Example of Calculator Constructor for https://github.com/blazingcloud/javascript_lessons/tree/master/05_calculator

calc.js

// Calculator Constructor
var Calculator = function() {

    //private
    var q = [0]; 

    var makeRpnFunction = function(op) {
        return function() {
            var v1 = q.pop();
            if (v1 == undefined) {
                throw new Error('calculator is empty');
            } else {
                var v2 = q.pop();
                if (v2 == undefined) {
                    q.push(v1); /* put v1 back if we fail to get a v2 */
                    throw new Error('calculator is empty');
                } else {
                    q.push(op(v2, v1));
                };  
            };  
        };    
    };  

    //public
    this.value = function() {
        return q[q.length-1];
    };  

    this.add = function(v) {
        q[q.length-1] += v;
    };  

    this.subtract = function(v) {
        q[q.length-1] -= v;
    };  

    this.push = function(v) {
        q.push(v);
    };  

    this.plus = makeRpnFunction( function(a, b) { return a + b; } );
    this.minus = makeRpnFunction( function(a, b) { return a - b; } );
    this.divide = makeRpnFunction( function(a, b) { return a / b; } );
    this.times = makeRpnFunction( function(a, b) { return a * b; } );
};


// I've added one more item to the spec - the implementation above meets this requirement also:

describe("using reverse polish notation", function() {
    // ...
    it("does not change value if an RPN operation fails when < 2 values are on the stack", function() {
        expect(function() {
            calculator.plus();
        }).toThrow("calculator is empty");
        expect(calculator.value()).toEqual(0);
    });
});