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Message-ID: <1553168.V9fml2jhPD@PointedEars.de>
From: Thomas 'PointedEars' Lahn <PointedEars@web.de>
Reply-To: Thomas 'PointedEars' Lahn <cljs@PointedEars.de>
Organization: PointedEars Software (PES)
Date: Sat, 14 Sep 2013 01:49:11 +0200
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Subject: Re: loop through an arry
Newsgroups: comp.lang.javascript
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Daniel Pitts wrote:

> On 9/12/13 5:45 PM, Ken wrote:
>> On 12.09.2013 18:39, Daniel Pitts wrote:
>>> On 9/12/13 2:35 PM, Ken wrote:
>>>> need to squeeze out all you can of the browser (which in case you would
>>>> use a *while* loop in any case) there is no real problem using your
>>>> approach.
>>> while and for loops should be identical in performance. They are merely
>>> different syntax for the same operations.
>>
>> Well.. it do depend on browser and implementation (such as JIT) but in
>> general while loops are faster as seen here (with most browsers):
>>
>> http://jsperf.com/fastest-array-loops-in-javascript/32
>>
>> Disclaimer: I didn't make the test.
> 
> So many of those tests are flawed. They don't even perform the same
> operation!

ACK.

> Anyway, for my browser, the fastest one was a test I created using a for
> loop:
> 
> for (var l = arr.length; l--;) {
>    someFn(arr[l]);
> }

I expected this; I have recommended this approach before (where reverse 
order is acceptable).  Those are, after all, the fewest operations.

> The second fastest one was a loop that was already there using a while
> loop:
> 
> var l = arr.length;
> while(l--) {
>    someFn(arr[l]);
> }
> 
> The difference in speed was close to the margin-of-error, so they are
> practically equivalent.

That is because they are compiled to the same bytecode or native code.
In pseudo-bytecode:

-------------------------------------------
line   #  operation    return  operands
-------------------------------------------
   1   0  DECL_VAR        $0   "l"
       1  FETCH_VAR       $1   "arr"
       2  FETCH_OBJ_R     $2   $1, "length"
       3  ASSIGN          $3   $0, $2
   2   4  FETCH_VAR       $4   $0
       5  JMPZ                 $4, ->13
       6  POST_DEC        $6   $0
   3   7  FETCH_VAR       $7   $1
       8  FETCH_VAR       $8   $0
       9  FETCH_OBJ_R     $9   $7, $8
      10  FETCH_VAR       $10  "someFn"
      11  CALL_FUNC       $11  $10, $9
      12  JMP                  ->4
      13  RETURN               0
-------------------------------------------

The “for” statement only differ from the “while” statement in that the 
operations #0 to #6 are produced by the same line.
 
> Testing in Chrome 29.0.1547.65 on OS X 10.8.4

Thanks.

-- 
PointedEars

Twitter: @PointedEars2
Please do not Cc: me. / Bitte keine Kopien per E-Mail.