group-by.md

Group By

Task: "Group this array of objects by a key."

Without Ponytail

npm install lodash

import { groupBy } from "lodash";

const byStatus = groupBy(orders, "status");
// → { pending: [...], shipped: [...], delivered: [...] }

Or the hand-rolled version:

const byStatus = orders.reduce((acc, order) => {
  (acc[order.status] ??= []).push(order);
  return acc;
}, {});

With Ponytail

// ponytail: Object.groupBy does this
const byStatus = Object.groupBy(orders, order => order.status);
// → { pending: [...], shipped: [...], delivered: [...] }

1 dependency (or a reduce) → 1 built-in. Object.groupBy shipped in Chrome 117, Firefox 119, Safari 17.4, Node.js 21. If you need a Map instead of a plain object: Map.groupBy(orders, o => o.status). Check your target runtime; if you need IE11 or old Node, the reduce one-liner is still the right call, not lodash.