My another Caesars Cipher algorithn

[wildan]

So i try to solve caesars cipher with for looping and indexOf array properties. But, i still confused in this part :

alpha[(ind + 13)  % 26

if you guys have some ref for me learn more deep about part that i confused, please share to me.
tis is the codes :

function rot13(str) {
  let alpha = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
  let arr = [];
  let spl = str.split("");
  for(let i = 0; i < spl.length; i++) {
    let ind = alpha.indexOf(spl[i]);
    if(ind !== -1 && ind !== undefined) {
       arr.push(alpha[(ind + 13)  % 26])
    } else {
      arr.push(spl[i])
    }
  }

  return arr.join("")
}

[JeremyLT]

It looks like a working solution, so I added spoiler tags.

[joshua.rod]

You need to check how modulus works on javascript

Just some testing, imagine that in one letter of the string you have “a”, so the index will be 0. Applying alpha[(ind + 13) % 26] will result in alpha[(0+13) % 26]

0 + 13 = 13
13 % 26 = 13 (remainder)

Now imagine we got the letter “z” in the string, so the index will be 25 (26 0-indexed). In this case we have:

25 + 13 = 38
38 % 26 = 12 (remainder and perfect position)
// That's because we need to start again 
// once we get to the end