function solution(s, n) {
    
    let upper = [];
    let lower = [];
    let answer = '';
    
    for (let i = 'A'.charCodeAt(); i<='Z'.charCodeAt(); i++) {
        upper.push(String.fromCharCode(i));
    }
    for (let i = 'a'.charCodeAt(); i<='z'.charCodeAt(); i++) {
        lower.push(String.fromCharCode(i));
    }
    
    for (let i = 0; i< s.length; i++) {
        if(s[i] === ' ') {
            answer += s[i];
            continue
        }
        if(upper.includes(s[i])) {
           let Index = upper.indexOf(s[i]);
            answer += upper[(Index + n) % 26];
            continue;
        }
       let Index = lower.indexOf(s[i]);
        answer += lower[(Index + n) % 26];
    }
    return answer;
  }

참고

https://velog.io/@michael00987/javascript%ED%94%84%EB%A1%9C%EA%B7%B8%EB%9E%98%EB%A8%B8%EC%8A%A4-%EC%8B%9C%EC%A0%80-%EC%95%94%ED%98%B8