title: What is a question mark for in the next expression in an angular 2 app
body: <p>Just wondering what does the question mark means in the next expression in an angular 2 app.</p>
  
  <p><div class="snippet" data-lang="js" data-hide="false" data-console="true" data-babel="false">
  <div class="snippet-code">
  <pre class="snippet-code-html lang-html prettyprint-override"><code>&lt;h1&gt;
   Time Remaining: {{ currentExercise.duration? - exerciseRunningDuration }}
  &lt;/h1&gt;</code></pre>
  </div>
  </div>
  </p>