arguments extending Array?

Brendan Eich brendan at
Fri Feb 2 11:47:19 PST 2007

On Feb 2, 2007, at 10:52 AM, liorean wrote:

> On 2/2/07, Hallvord R. M. Steen <hallvord at> wrote:
>> In Opera's implementation the arguments object extends Array,  
>> meaning that
>> it has all the Array-related methods and also methods and  
>> properties set
>> on Array.prototype . This has caused an incompatibility with a  
>> recent RC
>> of Prototype and is considered a bug - but from blog and E-mail
>> correspondence regarding this issue it seems several[1] authors  
>> consider
>> it a nice feature. The arguments object is already "array-like"  
>> and apart
>>  from the Prototype issue our implementation has caused us no known
>> problems (the workaround for Prototype is relatively simple and  
>> they have
>> already implemented it AFAIK).
> I think it's among the most wanted features for adding to the
> language. And Prototype has gotten a lot of criticism for breaking
> Array objects. (Including from me...)

See the first bullet at 
bug_fixes.html -- ES4 indeed proposes to make arguments objects  
delegate to Array.prototype.

>> Would it be an idea for ES4 to make arguments extend Array?
> It's a wanted feature, but not necessarily a good one to add as-is.
> For example, formal parameters are joined with their corresponding
> number in arguments. Change either and both are changed. So something
> like this: arguments.sort(function()(Math.random()-.5)) would do evil
> things with formal parameter values.

It's true that arguments[i] aliases the i'th formal parameter for i <  
(number of formals declared). But you can do confusing things via  
this aliasing by mutation already, and could since JS1 in 1995.  
Delegating to Array.prototype does not change anything except by  
adding convenience, which will be used for good more than for ill, we  

All tools can be misused. If the bug here is the aliasing, then it's  
too late to fix without breaking backward compatibility, and we do  
not propose to do that for arguments[i] aliasing the i'th formal.

Of course, breaking Prototype shows that we are taking chances with  
backward compatibility. But so is Prototype with forward  
compatibility, by hacking on the standard objects' prototypes and  
assuming more than it needs (or needed -- glad to hear it is fixed to  
work with Opera) to.


More information about the Es4-discuss mailing list