const arr = [
  { id: 1, parentid: 0 },
  { id: 4, parentid: 2 },
  { id: 3, parentid: 1 },
  { id: 5, parentid: 0 },
  { id: 6, parentid: 0 },
  { id: 2, parentid: 1 },
  { id: 7, parentid: 4 },
  { id: 8, parentid: 1 },
  { id: 9, parentid: 7 },
  { id: 10, parentid: 9 },
  { id: 11, parentid: 3 }
];

function unflatten(arr) {
  const map = arr.reduce((pre, item) => {
    pre[item.id] = item;
    pre[item.id].children = [];
    return pre;
  }, {});

  const tree = [];
  Object.keys(map).forEach(id => {
    const value = map[id];
    if (value.parentid) {
      map[value.parentid].children.push(value);
    } else {
      tree.push(value);
    }
  });
  return tree;
}

const tree = unflatten(arr);