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From: Thomas 'PointedEars' Lahn <PointedEars@web.de>
Newsgroups: comp.lang.javascript
Subject: Re: Element visible
Date: Tue, 15 Jul 2014 15:08:40 +0200
Organization: PointedEars Software (PES)
Lines: 59
Message-ID: <1503828.jIctB0555y@PointedEars.de>
References: <eoidnRvwreMiFSHOnZ2dnUVZ_q-dnZ2d@westnet.com.au> <visibility-20140709041836@ram.dialup.fu-berlin.de> <j9idnT3Ozq34JSHOnZ2dnUVZ_hydnZ2d@westnet.com.au> <13026944.5APpuSVIFp@PointedEars.de> <wNmdnUw9Ar6cRyDOnZ2dnUVZ_sudnZ2d@westnet.com.au> <2118762.fOI3iGdBDC@PointedEars.de>
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Thomas 'PointedEars' Lahn wrote:

> I can only see two possibilities, other than those that you have excluded,
> that can cause an element to be not visible:
> 
> One, the element may not be part of a real document but exist only as
> element object in memory.  In that case the value of the “parentNode”
> property of that object would be “null” or at least a false-value because
> it has no parent node.
> 
> However, it may also be a descendent node of a node that only exists in
> memory.  In that case the former’s “parentNode” property value would not
> be “null”, but following up to the next ancestor until the “parentNode”
> property value of the target node is “null” should be sufficient in most
> cases.
> 
> Yet, with this approach there is a potential for false positives because
> the root element node’s “parentNode” property value is always “null” (or a
> false-value).  I have just found out (in Chromium 34) that even the value
> of the “ownerDocument” property of a node object can be not “null” if the
> represented node was previously part of a real (HTML5) document.  So I
> have yet to find a reliable test for this possibility.

Correction: At least the root element node’s “parentNode” property value is 
not “null” (or a false-value) if, and only if, that element is currently 
part of a document.  Hpwever, a document object’s “parentNode” property 
value is always “null” (or a false-value).  This is the reliable test I was 
looking for:

  function inDocument (e, doc)
  {
    /* null or undefined */
    if (doc == null)
    {
      doc = e.ownerDocument;
    }

    while ((e = e.parentNode))
    {
      if (e == doc)
      {
        return true;
      }
    }

    return false;
  }

Note: This method cannot tell whether “doc” is visible.  However, there 
appears to be the “visibilityState” property for that, and it is a W3C 
Recommendation:

<https://developer.mozilla.org/en-US/docs/Web/Guide/User_experience/Using_the_Page_Visibility_API>

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