Saturday, August 22, 2015

JavaScript private methods and variables

Often I am told that JavaScript has no real private methods and variables the same way as this is for Python or Perl.

Although, that is true for Python and Perl (all scopes and their content are publicly accessible and there is always a way to list and access the content) it is not true for JavaScript.

JavaScript have one of the most powerful scoping technique available for a main-stream programing language and provides you with the ability to hide and make private some content if needed.

Let me show you an example:

function MyClass() {
    var fullyPrivateVar = 'xxxx';
    this.notFullyPrivateVar = 'yyyy';

    function privateMethod() {
        return fullyPrivateVar;

    this.publicMethodWithPrivateAccess = function() { return privateMethod() }

MyClass.prototype.publicMethodWithNoPrivateAccess = function() {
    return this.notFullyPrivateVar;

x = new MyClass();

I think the example is self explanatory, but let me say few words on it:
In JavaScript you can have hierarchy of scopes and you can define unlimited amount function in functions levels (and scope levels) in difference to other programming languages where you have limited / fixed scope levels.

So I have a privateMethod and fullyPrivateVar defined within MyClass. Because of it, only things defined in this scope will have access to them. Like publicMethodWithPrivateAccess that I assign with the constructor to the newly constructed object. However,  publicMethodWithNoPrivateAccess is defined outside of the scope and has no access to those values. But it has access to everything in the this scope. One could argue, that this approach takes more memory for the methods because I define a new method with every initialization of the class (the function assignment). But that is not true. The function in the function is having a static code (it is not part of eval+variable) and is compiled during the initial javascript processing of the JS virtual machine. So it will not take extra memory. Only during execution the function will be dynamically assigned with the private scope of the constructor, something you want anyway.