Path: csiph.com!x330-a1.tempe.blueboxinc.net!usenet.pasdenom.info!weretis.net!feeder4.news.weretis.net!news.mixmin.net!feeder.erje.net!news.internetdienste.de!news.tu-darmstadt.de!news.belwue.de!newsfeed.arcor.de!newsspool4.arcor-online.net!news.arcor.de.POSTED!not-for-mail
Content-Type: text/plain; charset="UTF-8"
Message-ID: <1783522.fYvYJRLvp5@PointedEars.de>
From: Thomas 'PointedEars' Lahn <PointedEars@web.de>
Reply-To: Thomas 'PointedEars' Lahn <cljs@PointedEars.de>
Organization: PointedEars Software (PES)
Date: Thu, 13 Oct 2011 21:21:33 +0200
User-Agent: KNode/4.4.11
Content-Transfer-Encoding: 8Bit
Subject: Re: Objects comparaison ?
Newsgroups: comp.lang.javascript
References: <1k93ek2.1xghqcy1upjvv5N%unbewusst.sein@fai.invalid> <j77bob$rq6$1@dont-email.me>
Followup-To: comp.lang.javascript
MIME-Version: 1.0
Lines: 75
NNTP-Posting-Date: 13 Oct 2011 21:21:33 CEST
NNTP-Posting-Host: d6fab8d7.newsspool2.arcor-online.net
X-Trace: DXC=PO8fL9`H^^CE4ZB2flKORAA9EHlD;3YcB4Fo<]lROoRA8kF<OcfhCOKL5NU2=\P19JDZm8W4\YJNLb@mF9jNikdE_dXMBNGELEAX`I5II4M[bH
X-Complaints-To: usenet-abuse@arcor.de
Xref: x330-a1.tempe.blueboxinc.net comp.lang.javascript:7361

Jukka K. Korpela wrote:

> 13.10.2011 21:20, Une Bévue wrote:
>> however doing :
>> var o = { 'huit': 8};
>> b = (o == o)? true : false;
>> gives obviously true meaning objects are compared by reference ?
> 
> […] basically what you are comparing here is values of expressions, and
> the value of "o" is the same as the value of "o", namely a reference to an
> object.

The *evaluation* value of the expression `o' is a reference to one object.  
Equal references compare equal, like any other value.  But, as references 
are not type-converted in that context, by contrast it does not matter 
whether you use loose or strict comparison.

>> not true for strings :
>>
>> var a = 'toto';
>> var c = 'toto';
>> var b = (a===c)? true : false;
>>
>> gives true...
> 
> Right, because strings are not objects. […]

*Primitive* *string* *values* are not objects (they are converted to them if 
context requires it).  Change this to

  var a = new String('toto');
  var c = new String('toto');
  var b = (a === c)? true : false;

and it will yield `false' (with or without the strict comparison), because 
`a' and `c' would store different references (references to different 
objects).

BTW,

  var b = (a === c);

is semantically equivalent, but more efficient.  Don't make booleans if you 
already have them.

>> is there a function like equal for objects ???
>> or does I need to implement it ?
> 
> If you wish to compare objects by equality of property values, you'll
> have to do it yourself (directly or via libraries). This is not quite as
> trivial as it may sound, as you need to decide whether it suffices that
> all properties of the first object exist in the second object too, with
> identical values (with some value for "identical"), or whether you also
> wish to require that the objects are intuitively copies of the same
> stuff, so that the second object has no properties that the first one
> hasn't.

Java's equals() method – which exists precisely for that purpose – should 
provide a good start for an implementation in ECMAScript.  But as the 
existence of properties cannot be reliably tested in a 
(backwards-)compatible way in ECMAScript implementations, this only leaves 
either comparing the results of property accesses for the same property name 
or losing that compatibility.

However, practice shows that it is seldom necessary to compare objects for 
equality; it usually suffices, and is more efficient, to compare key 
property values or object references instead.


PointedEars
-- 
var bugRiddenCrashPronePieceOfJunk = (
    navigator.userAgent.indexOf('MSIE 5') != -1
    && navigator.userAgent.indexOf('Mac') != -1
)  // Plone, register_function.js:16