Flat array to entries (pairs of key-val)

arr-to-entries.js

const arr = [1, 2, 3, 4, 5, 6]; // -> [[1,2],[3,4],[5,6]]

arr.reduce((acc, cur, i) => {
  const pos = ~~(i / 2);
  acc[pos] = acc[pos] ?? [];

  if (i % 2 === 0) acc[pos].push(cur);
  else acc[pos].unshift(cur);

  return acc;
}, []);

arr.reduce(
  (acc, cur, i, pos) => (
    (pos = ~~(i / 2)), (acc[pos] = acc[pos] ?? []), (acc[pos][i % 2] = cur), acc
  ),
  [],
);

arr.reduce((acc, cur, i) => {
  if (i % 2 === 0) acc.push([cur]);
  else acc.at(-1).push(cur);
  return acc;
}, []);

arr.reduce(
  (acc, cur, i) => (i % 2 === 0 ? acc.push([cur]) : acc.at(-1).push(cur), acc),
  [],
);

Array.from({ length: Math.ceil(arr.length / 2) }, (_, i) =>
  arr.slice(i * 2, i * 2 + 2),
);