QGIS/resources/function_help/text/IS

<h3>IS expression</h3>
Returns 1 if a is the same as b.

<h4>Syntax</h4>
<pre>a IS b</pre>

<h4>Arguments</h4>
None

<h4>Example</h4>
<pre> 'A' IS 'A'  &rarr; returns 1 </pre>
<pre> 'A' IS 'a'  &rarr; returns 0 </pre>
<pre> 4 IS 4      &rarr; returns 1 </pre>
<pre> 4 IS 2+2    &rarr; returns 1 </pre>
<pre> 4 IS 2      &rarr; returns 0 </pre>
<pre> $geometry IS NULL  &rarr; returns 0, if your geometry is not NULL</pre>