Chunks

const chunk = (arr, size) => arr.reduce((acc, e, i) => (i % size ? acc[acc.length - 1].push(e) : acc.push([e]), acc), []);

chunk([1,2,3,4,5,6,7,8,9],3) // [[1,2,3],[4,5,6],[7,8,9]]
chunk([2,3,4,5,6],3) // [[2,3,4],[5,6]]