Split a string every N characters (starting from the end)

splitEveryN.js

function splitEveryN(str, n) {
  const len = str.length
  if (len > n) {
    let i = len - n
    const parts = []
    while (true) {
      parts.push(str.substr(i, n))
      if (i <= n) {
        parts.push(str.substr(0, i))
        return parts.reverse()
      }
      i -= n
    }
  }
  return [str]
}

A nicer API would be positive n to indicate "start from beginning" and negative n to indicate "start from end".

splitEveryN('abcde', 2) // => ['ab', 'cd', 'e']
splitEveryN('abcde', -2) // => ['a', 'bc', 'de']

splitEveryNth may be a better name