Object.getPrototypeOf throws on primitive-type parameter

Brendan Eich brendan at mozilla.com
Mon Aug 10 16:02:23 PDT 2009

Oliver Hunt pointed out the bug in SpiderMonkey's implementation (in  
Firefox 3.5) of Object.getPrototypeOf, where a primitive value does  
not cause a TypeError to be thrown as specified in ES5 draft. This bug  
was based on some miscommunication [1] and it will be fixed ASAP [2].

Throwing on non-Object (Object the spec type, not only instances of  
the Object native constructor) is as specified for all of the new  
Object metaprogramming APIs. But throwing differs from the behavior on  
primitives of the three ES3 Object.prototype additions,  
hasOwnProperty, getPrototypeOf, and propertyIsEnumerable, which  
convert |this| using the ToObject spec-internal helper function.

I bring this up not to lobby for a change to ES5 as drafted, just to  
note the inconsistency. This came up at a TC39 meeting a while ago and  
my notes say the committee was willing to use ToObject for  
getPrototypeOf, but the change was not made. Again, probably for the  

It would be great in Harmony to remove more of the Java-inspired  
primitive type "specialness", compatibly if possible but in stricter  
strict mode if necessary. I'm not sure how to do it in a way that  
helps getPrototypeOf, but even without a specific proposal, it helps  
to note that it's easier to relax an error condition in the future  
than to try to impose one.

So again, no ES5 change needed, just discussion fodder.


[1] https://bugzilla.mozilla.org/show_bug.cgi?id=444787#c7
[2] https://bugzilla.mozilla.org/show_bug.cgi?id=509510
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