/** Groups items by a property key; groups absent from the input remain absent. */
export declare function groupBy<K extends PropertyKey, T extends Record<K, PropertyKey>>(arr: T[], key: K): Partial<Record<T[K], T[]>>;
export declare function groupBy<K extends PropertyKey, T extends Record<K, PropertyKey>, R>(arr: T[], key: K, transform: (item: T) => R): Partial<Record<T[K], R[]>>;
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