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Zenon Page Templates - JS (ZPT-JS)
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<h1>ZPT-JS reference - Exists expressions</h1>
<ul>
<li><a href="#syntax">Syntax</a>.</li>
<li><a href="#description">Description</a>.</li>
<li><a href="#differences">Differences with ZPT</a>.</li>
<li><a href="#examples">Examples</a>.</li>
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<h2 data-attributes="id 'syntax'">Syntax</h2>
<pre class="syntax">
exists_expression ::= 'exists:' expression
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<h2 data-attributes="id 'description'">Description</h2>
<p>
<em>Exists</em> expressions test for the existence of paths. An exists expression returns <code>true</code> when the path expressions following its expression returns a value and it is not <code>undefined</code>. It is <code>false</code> when the path expression cannot locate an object or if it evaluates to <code>undefined</code>.
</p>
<p>
Note: some parts extracted from <a href="https://zope.readthedocs.io/en/latest/zopebook/AppendixC.html#tales-exists-expressions">Zope Page Templates Reference</a>.
</p>
<h2 data-attributes="id 'differences'">Differences with ZPT</h2>
<p>
In ZPT if an expression evaluates to <code>undefined</code> exists returns <code>true</code>; ZPT-JS returns <code>false</code>.
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<h2 data-attributes="id 'examples'">Examples</h2>
<p>
Testing for the existence of a property in an object:
</p>
<pre class="brush: html">
<div data-condition="not: exists: myObject/name">
Name not set!
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</pre>
<p>
Note: some parts extracted from <a href="https://zope.readthedocs.io/en/latest/zopebook/AppendixC.html#tales-exists-expressions">Zope Page Templates Reference</a>.
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