Private Variables in Module Pattern

Question

I am not clear about the concept of private, if I can still access it through the public method and redefine the properties of the module. I mean, I can perfectly do:

var aModule = (function() {

    var privateVar = 1;

    return {
        publicFunction: function() {
            return privateVar;
        }
    }

})();

aModule.publicFunction = function() {
    privateVar = function() {
        console.log('a new content');
    }
    privateVar();
};

aModule.publicFunction(); // 'a new content'

I understand that this is not possible if I write it in ES6 with let or const, because it would give me error try to overwrite the value of the private variable, but what sense does it have in ES5?

Answer 1

A private variable cannot be accessed or changed by code that's outside the module or class that owns the private variable.

For example, you can't do aModule.privateVar and expect it to give you anything back.

What your publicFunction is is what the Java (and other programming languages) world would call a "getter". Simply put it gives access to the value of the private variable, without allowing write access to it.

In your last example, you're not actually overwriting the private variable. You're just creating a new variable within publicFunction's scope and assigning a value to that. Just because it's also named privateVar doesn't mean it's the same area of memory.

I've added to your code to demonstrate this

var aModule = (function() {

        var privateVar = 1;

        return {
            publicFunction: function() {
                return privateVar;
            },
            getPrivateVar() {
                return privateVar;
            }
        }

    })();

    aModule.publicFunction = function() {
        privateVar = function() {
            console.log('a new content');
        }
        privateVar();
     };

     aModule.publicFunction(); // 'a new content'
     console.log(aModule.getPrivateVar()); //outputs 1

To give more detail on why this is, it's all about scope. privateVar exists in an anonymous function's scope. This anonymous function returns an object with several functions defined on it. When the function is called, this object is assigned to aModule, but retains access to privateVar because they share scope.

However, outside of that function we're at a different scope, which doesn't have access to the aModule's variables, except those exposed in the returned object