title: Does specific element exist event
body: <p>Is there any event to know when a specific element "starts to exist" in raw javascript?
  For example I have</p>
  
  <pre><code>&lt;div class="parent"&gt;
    &lt;div class="child"&gt;&lt;/div&gt;
  &lt;/div&gt;
  </code></pre>
  
  <p>And I want to do something when .parent and only the .parent (not the .child) "starts to exist" using an event instead of putting js code inside of it. I tried to setInterval and check whether the .parent exists.</p>