/**
 * Groups elements of an array into an object, using a key-generating function.
 *
 * Examples:
 *   groupBy(['Hello', 'World', 'Hello'], a => a) // { Hello: ['Hello', 'Hello'], World: ['World'] }
 *
 * @param arr - The array to group
 * @param toKey - Function to generate a key for each element
 * @returns An object mapping keys to arrays of grouped elements
 */
export default function groupBy<ArrayType extends any[], ElementType extends ArrayType extends (infer U)[] ? U : never>(arr: ArrayType, toKey: (a: ElementType) => string): Record<string, ArrayType>;