gen+map+filter+reduce.js

function reduce(f, acc, iter) {
  if (arguments.length == 2) {
    iter = acc[Symbol.iterator]();
    acc = iter.next().value;
  }
  for (const a of iter) acc = f(acc, a);
  return acc;
}

function *map(f, iter) {
  for (const a of iter) yield f(a);
}

function *filter(f, iter) {
  for (const a of iter) if (f(a)) yield a;
}

const add = (a, b) => a + b;

const list = [-1, 2, -3, 4];

reduce(add, map(Math.abs, list));
// 10
reduce(add, filter(a => a % 2, map(Math.abs, list)));
// 4