Testing private functions using eval

I'm writing a node module and I want to be able to unit test all my functions, however I have not exposed many of them (shown below).

module.exports = function() {
    var privateFunction = function() {
         // Do Stuff Here
    };
    return {
        publicFunction: function() { 
            // do stuff
            privateFunction();
        } 
    }
} 

I read on another stack question that eval could be used if my private functions were moved to the global scope, that way my functions would be interpreted and could be tested. Despite eval being generally considered bad practice, I can't really see another method of testing these functions.

Does anyone know of a better method to test these.