A method to create a truly private method in Javascript

Private Method in JS

var MyClass;

function() {
    MyClass = function() {
        this.prop = {};
    }
    
    //private method
    function getProp() {
        return this.prop;
    }
    
    //public method
    MyClass.prototype.foo = function() {
        return "foo " + getProp.call(this); //how to call the private method.
    }
}