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From: Thomas 'PointedEars' Lahn <PointedEars@web.de>
Newsgroups: comp.lang.javascript
Subject: Re: Why: X[i] = x[i] = undefined when x[i] defined?
Date: Fri, 17 Oct 2014 21:29:01 +0200
Organization: PointedEars Software (PES)
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Ed Jay wrote:

> On Fri, 17 Oct 2014 09:29:54 -0700, Ed Jay <edMbj@aes-intl.com> wrote:
>>Code - function1:
>>
>>x_pos_array = [];
>>y_pos_array = []
>>function make_div(e) { //mouse event
>>var marker_cnt = x_pos_array.length;
>>var x_pos = e.x;
>>var y_pos = e.y;
>>if(some condition is met) {
>>var div = document.createElement('div');
>>image_holder.appendChild(div);
>>div.id = 'mrkr'+marker_cnt;
>>//style the div and add onmouseover listener
>>x_pos_array.push(x_pos);
>>y_pos_array.push(y_pos);
>>}}
>>
>>Code function 2
>>x_pos_out = [];
>>y_pos_out = [];
>>function() {
>>var marker_cnt = x_pos_array.length;
>>for (var i = 0; i < marker_cnt; i++) {
>>if (x_pos_array[i] > 0) {
>>x_pos_out[i] = x_pos_array[i];
>>y_pos_out[i] = y_pos_array[i];
>>alert(i + ")  x_pos_out[i] = " + x_pos_out[i]);
>>//undefined for x_pos_array[i] > 0
>>
>>}
>>}
> Solved.
> //do something with x_pos_out and y_pos_out
> Neglected to declare div as an array.

The relevant ECMAScript implementations use dynamic type-checking.  
Therefore, you cannot declare something as an array; you can only declare a 
variable and assign to it a reference to an Array instance, after which you 
can assign any other value to it.
 
What you actually have neglected is to declare “x_pos_array” and 
“y_pos_array” variables by using the “var” keyword:

  var x_pos_array = [];
  var y_pos_array = [];

Not doing this causes your code to be more error-prone than it needs to be.  
You might be accidentally accessing a property of the global object, and 
your code will maybe compile, but certainly not run in ES5+ strict mode.


The code that you have posted is hard to read.  If you wish your code to be 
reviewed here by many, pretty printing, e.g. indentation, is advised.  It 
will also help you to see any errors in your code more easily.  See the FAQ 
for details.

Please provide a working contact e-mail address in the From header field, 
see also <http://www.interhack.net/pubs/munging-harmful/>.

-- 
PointedEars
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