Pure JavaScript - Private and public methods

Have you been creating your own classes in pure JavaScript? What if you neeed a private method? How to create them?

var MyClass = (function() {
        function MyClass() {
        }

        var prvMethod = function() {
               console.log('method');
        };

        MyClass.prototype = {
            init: function() {
                console.log('init ');
                prvMethod();

            }

        };

        return MyClass;
    })();

var mc = new MyClass();
mc.init();

So finally you can access init() but cannot access prvMethod(). It has only access into MyClass() function scope.