시저 암호 [Javascript]

내가 작성한 코드

function solution(s, n) {
    const eng_up = "ABCDEFGHIJKLMNOPQRSTUVWXYZABCDEFGHIJKLMNOPQRSTUVWXYZ".split("")
    const eng_lo = "abcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyz".split("")
    const sSplit = s.split("")
    
    const box = sSplit.map(elm=>{
        if(eng_up.includes(elm)){
            return elm = eng_up[eng_up.indexOf(elm)+n]
        } 
        if(eng_lo.includes(elm)){
            return elm = eng_lo[eng_lo.indexOf(elm)+n]
        }
        return elm
    })
    return box.join("")
}