[1,2,3,4,5] to [[1,2], [3,4], [5,null]]

a.js

const a = [1,2,3,4,5]
const b = a.reduce((acc, v, i, a) => {
  const [left, right] = acc
  if (i % 2 == 0) {
    left.push(v)
    if (i === a.length - 1) {
      right.push(null)
    }
  } else {
    right.push(v)
  }
  return acc
}, [[],  []])
console.log(b)