title: Cannot set property 'display' of undefined
body: <p>I'm writting script to hide/show menu but I get some troubles.</p>
  
  <pre><code>    function displayMenu() {
  //var classMenu = event.target.className;
  //classMenu += 'Menu';
      //document.getElementsByClassName(classMenu).style.display = 'block';
  document.getElementsByClassName('btn-pageMenu').style.display = 'block';
      }
  </code></pre>
  
  <p>In comment what I want to do finally, but even if I try with static var it's not working.
  In the CSS :</p>
  
  <pre><code>    fieldset.toolsbox ul.btn-pageMenu {display:none;}
  </code></pre>
  
  <p>I try like this too :</p>
  
  <pre><code>    .btn-pageMenu {display:none;}
  </code></pre>
  
  <p>No more success. Anybody have a suggestion ? I'm learning JS and I not finding errors when I compare with other similar scripts.</p>
  
  <p>Thanks for your help :)</p>