title: Set certain data to firstChild "data-id" attrbute?
body: <p>How can I get/set the firstChild's attribute "data-id"?
  I can set name and inner text of a textarea :</p>
  
  <pre><code>el.firstChild.name='txt_'+j;
  el.firstChild.innerText = 'Textarea '+j;
  </code></pre>
  
  <p>what I am trying to do is to change "data-id' value like this (pseudocode):</p>
  
  <pre><code>el.firstChild.data('id')=j; 
  </code></pre>
  
  <p>Just to clarify  - a bit of HTML:</p>
  
  <pre><code>&lt;div id="5" class="textarea_cloned"&gt;
  &lt;textarea name="txt_5"&gt;Textarea 5&lt;/textarea&gt;
  &lt;div class="remove" data-id="5"&gt; X &lt;/div&gt;
  &lt;/div&gt;
  </code></pre>
  
  <p>Thank you</p>