title: How to get all middle items in cards of 3 columns?
body: <p>I have an array of cards and want to get all the middle cards on the list and apply background color for them.</p>
  <p><a href="https://i.stack.imgur.com/FbZoz.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/FbZoz.jpg" alt="shot" /></a></p>
  <p>if we explain it we have the index
  like this</p>
  <pre><code>0 1 2
  3 4 5
  6 7 8
  </code></pre>
  <p>so the targetted index is</p>
  <pre><code>1 4 7 etc
  </code></pre>
  <p>So I guess there's a formula to get it but I don't get it.</p>
  <p>the current example code here</p>
  <pre><code>backgroundColor: index % 2 === 1 ? '#f00' : '#0f199a',
  </code></pre>