function lcs(s1, s2) {
  let m = Array(s2.length + 1).fill(null).map(() => Array(s1.length + 1).fill(null));

  for (let i = 0; i <= s1.length; i += 1) {
    m[0][i] = 0;
  }

  for (let i = 0; i <= s2.length; i += 1) {
    m[i][0] = 0;
  }

  for (let i = 1; i <= s2.length; i += 1) {
    for (let j = 1; j <= s1.length; j += 1) {
      if (s1[j - 1] === s2[i - 1]) {
        m[i][j] = m[i - 1][j - 1] + 1;
      } else {
        m[i][j] = Math.max(m[i - 1][j], m[i][j - 1]);
      }
    }
  }

  return m;
}

console.log(lcs('hello', 'hello'));
console.log(lcs('hello', 'hullo'));