@Rob: Jinx!
-- T.J. ;-)
On Jun 10, 4:22 am, RobG <[EMAIL PROTECTED]> wrote:
> On Jun 8, 11:45 pm, "Frederick Polgardy" <[EMAIL PROTECTED]> wrote:
>
> > On Sun, Jun 8, 2008 at 4:22 AM, T.J. Crowder <[EMAIL PROTECTED]> wrote:
>
> > > Hi Fred,
>
> > > No, not in standard JavaScript. The interpreter will work its way up
> > > the scope chain looking for "Prototype"; if it reaches the global
> > > object (which is always the root of the scope chain; the global object
> > > in browser apps is the window object), it assumes the reference is a
> > > property of that object. The property will have the value undefined.
>
> No, it won't. There is a big difference between a property whose
> value is undefined (as in a declared variable that has not been
> assigned a value) and an indetifier that has not been defined at all
> (i.e. not declared).
>
> Read the ECMAScript spec, section 10.1.4. To summarise, if a property
> with the same name as the identifier is not found on the scope chain,
> then return "a value of type Reference whose base object is null and
> whose property name is the Identifier".
>
> Having resolved the identifier, it now must be evaluated, section 12.4
> says:
>
> 1. Evaluate Expression.
> 2. Call GetValue(Result(1)).
> 3. Return (normal, Result(2), empty).
>
> When it gets to step 2 and calls GetValue... Section 8.7.1 getValue
> says:
>
> 1. If Type(V) is not Reference, return V.
> 2. Call GetBase(V).
> 3. If Result(2) is null, throw a ReferenceError exception.
>
> Ta da.
>
> > It seems like that would be the case based on JavaScript's lexical scoping
> > rules, but in Firefox I get a JavaScript error ("ReferenceError: Prototype
> > if not defined") when I try to alert(Prototype) or if(Prototype) { ... }.
>
> > Didn't make sense to me either. :-)
>
> Hopefully this order of posing makes more sense. :-)
>
> It makes sense if you realise that undeclared variables are created as
> global variables *only* when code is executed that assigns some value
> to them.
>
> e.g. the statement:
>
> Prototype;
>
> creates a reference error because when attempting to resolve the
> identifier "Prototype", it can't be found and so returns a null
> reference.
>
> ECMAScript spec sections 12.4, and
>
> Since JavaScript is a forgiving language, using:
>
> Prototype = 'foo';
>
> creates a Prototype property of the global object *when the statement
> is executed* and the value 'foo' is assigned to it (sections 11.13.1
> and 8.7.2 explain).
>
> --
> Rob
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