title: ! operator in typescript after object method
body: <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>