JavaScript notes: a Firebug puzzle

There it stood, written before me on the page "Globals created with var (those created in the program outside of any function) cannot be deleted."¹

Yet in Firebug

var a = 10;
delete a;
console.log("this is a: " + a);

consistently returned:

ReferenceError: a is not defined (Fool!)²

How could this be happening? 

If I reworked the code and ran it as a script in a web page, it gave me "this is a: 10", as expected.

After a bit of research (thank you, Google),  I found the answer. Ironically in a blog entry about another book by the same author that was puzzling me now...

The solution is well worth reading, but I'll see if I can summarize it:

• Variables are in fact properties with a "don't delete" flag set.
• Variables declared in JavaScript code that would otherwise be global, when executed by an eval call are in fact deletable (the flag isn't set)!
• Those deletable variables are added to the calling object's variable object, for want of a better place to put them.

Firebug appears to be executing code entered into the console via an eval call, hence my strange power to be able to delete global variables. No black magic here!

 Of course this answer poses another question: why are global variables that are in code executed in an eval context deletable?

I think that the following code might propose an answer:

(function test() {
   eval('var wtf = "wtf";');
   console.log(wtf);
   console.log(delete wtf);    // true
})();

When you evaluate code you will possibly have variables added to your calling context that you might not want there. By marking them as deletable, you have a chance to clean them up. Now to work out how you can do this automatically...

Footnotes
1 Page 12, JavaScript Patterns by Stoyan Stefanov, September 2010: First Edition.
2 Fool! wasn't really part of the message: that is just how I felt when I saw the error.