Fixing instanceof / typeof

Brendan Eich brendan at
Mon Oct 15 17:59:58 PDT 2012

Just curious: why did you need a nominal type test, rather than a "duck 
type" test?


Irakli Gozalishvili wrote:
> Hi,
> Today I had to deal with a value type detection in JS and that 
> reminded me of all the pain associated with it, especially when 
> dealing with values that may have come across the diff compartment / 
> frame / context. `instanceof` is useless when dealing with objects 
> from diff contexts, `typeof` is also pretty limited in a sense that it 
> can detect weather value is array, error, map, set, etc.. not to 
> mention it's awkwardnesses…
> Only solution that seems to work toady that is adopted by most popular 
> JS libraries is:
> function type(value) {
>   return' 
> ')[1].split(']')[0].toLowerCase()
> }
> I don't actually know if that's supposed to work according to spec or 
> if it's just an implementation coincidence that happen to
> work. Either way I would really welcome some standard 
> solution preferably  via function form (so that can be polyfilled today)
> to do type detection for all the built-ins and host objects.
> Aside main issue, it would be great if solution was compatible with 
> mozilla's xpcom components too where one object my be
> queried with multiple interfaces and there for have multiple types. 
> Also, above hack does not works with XPCOMs as 
>  Object.prototype.toString returns "[object 
> XPCWrappedNative_NoHelper]", while actual toString returns something 
> little more useful "[xpconnect wrapped (nsISupports, nsIURI, nsIURL)]"
> Regards
> --
> Irakli Gozalishvili
> Web:
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> es-discuss at

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