<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; { if (items.length === 0) { return { } as any; } else if (!Array.isArray(items)) { throw new Error(`${items} is not an array`) } else if (items === null) { return items; } const len = items.length; const result: any = {}; result.__proto__ = null; for (let i=0; i &lt; len; i++) { const key = callback(items[i]); if (typeof (result as any)[key] === 'undefined') { result[key] = []; } result[key].push(items[i]); } return result;}const items = [ { id: 1, kind: 'a', }, { id: 2, kind: 'b', }, { id: 3, kind: 'a', }];</code></pre>

Best case: O(n) | Worst case: O(n^2)

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: …