mapObject.js

function *entriesLazy(obj) {
  for (const k in obj) yield [k, obj[k]];
}

[...entriesLazy({ a: 1, b: 2 })];
// [['a', 1], ['b', 2]]

function *mapEntriesLazy(f, iter) {
  for (const [k, a] of iter) yield [k, f(a)];
}

[...mapEntriesLazy(a => a + 1, [['a', 1], ['b', 2]])];
// [['a', 2], ['b', 3]]

const m = new Map([['a', 1], ['b', 2]]);
new Map([...mapEntriesLazy(a => a + 1, m.entries())]);
// Map(2) {"a" => 2, "b" => 3}

const object = iter =>
  reduce((obj, [k, v]) => (obj[k] = v, obj), {}, iter);

object([['a', 1], ['b', 2]]);
// { a: 1, b: 2 }

const mapObject = (f, obj) =>
  object(mapEntriesLazy(f, entriesLazy(obj)));

mapObject(a => a + 1, { a: 1, b: 2});
// { a: 2, b: 3 }

mapObject(a => a + 1, new Map([['a', 1], ['b', 2]]).entries());
// { a: 2, b: 3}