<pre class="shiki github-dark shaku"><code>/** * @template T * @template { keyof any } K * @param { Array&lt;T&gt; } items * @param { (item: T) =&gt; K } callback * @returns { Record&lt;K, Array&lt;T&gt;&gt; } */function ObjectGroupBy(items, callback) { const map = new Map(); for(const item of items) { const value = callback(item); if(!map.has(value)) { map.set(value, []); } const existing = map.get(value); map.set(value, [...existing, item]) } // Create a null-prototype object and transfer the map entries const result = Object.create(null); for (const [key, value] of map.entries()) { result[key] = value; } return result;}</code></pre>

Time Complexity: O(n²) in the worst-case scenario + Space 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: …