Question

I have the following javascript

function person() {
  //private Variable
  var fName = null;
  var lName = null;

  // assign value to private variable
  fName = "Dave";
  lName = "Smith";
};

person.prototype.fullName = function () {
  return this.fName + " " + this.lName;
};

var myPerson = new person();
alert(myPerson.fullName());

I am trying to get an understanding of object orientated techniques in javascript. I have a simple person object and added a function to its prototype.

I was expecting the alert to have “Dave Smith”, however I got "underfined underfined". why is that and how do I fix it?

Answer 1

Unfortunately you can’t access a private variable. So either you change it to a public property or you add getter/setter methods.

function person() {

    //private Variable
    var fName = null;
    var lName = null;

    // assign value to private variable
    fName = "Dave";
    lName = "Smith";

    this.setFName = function(value){ fName = value; };
    this.getFName = function(){ return fName; }
};

see javascript – accessing private member variables from prototype-defined functions

---

But actually this looks like what you are looking for:
Javascript private member on prototype

from that SO post:

As JavaScript is lexically scoped, you can simulate this on a per-object level by using the constructor function as a closure over your ‘private members’ and defining your methods in the constructor, but this won’t work for methods defined in the constructor’s prototype property.

in your case:

var Person = (function() {
    var store = {}, guid = 0;

    function Person () {
        this.__guid = ++guid;
        store[guid] = { 
            fName: "Dave",
            lName: "Smith"
        };
    }

    Person.prototype.fullName = function() {
        var privates = store[this.__guid];
        return privates.fName + " " + privates.lName;
    };

    Person.prototype.destroy = function() {
        delete store[this.__guid];
    };

    return Person; 
})();


var myPerson = new Person();

alert(myPerson.fullName());

// in the end, destroy the instance to avoid a memory leak
myPerson.destroy();

Check out the live demo at http://jsfiddle.net/roberkules/xurHU/