chunk an array of numbers

arrayChunker.js

const chunker = (partial, current) => {
  const end = partial.length - 1;

  if (end >= 0 && current === partial[end][0]) {
    return [
      ...partial.slice(0, end),
      [...partial[end], current]
    ];
  }

  return [...partial, [current]];
};

console.log(
  JSON.stringify(
    [1, 1, 1, 2, 3, 3].reduce(chunker, [])
  ) === '[[1,1,1],[2],[3,3]]'
);

console.log(
  JSON.stringify(
    [1, 1, 2, 2, 1, null, 2, 2].reduce(chunker, [])
  ) === '[[1,1],[2,2],[1],[null],[2,2]]'
);