const input = 'abrkaaabcdefghijkjxxx';
let max = 0;
let index;

function findSubStr(startIndex) {
  let obj = Object.assign({});
  let len = 0;
  while (input[startIndex]) {
    if (!obj[input[startIndex]]) {
      obj[input[startIndex]] = input[startIndex];
      startIndex += 1;
      len += 1;
    } else {
      break;
    }
  }
  return len;
}

for (let i = 0; i < input.length; i++) {
  let newMax = findSubStr(i);
  if (newMax > max) {
    index = i;
    max = newMax;
  }
}

console.log(max, index);