Callable RegExp vs. typeof (was: Re: Draft of Function.prototype.bind.)

Brendan Eich brendan at
Thu Aug 13 13:44:08 PDT 2009

On Aug 13, 2009, at 1:28 PM, Juriy Zaytsev wrote:

> There was a discussion of this ticket on Hacker News this morning  
> and we had this slight confusion on whether giving RegExp objects a  
> [[Call]] property is permitted by spec < 
> >. I thought it was, since section 2 clearly states that  
> implementation can introduce its own extensions, but then someone  
> made a point about exact wording in that section and how it seems  
> that implementation can only add properties that spec doesn't  
> mention (i.e. additional ones) and not those that spec mentions  
> (such as [[Call]]) but doesn't specify them on certain objects.
> Could someone please clarify this?

Good point. I seem to recall that David-Sarah Hopwood made it already,  
but I'm having trouble finding his message right now.

ES3 Chapter 16 says:

"An implementation may provide additional types, values, objects,  
properties, and functions beyond those described in this  
specification. This may cause constructs (such as looking up a  
variable in the global scope) to have implementation-defined behaviour  
instead of throwing an error (such as ReferenceError)."

As the ycombinator thread correspondent says, this is ambiguous: does  
"may provide ... properties ... beyond those described by this  
specification" mean [[Call]] on a native object specified without  
[[Call]] by ES3 is ok? Or does it mean that only properties whose  
identifiers are not defined at all for any object may be added? Or  
should internal properties be excluded in any event?

The practical issue of typeof /a/ == "function" being misleading and  
causing compatibility problems is paramount. Fixing that by ruling  
that extending RegExp to have a [[Call]] internal property violates  
ES3 is plausible in theory, but again in practice hard if enough  
content depends on /a/(s).


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