Subj : JavaScript Variable Scoping and Execution Contexts
To   : netscape.public.mozilla.jseng
From : Greg Swindle
Date : Sat Sep 04 2004 01:11 pm

Hello,
I have a question about how prototyping relates to variables and their
scope.

Given the following code:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
var ParentObject = function()
{
    var objectName = "ParentObject";

    this.getObjectName = function()
    {
        return objectName;
    }
    this.setObjectName = function( newObjectName )
    {
        objectName = newObjectName;
    }
}

var ChildObject = function()
{
    ParentObject.apply(this, arguments);

    this.tryToReferenceObjectNameVar = function()
    {
        var text = null;
        // Attempt to reference the variable "objectName",
        // which was created in ParentObject.
        try
        {
            text = "ChildObject's \"objectName\"" +
                " variable is \"" + objectName + "\"";
        }
        catch( error )
        {
           alert( "Error: " + error.name +
               " at " + "line: " + error.lineNumber + "\n" +
               "\"" + error.message + "\"\n" +
               "Stack:\n" + error.stack + "\n" );
        }
        return text;
    }
}

var child = new ChildObject();
child.tryToReferenceObjectNameVar();
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The last call throws this ReferenceError:

Error: objectName is not defined

Is this because prototyping, which I have done by calling the "apply"
method of a constructor, creates an independent execution context
that cannot be accessed from the "child" object's execution context? In
other words,
when I call

    child.getObjectName();
    child.tryToReferenceObjectNameVar();

are two (or several) different execution contexts being referenced? How is
the
JavaScript interpreter handling this?

TIA,
Greg

.