type MapOptions<T, U = T> = {
    /**
     * Allow merging existing keys or not, if not keys will be overriden if they exist
     * (default=false)
     *
     * Example:
     *  mapFn([{uid: 12, a: 'hi'}, {uid: 12, b: 'ho'}], el => el.uid, {merge: true})
     * Output:
     *  {12: {uid: 12, a: 'hi', b: 'ho'}}
     * Output if merge is false
     *  {12: {uid: 12, b: 'ho'}}
     */
    merge?: boolean;
    /**
     * A custom transform function that is applied to each element before mapping.
     * Its output type `U` becomes the value type of the resulting map.
     */
    transform_fn?: (el: T) => U;
};
type MapFn<T extends Record<string, any>> = (entry: T) => (string | number | boolean);
/**
 * Map an object array into a kv-object through a function that generates a key. Returning a non-string,
 * non-numeric value from the function (eg: false) will filter out the object.
 *
 * Example:
 *  mapFn([{uid: 12, name: 'Peter'}, {uid: 15, name: 'Jonas'}], el => el.uid);
 * Output:
 *  {12: {uid: 12, name: 'Peter'}, 15: {uid: 15, name: 'Jonas'}}
 *
 * @param {Record<string, any>[]} val - Array to map
 * @param {MapFn} fn - Handler function which is run for each of the objects and should return a string or number
 * @param {MapOptions?} opts - Options object to override built-in defaults
 */
declare function mapFn<T extends Record<string, any>, U extends Record<string, any> = T>(arr: T[], fn: MapFn<T>, opts?: MapOptions<T, U>): Record<string, U>;
export { mapFn, mapFn as default };