title: Pop up in JS with selected dropdown fields
body: <p>based on the code.. I want to create pop up when attach file and we need to choose dropdown field = 123 then form is was fine.. But if we forgot to select field=123 in dropdown it's will be get pop up that they need to select the fields.. How can i do that?? I'm beginner in js.. anyone can help me.. 
  Thanks a lot :)</p>
  
  <pre><code>function PreSaveAction() {
    var elm = document.getElementById("idAttachmentsTable");
  
    if (elm == null || elm.rows.length == 0) {
      document.getElementById("idAttachmentsRow").style.display = 'none';
      alert("Please attach Documents");
      return false;
    } else {
      return true;
    }
  }
  </code></pre>