Cross-global instanceof

Andrea Giammarchi andrea.giammarchi at
Thu Oct 31 14:56:42 PDT 2013

Sorry, secure was actually "reliable", as being secure about the type, not
about security itself.

A case where I do use instanceof is, example, with new String to easily
decide later on what to do with such object that should not be treated as

The script does resolve circular dependencies backward and forward out of
JSON.stringiy and JSON.parse that do create objects from the current realm.

Nothing else than `instanceof` is as usable/useful in current ESX spec for
same purpose in terms of performance and reliability.

Just one case, still `instanceof` is used a lot and usually for good
reasons, not because we are all bad developers (I hope ...)


On Thu, Oct 31, 2013 at 2:32 PM, Allen Wirfs-Brock <allen at>wrote:

> On Oct 31, 2013, at 1:58 PM, Andrea Giammarchi wrote:
> I think the question is legit and this is a weak check
> `'throw' in possibleGenerator`
> specially if you have Dictionary like objects around ...
> And why would such an object be passed into a context where you expect to
> have a generator?  That would probably be an upstream bug and cause a
> downstream failure.  Presumably you should have tests that look for such
> failures.
> 'length' in object does not tell you much, does it?
> who is that relevant?  If you need to condition something on the existence
> of a length property, you can.  But in most situations you don't want to do
> such tests.  Just access the property and use it/call it.  It it is the
> right kind of object things will work.  If it is the wrong kind, it is
> somebody's bug.
> `Object.prototype.toString` is abused all over client/server
> libraries/utilities since ducks sometimes might look like gooses too.
> Any, quite likely, most commonly used for unnecessary nominal type tests.
> So eventually would you don't want developers to really know what they are
> dealing with and promote weak feature detections instead? It might work,
> but that ain't secure.
> Yes, no. Good OO is about letting go of knowledge of an objects
> implementation.  In most cases you should only be concerned about the
> object behavior you will use and not care about who implemented it or how
> or whether or not there are multiple interoperable implementations
> operating.  But you should be feature testing for the behavior you depend
> upon.  Just invoke it.  It the method need isn't there you are usually
> going to get an exception, typically sooner that latter.   Feature
> detection shouldn't be  used as poor-man nominal type checking (where you
> throw if it fails).  You should only use feature checking, when you have
> real alternative paths to take depending upon the presence of the feature.
> "Secure"?  What kind of security for what purpose? Every little about JS
> is secure.
> Of course, there really are some places where you really do need to do a
> brand check for a specific object implementation.   In those cases, the
> implementation of the object should include brand checking in its design
> and expose an appropriate function for doing the check.  But a general
> purpose mechanism is an attractive nuisance, and probably doesn't actually
> do the check you need for your specific use case.
> I still prefer `instanceof` for better performance since I don't usually
> work with different realm but I also would like to have an
> `, object);` facility instead of the boring, slow, and
> inappropriate `{}` check.
> Show me real use cases and I'll try to explain with the check isn't needed.
> Allen
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