A function to build two-dimensional array of sequential numbers

build-matrix.js

buildMatrix(3, 4, 5, 'right-up') 
// [
//   [1, 2, 3, 4],
//   [2, 3, 4, 5],
//   [3, 4, 5, 6],
// ]

function buildMatrix0(rCount, cCount, countStart, direction) {
  const [first, then] = direction.split('-')
  const matrix = Array(rCount).fill().map(() => [])
  const counts = {r: rCount, c: cCount}
  const i = {}
  let [d1, d2] = ['up', 'down'].includes(first)
    ? ['c', 'r'] : ['r', 'c']

  for (i[d1] = 0; i[d1] < counts[d1]; i[d1]++)
    for (i[d2] = 0; i[d2] < counts[d2]; i[d2]++)
      matrix[i.r][i.c] = countStart++

  if ([first, then].includes('up')) matrix.reverse()
  if ([first, then].includes('left')) matrix.forEach(row => row.reverse())

  return matrix
}

function buildMatrix(rCount, cCount, countStart, direction) {
  const matrix = []
  const calc = getCalc(countStart, direction, rCount, cCount)

  for (let r = 0; r < rCount; r++) {
    matrix[r] = []
    for (let c = 0; c < cCount; c++) {
      matrix[r][c] = calc(r, c)
    }
  }

  return matrix
}

function getCalc(start, dir, rCount, cCount) {
  return dir == 'right-down' ? (r, c) => start + r * cCount + c
    : dir == 'right-up' ? (r, c) => start + (rCount - r - 1) * cCount + c
    : dir == 'left-up' ? (r, c) => start + (rCount - r - 1) * cCount + (cCount - c - 1)
    : dir == 'left-down' ? (r, c) => start + r * cCount + (cCount - c - 1)
    : dir == 'down-right' ? (r, c) => start + c * rCount + r
    : dir == 'down-left' ? (r, c) => start + c * rCount + (rCount - r - 1)
    : dir == 'up-left' ? (r, c) => start + (cCount - c - 1) * rCount + (rCount - r - 1)
    : dir == 'up-right' ? (r, c) => start + (cCount - c - 1) * rCount + r
    : null
}

const directions = [
  'right-down',
  'right-up',
  'left-up',
  'left-down',
  'down-right',
  'down-left',
  'up-left',
  'up-right',
]

for (const direction of directions) {
  console.log(direction)
  console.log(buildMatrix(3, 4, 5, direction))
  console.log()
}