Increment Excel column letters

incrementColumnLetters.js

// Only supports positive numbers for now
function column (letters, numberOfColumnsAway=0) {
  let newColumnLetters = letters.trim().toUpperCase()

  for (let i = 0; i < Math.abs(numberOfColumnsAway); i++) {
    newColumnLetters = incrementColumnLetters(newColumnLetters)
  }

  return newColumnLetters
}

function incrementColumnLetters (letters) {
  const INPUT_ERROR_MSG = 'Input is not a string of letters.'
  if (!letters) throw new TypeError(INPUT_ERROR_MSG)

  const startLetter = 'A'
  const endLetter = 'Z'
  const endLetterCode = endLetter.charCodeAt()
  const letterRangeRE = new RegExp(`[${startLetter}-${endLetter}]`,'g')
  const lettersList = letters.trim().toUpperCase().match(letterRangeRE)

  if (!Array.isArray(lettersList)) throw new TypeError(INPUT_ERROR_MSG)

  let i,c,letter
  const incrementedLettersList = []
  for (i=lettersList.length-1,c=1; i>=0||c>0; i--) {
    if (lettersList[i]) letter = lettersList[i]
    else letter = startLetter, c--
    if (c>0) {
      letter = String.fromCharCode(letter.charCodeAt() + c--)
      if (letter.charCodeAt() > endLetterCode) letter = startLetter, c++
    }
    incrementedLettersList.unshift(letter)
  }

  return incrementedLettersList.join('')
}