Discussion on: Public Solving: Caesar decipher in JavaScript

An interesting property of latin alphabetical characters is that their 5th bit (32) being set or unset is the only difference between a large or a small character and the 6th bit (64) is set for both. We can also employ binary operations like | and & to filter certain bits.

characterCode & 31 = 1 for 'A', 2 for 'B'...
characterCode & 96 = 64 for upper case, 96 for lower case

That allows us to create a letterShift function that returns a function that directly works as a callback to string. replace.

const letterShift = (shift) => (letter) => {
  const charCode = letter.charCodeAt(0)
  // 64 for upper case, 96 for lower case
  const casePart = charCode & 96
  // 1 for a, 2 for b, ...
  const charPart = charCode & 31
  // use modulo for wraparound
  const shiftedCharPart = ((charPart - 1 + shift) % 26) + 1
  return String.fromCharCode(casePart | shiftedCharPart)
}

P.S. I wrote this on my cellphone, so there are probably a few typos in there.