longestCommon.js

tests = [["ABAZDC", "BACBAD"], ["AA", "AAAA"], ["AGGTAB", "GXTXAYB"]];

function findCommon(a, b) {
  arrB = [...b];
  return [...a].filter((c, index) => {
    if (!arrB.includes(c)) {
      return false;
    }

    const match = arrB.indexOf(c, Math.min(index, arrB.length - 1));

    if (match !== -1) {
      return true;
    }
  });
}

function longestCommon(s1, s2) {
  const maxLength = Math.min(s1.length, s2.length);

  f1 = findCommon(s1, s2).join("");
  f2 = findCommon(s2, s1).join("");

  const long = Math.max(f1.length, f2.length) === f1.length ? f1 : f2;
  const short = long === f1 ? f2 : f1;

  return long.length <= maxLength ? long : short;
}

tests.map((pair, index) => {
  console.log("Winner", index, longestCommon(pair[0], pair[1]));
});