SOURCE

const str = 'scasdcdddsssdsdsdd';
let res = '';



const lengestPalindrome = function(s){
    let len = s.length-1;
    if(len < 2){
        return s;
    }

    for(let i = 0; i <= len; i++){
        helpFn(i,i);
        helpFn(i,i+1)
    }

    function helpFn(m,n){
        while(m >= 0&&m <= len&&s[m] == s[n]){
            m--;
            n++;
        }
        if(n - m - 1 > res.length){
            res = s.slice(m+1,n)
        }
    }
    return res;
}

console.log(lengestPalindrome(str))