Map function in TypeScript that works for Arrays as well as Objects.

map.ts

/**
 * Unknown array type alias.
 */
type TArray = unknown[];

/**
 * Unknown object type alias.
 */
type TObject = Record<string | symbol, unknown>;

/**
 * Mapable collection.
 */
export type Mapable = TArray | TObject;

/**
 * Mapper function.
 */
export type Mapper<T extends Mapable, U> = T extends TArray
  ? (value: T[number], key: number, collection: T) => U
  : (value: T[keyof T], key: keyof T, collection: T) => U;

/**
 * Map arrays and objects alike.
 */
export function map<T extends TArray, U>(collection: T, map: Mapper<T, U>): U[];
export function map<T extends TObject, U>(
  collection: T,
  map: Mapper<T, U>
): Record<keyof T, U>;
export function map<T extends Mapable, U>(
  collection: Mapable,
  map: Mapper<T, U>
) {
  if (Array.isArray(collection)) {
    return collection.map(map as Mapper<TArray, U>);
  }

  return Object.fromEntries(
    Object.entries(collection).map(([key, value]) => [
      key,
      (map as Mapper<TObject, U>)(value, key, collection),
    ])
  );
}