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Re: Who's eaten my apple and why?

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On 2012-09-27 02:59, Gene Wirchenko wrote:
> On Thu, 27 Sep 2012 01:35:17 +0200, Christoph Becker
> <cmbecker69@gmx.de> wrote:
> 
>>justaguy wrote:
>>> 	function dSelect() {
>>> 		var sel = document.getElementById('mySel');
>>> 		var o = document.createElement('option');
>>> 
>>> 		o.value="a";
>>> 		o.text="apple";
>>> 		sel.options.add(o);
>>> 		alert(sel.length);
>>> 
>>> 		o.value="ora";
>>> 		o.text="orange";
>>> 		sel.options.add(o);
>>> 		alert(sel.length);
>>> 	}
>>
>>Your're adding the *same* object to the HTMLSelectElement twice.  What's
> 
>      Aren't the values simply taken?  If I set o to null after both
> adds, it does not affect the select options so it presumably is not
> referring to o after it is used to add an option.

No, Christoph has it right. The entries in 'options' never referred to
'o'. They, and the 'o' variable, hold a reference to the same option
object. If you assign something else to 'o' (like null), then the 'o'
variable will contain something else, but that doesn't affect the object
itself.

When you write 'options.add(o)', you're not just passing the 'value' and
'text' properties, you're passing (a reference to) the entire object.

Here's a simpler but similar example:

  var arr = [];
  var o = {foo: "bar"};

  arr.push(o);
  o.foo = "qux";
  arr.push(o);
  o = null;

  print(arr[0].foo);
  print(arr[1].foo);

This will print "qux" twice. Even though the .foo property was "bar"
when we added the first element, and even though we nulled the 'o'
variable later.

You can still modify an object after adding it to an array, or passing
it to a method - as long as you have a reference to it. When we set 'o'
to null, we lose that reference, but not the object.

- stefan

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Thread

Who's eaten my apple and why? justaguy <lichunshen84@gmail.com> - 2012-09-26 14:43 -0700
  Re: Who's eaten my apple and why? Christoph Becker <cmbecker69@gmx.de> - 2012-09-27 01:35 +0200
    Re: Who's eaten my apple and why? justaguy <lichunshen84@gmail.com> - 2012-09-26 17:46 -0700
      Re: Who's eaten my apple and why? Stefan Weiss <krewecherl@gmail.com> - 2012-09-27 03:51 +0200
        Re: Who's eaten my apple and why? justaguy <lichunshen84@gmail.com> - 2012-09-26 19:44 -0700
          Re: Who's eaten my apple and why? Stefan Weiss <krewecherl@gmail.com> - 2012-09-27 22:06 +0200
    Re: Who's eaten my apple and why? Gene Wirchenko <genew@ocis.net> - 2012-09-26 17:59 -0700
      Re: Who's eaten my apple and why? Stefan Weiss <krewecherl@gmail.com> - 2012-09-27 04:16 +0200
        Re: Who's eaten my apple and why? Gene Wirchenko <genew@ocis.net> - 2012-09-26 19:38 -0700
          Re: Who's eaten my apple and why? Stefan Weiss <krewecherl@gmail.com> - 2012-09-27 05:44 +0200
            Re: Who's eaten my apple and why? Gene Wirchenko <genew@ocis.net> - 2012-09-27 09:28 -0700
              Re: Who's eaten my apple and why? Tim Streater <timstreater@greenbee.net> - 2012-09-27 17:40 +0100
                Re: Who's eaten my apple and why? Gene Wirchenko <genew@ocis.net> - 2012-09-27 16:24 -0700
              Re: Who's eaten my apple and why? Dr J R Stockton <reply1239@merlyn.demon.co.uk.invalid> - 2012-09-29 20:47 +0100
          Re: Who's eaten my apple and why? Christoph Becker <cmbecker69@gmx.de> - 2012-09-27 13:00 +0200
            Re: Who's eaten my apple and why? Gene Wirchenko <genew@ocis.net> - 2012-09-27 09:34 -0700
              Re: Who's eaten my apple and why? Christoph Becker <cmbecker69@gmx.de> - 2012-09-27 19:14 +0200
                Re: Who's eaten my apple and why? Stefan Weiss <krewecherl@gmail.com> - 2012-09-27 21:40 +0200
                Re: Who's eaten my apple and why? Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2012-09-28 15:39 +0200
                Re: Who's eaten my apple and why? Gene Wirchenko <genew@ocis.net> - 2012-09-27 16:25 -0700
      Re: Who's eaten my apple and why? Tim Streater <timstreater@greenbee.net> - 2012-09-27 08:52 +0100
  Re: Who's eaten my apple and why? Gene Wirchenko <genew@ocis.net> - 2012-09-26 17:50 -0700

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