Why do we need private variables?

Question

What are they for and how do we make it? Can you give me an example?

Answer 1

To avoid collisions with multiple libraries, for example.

Say they both use a variable commonly used such as data. If both libraries use private variables it's all fine:

var lib1 = (function() {
    var data;

    return {
        get: function()  { return data },
        set: function(v) { data = v }
    };
})();

// Supposed to do something different:
var lib2 = (function() {
    var data;

    return {
        get: function()  { return data },
        set: function(v) { data = v }
    };
})();

lib1.set(123);
lib2.set(456);
lib1.get(); // 123
lib2.get(); // 456

However suppose they don't use private variables but global ones like this:

var lib1 = (function() {
    return {
        get: function()  { return data },
        set: function(v) { data = v }
    };
})();

// Supposed to do something different:
var lib2 = (function() {
    return {
        get: function()  { return data },
        set: function(v) { data = v }
    };
})();

lib1.set(123);
lib2.set(456);
lib1.get(); // 456 - overwritten by lib2. lib1 might not work properly anymore.
lib2.get(); // 456

So lib1.get() will fetch the same data as lib2.get().

This example is too obvious of course but to stay safe it's a good practice to use private variables.

Answer 2

Variables are encapsulated within a class to stop their names colliding. These can be public or private. Sometimes there is a need to make sure that variables are only changed using the functions that set them. For example the parts of a date would need to be verified to stop someone setting an invalid date such aas February 45th.

var factorial = (function(){
    var precog = [1,1];// ===undefined for other indices, N = undefined || N
    return function(y){  
        return precog[y] || (precog[y]=y*arguments.callee(y-1));
    };
})();

Here is a JavaScript function with a private precog. This stores previously calculated values and it is private to stop them being manipulated.