map two item once

const arr = {
  children: [1,2,3]
};
arr.childrenGroup = [];
arr.children.reduce((r, e, index) => {
  const lenghtOfChildren = qcSort.children.length;
  if (index % 2 === 1) {
    arr.childrenGroup.push([r,e]);
  } else if (lenghtOfChildren % 2 === 1 && index === lenghtOfChildren - 1) {
    arr.childrenGroup.push([e]);
  }
  return e;
}, null);
console.log(arr.childrenGroup);