Create objects in object-oriented javascript

In short, you can use private variables when you return another scope when declaring a class.

1. function Cats() {
2.   var nameList = []; // private var
3.        
4.   // This is where you define another scope!
5.   return {
6.     add:function(name) {
7.       nameList.push(name);
8.     }
9.   }     
10. }

How does it work?

The magic lies in creating a different scope at the end of the class definition that does not include private variables. Then, private members are available in this scope and not outside of it, thanks to the power of closures.

Differences between private and public

These two classes definition shows the difference between the a class where all members are public versus a class where some members are private.
This is a class where all members are public.

1. function PublicCats() {
2.   // This is the list of cat names
3.   this.nameList = [];
4.  
5.   // This is a method that I would like to be private but can’t
6.   // It returns the last cat of the list
7.   this.lastCat = function() {
8.     return this.nameList[this.nameList.length-1];
9.   }
10.  
11.   // Return the list of names
12.   this.names = function() {
13.     return this.nameList;
14.   }
15.  
16.   // Add a name to the list
17.   this.add = function(name) {
18.     this.nameList.push(name);
19.    
20.     // Return the last cat just added
21.     return this.lastCat();
22.   }  
23. }

This is the corresponding class where some members are private.

1. function PrivateCats() {
2.   // This is the list of cat names
3.   var nameList = [];
4.  
5.   // This is a private method
6.   var lastCat = function() {
7.     // Note : I don’t use "this" to access private variables
8.     // thanks to the power of closures!
9.     return nameList[nameList.length-1];
10.   }
11.  
12.   // These are our public methods!
13.   // This is where we create another scope to
14.   // avoid external objects to use the private variables.
15.   return {
16.     add:function(name) {
17.       // Note : once again, I don’t use "this"
18.       // to access the private variables and methods
19.       nameList.push(name);
20.       return lastCat();
21.     },
22.     names:function() {
23.       return nameList;
24.     }
25.   }  
26. }

In the above code, line 15 makes all the difference between the two classes.