A variation that keeps the null-prototype behaviour of the real <code>Object.groupBy</code>:
<pre class="shiki github-dark shaku"><code>function ObjectGroupBy(items, callback) { const out = Object.create(null) for (const item of items) { const key = callback(item) if (!out[key]) out[key] = [] out[key].push(item) } return out}</code></pre>
Two details that bite in the tests:
<ul>
<li>the result must not inherit from <code>Object.prototype</code>, so <code>Object.create(null)</code> rather than <code>{}</code> — otherwise a key like <code>toString</code> collides with an inherited method</li>
<li>keys go through string coercion, so <code>1</code> and <code>'1'</code> land in the same bucket</li>
</ul>
We use this exact shape to bucket render jobs by aspect ratio at <a href="https://ravvi.ai">Ravvi</a>, and the null-prototype part is what stopped a <code>constructor</code> key from blowing things up.

Null-prototype version + the two edge cases that break naive solutions

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: …