프로그래머스 | 압축

function solution(msg) {
    var answer = [];
    let arr=new Array(26).fill(0).map((e,i)=>String.fromCharCode(i+65));
    
    for(let i=0;i<msg.length;i++){
        let s=msg[i];
        let ns=msg[i+1];
        
        while(arr.includes(s+ns)&&i<msg.length-1){
            i++;
            s=s+ns;
            ns=msg[i+1];
        }
        
        answer.push(arr.indexOf(s)+1);
        if(ns!==undefined) arr.push(s+ns)
        
    }
    return answer;
}