! operator in typescript after object method
<p>I have an object <code>X</code> with a method <code>getY()</code> returning an object <code>Y</code> with a method <code>a()</code>, in typescript.
What does it mean an expression like this one:</p>

<pre><code>X.getY()!.a()
</code></pre>

<p>I guess the <code>!</code> operator is used to check against null, but how does it work concretely? Where is defined in the language?</p>