Append Characters to String to Make Subsequence

export function appendCharacters(s: string, t: string): number {
  const [m, n] = [s.length, t.length];
  let [sIndex, tIndex] = [0, 0];
 
  while (sIndex < m && tIndex < n) {
    tIndex += s[sIndex] === t[tIndex] ? 1 : 0;
    sIndex += 1;
  }
  return n - tIndex;
}