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From: Ben Bacarisse <ben.usenet@bsb.me.uk>
Newsgroups: comp.lang.javascript
Subject: Re: Reverse elements in array, they can be integers or strings.
Date: Sun, 17 May 2015 00:28:22 +0100
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Denis McMahon <denismfmcmahon@gmail.com> writes:

> On Sat, 16 May 2015 15:00:42 +0200, Evertjan. wrote:
>
>> jonas.thornvall@gmail.com wrote on 16 mei 2015 in comp.lang.javascript:
>
>>> var arrLength; 
>>> arrLength=thisArr.length;
>>> for (i=arrLength;i>=0;i--){
>>>      thisElement=thisArr[i];
>
>> this will give an ERROR,
>> as the highest indexed element has an index of i-1
>
> Will it?

The code is wrong, but whether it "gives and ERROR" depends on what
was meant by that.

> Given i starting at thisArr.length and executing an i-- before accessing 
> the array, I don't think it will give that error, the error will be on 
> the pass after i == 0, as it will then decrement i and execute the loop 
> with i having a value of -1, which is an illegal array index.

You've got the place where the decrement happens mixed up -- it is done
after each execution of the loop body, so the loop above initially
accesses an undefined element, and then accesses all of the defined
elements in sequence.  An index of -1 is not used (though i == -1 at the
end of the loop's execution).

> All of the following are valid in c-type languages where 0 is "falsey", 
> numbers other than 0 are "truthy" and arrays are 0 indexed:
>
> for (var i = arr.length; i > 0; i--) {
> // do something with arr[i]
> }
>
> for (var i = arr.length; i; i--) {
> // do something with arr[i]
> }

No, these are both wrong.  They access an undefined element first, and
neither accesses element 0.

> var i = arr.length;
> while (i--) {
> // do something with arr[i]
> }

This one is correct.

<snip>
-- 
Ben.