[].reduce vs. [].flatMap

index.js

const xs = [...Array(10000).keys()];

const prom = (n) => Promise.resolve(n);

(async () => {
  console.time("reduce");
  const t1 = xs.reduce(
    (memo, n) => (n % 2 === 0 ? [...memo, prom(n)] : memo),
    []
  );
  await Promise.all(t1);
  console.timeEnd("reduce");

  console.time("flatMap");
  const t2 = xs.flatMap((n) => (n % 2 === 0 ? [prom(n)] : []));
  await Promise.all(t2);
  console.timeEnd("flatMap");
})();