Ignoring special characters and spaces in Caeser Cipher in Javascript

I am creating a program in javascript which which asks the user to enter a sentence and then encrypt the sentence by replacing each letter with a letter 15 characters down the alphabet. I am unsure on how to have the input keep the special characters and spaces and not try to encrypt those.

let userInput = prompt("Please enter any text to encrypt it:");
let lowerInput = userInput.toLowerCase();



function startEncryption() {
  let encryptedMessage = "";
  let shift = 15;

for (letter of lowerInput) {
  encryptedMessage += shiftLetter(letter, shift);
 }  

 console.log(encryptedMessage);
  }
 startEncryption();

function shiftLetter(letter, shift) {
  let newLetter = "";

  let letterCode = letter.charCodeAt(0);
  let newLetterCode = letterCode + shift;

  if (newLetterCode < 97) {

    newLetterCode += 26;
  } else if (newLetterCode > 122) {

    newLetterCode -= 26;
  }

  newLetter = String.fromCharCode(newLetterCode);

  return newLetter;
 }

you could add a guard clause to your shiftLetter function, exit early if an invalid letter/symbol is found.

function shiftLetter(letter, shift) {
if (!letter.match(/^[a-z]$/i)) { return letter }
// the rest of your code
}