function fssn(str) { let arr = str.split(""); let res = []; let st = arr[0]; let indexs = 0; let length = 0; for (let i = 1; i < arr.length + 1; i++) { if (arr[i - 1] == arr[i]) { st += arr[i]; } else { res.push(st); st = arr[i]; } } res.forEach((item, index) => { if (item.length > length) { length = item.length; indexs = index; } }); return res[indexs]; }