group-by

groupBy walks an array once and bins each item under the string form of item[key]. Useful for building lookup tables, rendering grouped lists, or reducing flat data to a categorized shape.

export function groupBy<T>(arr: T[], key: keyof T): Record<string, T[]> {
	return arr.reduce<Record<string, T[]>>((acc, item) => {
		const group = String(item[key]);
		if (!acc[group]) acc[group] = [];
		acc[group].push(item);
		return acc;
	}, {});
}

Notes

• The key value is coerced via String(...) so numeric and boolean fields work, but at the cost of losing their original type as a record key.
• For the standard-library equivalent on modern runtimes, see Object.groupBy (ES2024).