function Action() {
    this.propertyPublic = 'public';
    var propertyPrivate = 'private';
    this.getPropertyPrivate = function() {
        return propertyPrivate;
    };
}

var someAction = new Action();
console.log(someAction.propertyPublic); // "public"
console.log(someAction.propertyPrivate); // undefined
console.log(someAction.getPropertyPrivate()); // "private"