JS: Simple OOP private/public methods and properties

oop_in_javascript.js

var helper = function (element) {
    /**
     * [private] Base HTML div
     * @type {HTMLDivElement}
     */
    var element = element;

    /**
     * [public] Count of base element childrens
     * @type {Number}
     */
    this.count = element.children.length;

    /**
     * [private] Returns a random token
     * @return {string} 
     */
    var encrypt = function () {
        return Date()
            .toString()
            .replace(/\W+/g, '')
            .toLowerCase();
    };

    /**
     * [public] Executes the private "encrypt" method
     * @return {string} A random token
     */
    this.e = function () {
        return encrypt();
    };
};


 // Usage:
 var mainElement = document.querySelector("#main");
 var subElement = document.querySelector("#sub");
 var mainHelper = new helper(mainElement);
 // -> helper {count: 12}
 var subHelper = new helper(subElement);
 // -> helper {count: 4}
 
 mainHelper.e();
 // -> "monsep192016205438gmt0500cot"
 
 subElement.e();
 // -> "monsep192016205847gmt0500cot"
 
 mainHelper.encrypt();
 // -> Uncaught TypeError: a.encrypt is not a function(…)