How do arguments objects print?

Brendan Eich brendan at
Sat Feb 28 00:40:21 PST 2009

On Feb 27, 2009, at 11:59 PM, Mark S. Miller wrote:

> In the 23feb09 draft, arguments objects have a [[Class]] of "Object"  
> and inherit their toString property from Array.prototype.  
> Array.prototype.toString in effect simply does "return  
> this.join();". However, Array.prototype.toString is not generic (it  
> insists that its this have [[Class]] "Array") even though  
> Array.prototype.join() is generic. Altogether, by the 23feb09 draft,  
> attempting to print an arguments object must result in a throw. I  
> doubt this is what anyone intends. Could we simply make  
> Array.prototype.toString generic, so that the it works when inherited?

Compatibility means returning "[object Object]" from (function f() 
{return arguments;})().toString(). At least for browsers other than  
Safari. I don't know of any web content that counts on this, but I  
haven't looked and it's a big web ;-).

> Btw, I notice on Safari that the [[Class]] of an arguments object is  
> "Arguments". Given how weird the arguments object is, if it is not  
> too late, I think Safari's special [[Class]] value would be a good  
> idea.

That's a better idea than what ES1-3 say, for sure. I wonder whether  
that goes back to the kjs dawn of time. Cc'ing Maciej.

We could try this out during implementation of 3.1, before finalizing  
the standard. Is it better than making the delegated  
Array.prototype.toString generic, though?


More information about the Es-discuss mailing list