A beautiful implementation of groupBy in typescript

groupBy.ts

export const groupBy = <X, K extends PropertyKey>(
  getKey: (x: X) => K,
  xs: readonly X[],
): { readonly [k in K]?: readonly (X | undefined)[] } => {
  const grouped: { [k in K]?: (X | undefined)[] } = {};

  for (const x of xs) {
    const key = getKey(x);
    grouped[key] ??= [];
    // we have initialised ^ so grouped[key] won't be undefined
    // eslint-disable-next-line @typescript-eslint/no-non-null-assertion
    grouped[key]!.push(x);
  }

  return grouped;
};