LongestSubseq.js

function longestSubseq(s1, s2) {
  const s1Arr = s1.split('');
  const s2Arr = s2.split('');
  
  let result = [];
  
  while (s1Arr.length) {
    const tmpResult = [];
    let startIndex = 0;
    s1Arr.forEach((s1Char, index) => {
      const currentIndex = s2Arr.indexOf(s1Char, startIndex);
      if (currentIndex !== -1) {
        tmpResult.push(s1Char);
        startIndex = currentIndex + 1;
      }
    })
    s1Arr.shift();
    if (tmpResult.length > result.length) {
      result = tmpResult;
    }
  }
  
  return result.join('');
}


const answer = longestSubseq('AGGTAB', 'GXTXAYB');
console.log(answer);