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From: Thomas 'PointedEars' Lahn <PointedEars@web.de>
Newsgroups: comp.lang.javascript
Subject: Re: Object prop missing?
Date: Thu, 13 Jun 2019 13:05:20 +0200
Organization: PointedEars Software (PES)
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Andrew Poulos wrote:

> var options = [
>                 {},
>                 {"id": "opt1", "on": false },
>                 {"id": "opt2", "on": false },
>                 {"id": "opt3", "on": false },
>                 {"id": "opt4", "on": false },
>                 {"id": "opt5", "on": false }
>                ];
> 
> for (var i = 6; i > 0; i--) {
>    // add stuff to options elements
>    options[i].h = 28;
> }
> 
> showOptions();
> 
> function showOptions() {
>    console.log(options[1]);        // 1.
>    console.log(options[1].h)       // 2.
>    console.log(typeof options[1]); // 3.
> 
>    // code that relies on .h values
> }
> 
> In Windows Chrome 75.0.3770.80 (64-bit):
> 
> 1.
>  > Object
> 
> when opened
> 
> v Object
>    > elem: div#opt1.opt
>      h: 28
>      id: "opt1"
>      maxw: 548
>      on: false
>    > pos: {x: 46}
>    > txt: p
>      __proto__: Object
> 
> 2. undefined
> 
> 3. object
> 
> h is in options[1] but when I try to get the value it's not.

I cannot reproduce your results.  If I run your code in

| > navigator.userAgent
| <- "Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) 
| Chrome/73.0.3683.75 Safari/537.36"

I get

| Uncaught TypeError: Cannot set property 'h' of undefined
|     at <anonymous>:12:17

because your code is logically in error: The referred Array instance has 6 
elements, so it does NOT have an element with index 6 (indexes start at 0)
and “options[6]” is evaluated to the *primitive* (non-object) “undefined” 
value.

Note that

  for (var i = 6; i > 0; i--)

is NOT equivalent to

  for (var i = 6; i--;)

The former checks whether the value of “i” is *greater than* zero; if so, 
the Block statement is executed.  *After* that “i” is decremented.

By contrast, the latter checks whether the value of “i” is *not equal to* 
zero, and decrements it *in any case* (to produce an *existing* index) 
*before* the Block statement is executed (only if “i” was not equal to 
zero).

The equivalent code would be (confusingly; do not do that)

  for (var i = 6; i-- > 0;)

-- 
PointedEars
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