partition.js

const partition = (list, partitioner) => {
  const toIndex = (el) => (partitioner(el)) ? 0 : 1; 
  
  const toPartition = (result, el) => {
    result[toIndex(el)].push(el)
    return result;
  } 
  return list.reduce(toPartition, [[],[]]);
}

partition([1, 2, 3, 4], n => n % 2)