title: What does "||" mean?
body: <p>I'm newbie and trying to study javascript myself.</p>
  
  <p>There is an example: </p>
  
  <pre><code> var scrollTop = document.documentElement.scrollTop || document.body.scrollTop
  </code></pre>
  
  <p>I just wonder what does the symbol " ||" do? Thank you! Appreciate your help.</p>