with_index.js

const createEach = (withIndex) => {
  return withIndex ?
    (arr, iterator) => arr.forEach((elem, index) => iterator(elem, index)) :
    (arr, iterator) => arr.forEach((elem) => iterator(elem));
};

const createMap = (withIndex) => {
  return withIndex ?
    (arr, iterator) => arr.map((elem, index) => iterator(elem, index)) :
    (arr, iterator) => arr.map((elem) => iterator(elem));
};

const each = createEach(false);
each.withIndex = createEach(true);

const map = createMap(false);
map.withIndex = createMap(true);

map(['10', '10', '10'], parseInt); // [ 10, 10, 10 ]
map.withIndex(['10', '10', '10'], parseInt); // [ 10, NaN, 2 ]

with_index_take2.js

const create = (forArrFn, withIndex) => {
  return withIndex ?
    (arr, iterator) => (forArrFn(arr)((elem, i) => iterator(elem, i))) :
    (arr, iterator) => (forArrFn(arr)((elem) => iterator(elem)));
};

const forArrEachFn = arr => arr.forEach.bind(arr);
const each = create(forArrEachFn, false);
each.withIndex = create(forArrEachFn, true);

const forArrMapFn = arr => arr.map.bind(arr);
const map = create(forArrMapFn, false);
map.withIndex = create(forArrMapFn, true);

map(['10', '10', '10'], parseInt); // [ 10, 10, 10 ]
map.withIndex(['10', '10', '10'], parseInt); // [ 10, NaN, 2 ]

with_index_take3.js

const arrayMethods = ['forEach', 'map', 'filter'];
const anExcellentModule = {};

arrayMethods.forEach((name) => {
  anExcellentModule[name] = (arr, iterator) => arr[name](elem => iterator(elem));
  anExcellentModule[name].withIndex = (arr, iterator) => arr[name]((elem, i) => iterator(elem, i));
});

anExcellentModule.map(['10', '10', '10'], parseInt); // [ 10, 10, 10 ]
anExcellentModule.map.withIndex(['10', '10', '10'], parseInt); // [ 10, NaN, 2 ]