Replace letters in a key with either uniform alpha or random letters

Replace random key

const randomInt = (from, to) => Math.floor(Math.random() * (to - from + 1) + from)

const rs = (input, random = false) => {
  
  const len = input.length
  const upper = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ'.split('')
  const lower = 'abcdefghijklmnopqrstuvwxyz'.split('')
  let lastUpper = -1
  let lastLower = -1
  let output = ''
  
  for (let i = 0; i < len; i++) {
    const char = input.charAt(i)
    if (!upper.includes(char) && !lower.includes(char)) {
      output += char
      continue
    }
    const isUpper = char === char.toUpperCase()
    if (random) {
      const ri = randomInt(0, upper.length - 1)
      output += isUpper ? upper[ri] : lower[ri]
    } else {
      if (isUpper) {
        lastUpper++
        if (!upper[lastUpper])
            lastUpper = 0
        output += upper[lastUpper]
      } else {
        lastLower++
        if (!lower[lastLower])
            lastLower = 0
        output += lower[lastLower]
      }
    }
    
  }
  
  return output
  
}

console.log(rs('irqrLyYWqBNjSDM3', true))