking more about those methods that do operate on the state.

Answer 1

Yes. I've edited this code so it's actually fully functional as you had intended it to work. It seems a bit redundant to me, but, it does provide you the ability to provide a public interface, but to keep your variables private and control the way the user interacts with them.

function MyStack(){ 
    var list = []; 
    var index = 0;

    this.getIndex = function(){
        return index;
    }
    this.setIndex = function(val){
        index = val;
    }
    this.list = function(val){
        if(val){
           // setter if a value was provided. Illustrating how you can control
           // index, which I assume is the point of having these things private 
           // to begin with
           return list[this.setIndex(this.getIndex() + 1)] = val;
        }

        // always return list - acts like a getter
        return list;
    }
}
MyStack.prototype = {
    insert: function(val){
            return this.list(val);
        },
    pop:function(){}
}

var stack1 = new MyStack();       
stack1.insert(5);
var stack2 = new MyStack();       
stack2.insert(11);

Answer 2

You should check out John Resig's Simple Javascript Inheritance. It is a great read, and it has been extended to provide support for privates, aptly called Privates.js;