This is a stack implemented without assignment statements.

gistfile1.js

var Stack = function () {
    return {
        pop: function() {
            throw "popped from an empty stack";
        },
        push: function(value) {
            return (function (self) {
                return _.extend({}, self._incrementCount(), {
                    head: value, 
                    pop: function () {
                        return self;
                    }
                }); 
            })(this);
        
        },
        _incrementCount: function () {
            return _.extend({}, this, { count: this._getCount() + 1 });
        },
        _getCount: function () {
            if (this.count) {
                return this.count;
            } else {
                return 0;
            }
        },
        peek: function () {
            return this.head;
        }
    };
};

var bob = new Stack;
var jane = bob.push(3).push(4).pop();
console.log(jane.peek());
console.log(jane.peek());

Here is a fiddle to play with.

WHO IS JANE AND WHAT IS SHE DOING WITH BOB????????