title: How calculate the height of div with definition 'height=auto'
body: <p>I need to calculate the height of this div with Javascript.</p>
  
  <p>When i wrote this script:</p>
  
  <pre><code>function getMainDivHeight() {
      var num = document.getElementById('up_container').style.height;
      return num;
  }
  </code></pre>
  
  <p>this script returns <code>auto</code></p>
  
  <p>I need the height of the div after it change.</p>
  
  <p>Thanks ! ! !</p>