How to concatenate this array : <p>How do i concatenate these two arrays to get the expected output?</p>

<pre class="lang-js prettyprint-override"><code>let bidder= [
  [1,2,3,4],
  [5,6,7,8]
]

let arr1 = [9,10,11,12]

</code></pre>

<p>expected result</p>

<pre class="lang-js prettyprint-override"><code>[
(4) [...],
(4) [...],
(4) [...],
]
</code></pre>
