Question

I came up with the situation below:

function Dog () {
    "use strict";
    this.age = 1;
    var name = "Fido";
    this.getName = function () { return name; }
}

And now I’m creating a new instance of the “Dog” class and printing the variable’s values.

var d = new Dog;
document.write('<strong>Dog age:</strong> ' +d.age); \\Outputs "1" as expected
document.write('<br/>');
document.write('<strong>Dog name:</strong> ' +d.name); \\Outputs "undefined" as expected, 'cause it's a private variable.
document.write('<br/>');
document.write('<strong>Get Dog name:</strong> ' +d.getName()); \\Outputs "Fido", as expected.

But let’s say I want to change the Dog’s name, like this:

d.name = "Stinky";
document.write('<br/>');
document.write('<strong>Dog name Again:</strong> ' +d.name); 
document.write('<br/>');
document.write('<strong>Get Dog name Again:</strong> ' +d.getName());

Based on this, I got a couple of questions:

1. Why on earth “d.name” didn’t showed me “undefined”? Isn’t “name” a private variable? I suppose you can’t change private variables values, am I right? I wonder if this process have created a new variable, but this time, a public one, and with the same name. If so, is there a way to prevent the creation of new variables every time I try to assign a new property with the same name? Is there a way to throw a “type error” or something (well, this was what I expected).
2. And at last: Why “getName” printed the original value, “Fido”, even after I assigned a new value to it ?

Any ideas?

Here’s a fid to make things easier.

http://fiddle.jshell.net/yZpfg/2/

Answer 1

d.name = "Stinky"; is adding a new (public) property to to the d object, which is an instanceOf Dog.

the getter still references the (private) variable with the value Fido.

If you want to allow a consumer to change a private variable, you need a setter as well:

function Dog () {
    "use strict";
    this.age = 1;
    var name = "Fido";
    this.getName = function () { return name; }
    this.setName = function (value) { name = value; }
}

var d = new Dog(); 
d.name; // undefined because there is no name public property
d.getName() // returns the internal private, "Fido"
d.setName('Stinky'); // the internal private is now "Stinky"