https://www.codewars.com/kata/arrange-matrix-by-diagonals-omg

diagonalKata.js

const getSize = matrix => ({
  width: matrix[0].length,
  height: matrix.length
})

const goDiagonally = (func, matrix, columnLimit) => (line, column) => {
  while (line >= 0) {
    matrix[line][column] = func()

    line--
    column++

    if (column > columnLimit) break
  }
}

const getEmptyMatrix = height => Array.from({ length: height }).map(_ => [])

const diagonaze = (matrix, func) => {
  const { width, height } = getSize(matrix)
  const newMatrix = getEmptyMatrix(height)
  const lineLimit = (height - 1)
  const columnLimit = (width - 1)
  const goDiagonal = goDiagonally(func, newMatrix, columnLimit)
  let currentLine = 0
  let currentColumn = 0

  goDiagonal(currentLine, currentColumn)
  
  while (currentLine < lineLimit || currentColumn < columnLimit) {
    if (currentLine < lineLimit) currentLine++
    else if (currentColumn < columnLimit) currentColumn++

    goDiagonal(currentLine, currentColumn)
  }

  return newMatrix
}

const flattern = matrix => 
  matrix.reduce((acc, array) => [ ...acc, ...array ], [])

const kataMaldito = matrix => {
  const string = flattern(matrix)

  return diagonaze(matrix, () => string.shift())
}

export default kataMaldito

tests.js

import expect from 'expect'
import kataMaldito from './diagonalKata'

expect(kataMaldito([
  ['c', 'h', 'a'],
  ['r', 'a', 'c'],
  ['t', 'e', 'r']
])).toEqual([
  ['c', 'a', 'c'],
  ['h', 'a', 'e'],
  ['r', 't', 'r']
])

expect(kataMaldito([
  ['J', 'A', 'V', 'A'],
  ['C', 'O', 'D', 'E'],
  ['T', 'A', 'C', 'O'],
])).toEqual([
  ['J', 'V', 'O', 'T'],
  ['A', 'C', 'E', 'C'],
  ['A', 'D', 'A', 'O']
])

expect(kataMaldito([
  ['T', 'h', 'i', 's', 'A', 'r', 'r', 'a', 'y'],
  ['i', 'S', 's', 'O', 'c', 'r', 'a', 'z', 'Y'],
  ['I', 't', 's', 'R', 'l', 'y', 'B', 'a', 'd']
])).toEqual([
  ['T', 'i', 'r', 'y', 's', 'r', 'Y', 's', 'y'],
  ['h', 'A', 'a', 'S', 'c', 'z', 't', 'l', 'a'],
  ['s', 'r', 'i', 'O', 'a', 'I', 'R', 'B', 'd']
])

expect(kataMaldito([
  ['c', 'h'],
  ['r', 'a'],
  ['t', 'e'],
  ['a', 'b']
])).toEqual([
  ['c', 'r'],
  ['h', 't'],
  ['a', 'a'],
  ['e', 'b']
])