&gt; we just use the index from the .map() loop to add a key to every element<p>Doing this could hide weird bugs. Let&#x27;s say we render a list of posts with &quot;remove&quot; button for each one. The user clicks on &quot;remove&quot; button, we modify the underlying this.state.posts. And on next re-render removed post key would be assigned to the next post! The next post could inherit just-deleted post handlers, for example.<p>It&#x27;s much better to use  unique keys like database ids.