Javascript Reduce Map

reduce-map.test.ts

import { reduceMap } from './reduce-map';

describe('reduceMap', () => {
  const sumValueAndSubtractKey = (result: number, value: number, key: number) => result + value - key;

  it('should return initial value for empty map', () => {
    expect(
      reduceMap(
        new Map(),
        sumValueAndSubtractKey,
        4,
      ),
    ).toEqual(4);
  });

  it('should return accumulated value for non-empty map', () => {
    expect(
      reduceMap(
        new Map([
          [5, 2],
          [7, 4],
        ]),
        sumValueAndSubtractKey,
        2,
      ),
    ).toEqual(-4);
  });
});

reduce-map.ts

export const reduceMap = <TMapKey, TMapValue, TResult>
  (
    map: Map<TMapKey, TMapValue>,
    callback: (accumulator: TResult, value: TMapValue, key: TMapKey) => TResult,
    accumulator: TResult,
  ) => {
  let result = accumulator;

  map.forEach((value, key) => result = callback(result, value, key));

  return result;
};