Normally, I've seen prototype functions declared outside the class definition, like this:

function Container(param) {
    this.member = param;
}
Container.prototype.stamp = function (string) {
    return this.member + string;
}

var container1 = new Container('A');
alert(container1.member);
alert(container1.stamp('X'));

This code produces two alerts with the values "A" and "AX".

I'd like to define the prototype function INSIDE of the class definition. Is there anything wrong with doing something like this?

function Container(param) {
    this.member = param;
    if (!Container.prototype.stamp) {
        Container.prototype.stamp = function() {
            return this.member + string;
        }
    }
}

I was trying this so that I could access a private variable in the class. But I've discovered that if my prototype function references a private var, the value of the private var is always the value that was used when the prototype function was INITIALLY created, not the value in the object instance:

Container = function(param) {
    this.member = param;
    var privateVar = param;
    if (!Container.prototype.stamp) {
        Container.prototype.stamp = function(string) {
            return privateVar + this.member + string;
        }
    }
}
var container1 = new Container('A');
var container2 = new Container('B');
alert(container1.stamp('X'));
alert(container2.stamp('X'));

This code produces two alerts with the values "AAX" and "ABX". I was hoping the output would be "AAX" and "BBX". I'm curious why this doesn't work, and if there is some other pattern that I could use instead.