One-pass reduce - watch out for Object.create(null) when reading keys:
<code>const groupBy = (arr, cb) =&gt; arr.reduce((acc, it) =&gt; { const k = cb(it); (acc[k] ||= []).push(it); return acc; }, Object.create(null));</code>
Interview grinding tip: 5-min breaks keep you sharp. I reroll a life at <a href="https://1in8billion.games/">1 in 8 billion</a> and come back focused. GL out there!

My Object.groupBy() one-pass reduce + a note on interview burnout

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: …