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From: Joao Rodrigues <groups_jr-1@yahoo.com.br>
Newsgroups: comp.lang.javascript
Subject: Re: arr.push(obj);
Date: Tue, 11 Mar 2014 06:59:34 -0300
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On 11/03/2014 01:19, jonas.thornvall@gmail.com wrote:
> Den tisdagen den 11:e mars 2014 kl. 04:28:25 UTC+1 skrev jonas.t...@gmail.com:
>> Den tisdagen den 11:e mars 2014 kl. 04:19:49 UTC+1 skrev jonas.t...@gmail.com:
>>
>>> Den tisdagen den 11:e mars 2014 kl. 01:38:12 UTC+1 skrev jonas.t...@gmail.com:

<snip>

>> I did find a page that told that arr[-1] get the last element of array, so it seem negative indexes not allowed.

That is a misconception.

>
> I was wrong it is allowed or at least working.
>
> <script type="text/javascript">
> var arr=[];
> for (i=10;i>=-20;i--) {arr[i]=i;}
> arr[-22]=-22;
> for (i=10;i>=-22;i--) {document.write(arr[i],",");}
> </script>

According to the section 15.4 of the ECMAScript 5.1 Specification, "A 
property name P (in the form of a String value) is an array index if and 
only if ToString(ToUint32(P)) is equal to P and ToUint32(P) is not equal 
to 232−1". So, I think this is why you can set negative indices to an 
array. But the array's length property value is always a nonnegative 
integer less than 232.

Thereby, your code creates a mutilated array, and some of the native 
array methods might not work as expected. For instance, arr.length will 
return 11, and arr.join(',') will yield 0,1,2,3,4,5,6,7,8,9,10


-- 
Joao Rodrigues