Question

The below doesnt work and im struggling to work out why….

function CommentHandler() {

    var Text;
}


var myCommentHandler = new CommentHandler();

myCommentHandler.prototype.SayHello = function () {

    document.write('Hello World');

}

I get the error : ‘myCommentHandler.prototype is undefined’

I just cant see why? I have declared a variable called myCommentHandler and copied in the object CommentHandler then Im trying to access the protoype of it and assign a function but I cant…Anyone know why?

Answer 1

The prototype belongs to the class, not the instance:

CommentHandler.prototype.MyFunction = ...

However you can also access the prototype via the instance’s constructor property:

myObj.constructor.prototype.MyFunction = ...

This will of course add that function to every instance of your CommentHandler.

If instead you only wanted to add that function to that one instance, you’d do:

myObj.MyFunction = ...

Note that in none of these variants would it be possible for MyFunction to access your private variable Text.

This is the compromise forced by the prototype inheritance model – variables and methods must be public (e.g. this.Text) if they’re to be accessed from outside, and in this case functions declared in the prototype are effectively “outside”.

The alternative (using closure-scoped variables and methods) seems more natural to OO programmers, but makes inheritance much harder.