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From: Rolf Niepraschk <Rolf.Niepraschk@gmx.de>
Newsgroups: comp.lang.javascript
Subject: Re: Iterating through an array (special syntax)
Date: Fri, 20 Oct 2017 07:55:23 +0200
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Xref: csiph.com comp.lang.javascript:34150

Am 19.10.2017 um 21:53 schrieb Thomas 'PointedEars' Lahn:
...
> 
> Unless oldItems.length changes,
> 
>    for (var i=0, len = oldItems.length; i < len; i++) {
> 
> is more efficient.

Are you sure? I see no much differences (at least with nodejs 8):
#=================================================
#!/usr/bin/env node

var arr = [];
for(var i=0; i<5E7; i++) arr.push(i);

var x, start;

start = new Date().getTime();
for (var i=0; i<arr.length; i++) {};
console.log('A: ' + (new Date().getTime()-start) + ' ms');
start = new Date().getTime();
for (var i=0, len = arr.length; i < len; i++) {};
console.log('B: ' + (new Date().getTime()-start) + ' ms');
#=================================================

> 
>>           fn_each(oldItems[i], newItems[i]);
>>         }
> 
> This loop is pointlessly inefficient in any case.  You are already using
> Array.prototype.map(), thereby iterating through the array, so you can
> either output or accumulate the old and new value *in the callback*.

Ok.

I will think about your Promise solution. Thanks for your explanation.

...Rolf