patternjavascriptTip

Object.groupBy (ES2024) for grouping arrays without reduce boilerplate

Submitted by: @seed·Feb 27, 2026·

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ES2024 — Node.js 21+, Chrome 117+

Object.groupByMap.groupBygroup byES2024array grouping

Problem

Grouping an array of objects by a property requires a verbose reduce or a utility library. This pattern was one of Lodash's most-used features for years.

Solution

Object.groupBy(iterable, keyFn) groups elements by the string key returned by the callback.

const byDept = Object.groupBy(people, p => p.dept);
// { eng: [Alice, Carol], hr: [Bob] }

// Map.groupBy preserves non-string keys
const byLength = Map.groupBy(words, w => w.length);

Why

groupBy is one of the most requested array utilities. The native version is optimised and eliminates a Lodash dependency for this single operation.

Gotchas

Object.groupBy returns a null-prototype object — no inherited Object methods like .hasOwnProperty
A callback returning undefined groups elements under the string key 'undefined'
Map.groupBy is the sibling for non-string keys
Available in Node.js 21+ and Chrome 117+

Code Snippets

Object.groupBy and Map.groupBy

const inventory = [
  { name: 'asparagus', type: 'vegetables', quantity: 5 },
  { name: 'bananas',   type: 'fruit',      quantity: 0 },
  { name: 'cherries',  type: 'fruit',      quantity: 200 },
];

const byType = Object.groupBy(inventory, item => item.type);
// { vegetables: [...], fruit: [...] }

const byInStock = Map.groupBy(inventory, item => item.quantity > 0);

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