/**
 * groups an array of elements into multiple arrays on an object where the
 * key is the string returned by the groupKey function.
 *
 * If the groupKey function returns undefined, the value won't be included
 * in the result. This can be used to filter out values from the initial array.
 *
 * There's also an ECMAScript proposal that will add a very similar function
 * on Object and Array: https://github.com/tc39/proposal-array-grouping
 *
 * This function can be deprecated once it's widely released.
 */
export declare const groupBy: <T, K extends string>(elements: T[], groupKey: (element: T) => K | undefined) => Record<K, T[]>;
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