/**
 * Calls a defined callback function on each element of an array, and
 * returns a new array of groups keyed by the return value of the callback
 * function, associated with an array of elements that have the same key.
 *
 * @param keyExtractFn - A function that accepts up to three arguments. The
 *  innerJoin method calls the keyFn function one time for each
 *  element in the array.
 * @param keyComparerFn - An optional function that accepts two arguments.
 *  The innerJoin method calls the keyComparerFn function to determine the
 *  equality of keys. Triple equal `===` comparison is used by default.
 *
 * @returns An array of group elements, each with an array of elements
 *  matching each key.
 */
export declare const innerJoin: <T, U>(other: readonly U[], on: (left: T, right: U) => boolean) => (arr: readonly T[]) => {
    left: T;
    right: U;
}[];