Generic groupBy with key inference
Group an array into a Record by a property or function while keeping the item type.
- TypeScript
- GreenSnippets
- MIT
- Executed with Node.js 22 (type stripping)
function groupBy<T, K extends string | number>(
items: readonly T[],
getKey: (item: T) => K,
): Partial<Record<K, T[]>> {
const result: Partial<Record<K, T[]>> = {};
for (const item of items) {
const key = getKey(item);
(result[key] ??= []).push(item);
}
return result;
}Example / expected output
const byRole = groupBy(users, (u) => u.role);
// Partial<Record<"admin" | "member", User[]>>How it works
The key function decides the group name, and the return type is a partial record because not every possible key necessarily appears. Object.groupBy exists in newer runtimes, but this version works everywhere and is easy to extend.
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