<pre class="shiki github-dark shaku"><code>function ObjectGroupBy&lt;T, K extends keyof any&gt;( items: Array&lt;T&gt;, callback: (item: T) =&gt; K): Record&lt;K, Array&lt;T&gt;&gt; { return items.reduce((acc, item) =&gt; { if (!(callback(item) in acc)) { acc[callback(item)] = []; } acc[callback(item)].push(item); return acc; }, Object.create(null))}</code></pre>

Typescript Solution using Array.reduce() | Time complexity O(n)

Object.groupBy() allows us to easily group array items, please try to implement it by yourself. const items = [ { id: 1, kind: 'a', }, { id: 2, kind: …