The specs below rely on Jasmine (http://pivotal.github.io/jasmine/). Just a little test with the Jasmine framework as I haven't really looked at it for more than a year now.

Currying.js

function addTwoNumbers(x, y) {
    var oldX = x, oldY = y;    

    if (typeof oldY === "undefined") {
        return function(newY) {
            return oldX + newY;
        };
    }

    return x + y;
}

CurryingSpecs.js

describe("Currying", function() {   

    it("should calculate sums correctly", function () {

    });

    describe("when 2 numbers are passed", function() {
        var result = addTwoNumbers(10, 20);

        it("the correct total should be calculated", function() {
            expect(result).toBe(30);
        });        
    });

    describe("when only 1 number is passed", function() {
        var result = addTwoNumbers(10);

        it("a function should be returned that will have the original passed in a closure", function () {
            var isAFunction = helpers.isFunction(result);

            expect(isAFunction).toBe(true);
        });

        it("if another number is passed to the function returned, it should return the sum of the partially applied original and the new number", function() {
            var newResult = result(10);

            expect(newResult).toBe(20);
        });
    });
    
    
});

var helpers = {
    isFunction: function(functionToCheck) {
        var getType = {};
        return functionToCheck && getType.toString.call(functionToCheck) === '[object Function]';
    }
};