A bit about the stuff I've done


Wednesday, 24 July 2013

Private members in javascript classes

Whoever told you this couldn't be done was lying.

By utilising closures we can create a class that holds its own private members which cannot be accessed outside of that class - even by adding a new function onto the class

Consider the following javascript function

function Outer() { var PrivateVar='PrivateValue'; function Inner() { PrivateVar='Some Value'; } }

This is known as a closure.
You can read more about them here: http://stackoverflow.com/questions/111102/how-do-javascript-closures-work

Put simply the inner function has access to the variables in the outer function.
These variables are "live" - that is they are not fixed when the inner function was created.

Now consider that the Outer and Inner functions are, in fact, constructors

function MyClassWithPrivateVars(params) { var PrivateVar; function MyClass(params) { PrivateVar='Initial Value'; } return new MyClass(params); } var Instance = new MyClassWithPrivateVars('whatever');

When you call the outer constructor you are actually calling the inner constructor.
The inner constructor has access to the vars within the outer constructor as before.

Let's add a method to the inner constructor.
This new method of course has access to the variables in the outer constructor.

In this instance the method is a "property".

function MyClassWithPrivateVars(params) { var PrivateVar; function MyClass(params) { PrivateVar='Initial Value'; this.PublicProperty = function(value) { if (typeof(value)!=="undefined") { PrivateVar=value; } return PrivateVar; } } return new MyClass(); }

Now if we create an instance of MyClassWithPrivateVars and invoke the property we'll see that it gets the value as expected.

var MyInstance=new MyClassWithPrivateVars(); console.log(MyInstance.PublicProperty());

Outputs "Initial Value" because the private variable was set to that value in the constructor.

MyInstance.PublicProperty(10); console.log(MyInstance.PublicProperty());

This time we get 10

So is the variable really private?

Well - let's try and access it directly:

console.log(MyInstance.PrivateVar);

This should output undefined as PrivateVar is not a member of the inner class.

Likewise

console.log(MyClassWithPrivateVars.PrivateVar);

is also undefined because PrivateVar is a member of the instance of the outer class - not of the constructor itself.

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