Массив разделов на два

const partition = (arr, fn) =>
  arr.reduce(
    (acc, val, i, arr) => {
      acc[fn(val, i, arr) ? 0 : 1].push(val);
      return acc;
    },
    [[], []]
  );

const partitionBy = (arr, fn) =>
  arr.reduce(
    ({ res, last }, v, i, a) => {
      const next = fn(v, i, a);
      if (next !== last) res.push([v]);
      else res[res.length - 1].push(v);
      return { res, last: next };
    },
    { res: [] }
  ).res;

const users = [
  { user: 'barney', age: 36, active: false },
  { user: 'fred', age: 40, active: true },
];
partition(users, o => o.active);
// [
//   [{ user: 'fred', age: 40, active: true }],
//   [{ user: 'barney', age: 36, active: false }]
// ]

const numbers = [1, 1, 3, 3, 4, 5, 5, 5];
partitionBy(numbers, n => n % 2 === 0); // [[1, 1, 3, 3], [4], [5, 5, 5]]
partitionBy(numbers, n => n); // [[1, 1], [3, 3], [4], [5, 5, 5]]