Get nth or last nth index of a pattern from a string

nth_index.js

function nthIndex(str, pat, n){
  let i= -1;
  while (n-- && i++ < str.length) {
      i = str.indexOf(pat, i);
      if (i < 0) break;
  }
  return i;
}

function lastNthIndex(str, pat, n) {
  let i = str.length;
  while (n-- && i-- > -1) {
    i = str.lastIndexOf(pat, i);
    if (i < 0) break;
  }
  return i;
}