Reduce an array into array of arrays

array-of-arrays.js

const expect = require('expect')

const groupItems = (array = [], count) =>
  array.reduce(
    (prev, cur) => {
      const lastIndex = prev.length - 1
      return lastIndex === -1 ? [[cur]]
        : prev[lastIndex].length === count ? [...prev, [cur]]
        : [...prev.slice(0, -1), [...prev[lastIndex], cur]]
    },
    []
  )

describe('groupItems', () => {

  it('should create a group of 3 elements', () => {
    const groups = groupItems([1, 2, 3], 3)
    expect(groups).toEqual(
      [
        [1, 2, 3]
      ]
    )
  })

  it('should create 2 groups of 3 elements', () => {
    const groups = groupItems([1, 2, 3, 4, 5, 6], 3)
    expect(groups).toEqual(
      [
        [1, 2, 3],
        [4, 5, 6]
      ]
    )
  })

  it('should create a group of 2 elements, last group with 1 element', () => {
    const groups = groupItems([1, 2, 3], 2)
    expect(groups).toEqual(
      [
        [1, 2],
        [3]
      ]
    )
  })

  it('should create no group from undefined', () => {
    expect(groupItems()).toEqual([])
  })
})

  groupItems
    ✓ should create a group of 3 elements
    ✓ should create 2 groups of 3 elements
    ✓ should create a group of 2 elements, last group with 1 element
    ✓ should create no group from undefined


  4 passing (7ms)