r.js

function insertInArray(string, index, arr) {
  const sliceBefore = arr.slice(0, index);
  const sliceAfter = arr.slice(index, arr.length);
  return sliceBefore.concat([string], sliceAfter);
}

function differ(arr1, arr2) {
  const diff = [];

  arr2.forEach((element, index) => {
    if (element !== arr1[index]) {
      diff.push({ matched: false, element, index });
    }
  });
  return diff;
}
function insert(s, t) {
  const string1 = s.split('');
  const string2 = t.split('');
  const diff = differ(string1, string2);

  let insertion = null;
  diff.every((item) => {
    if (
      String(insertInArray(item.element, item.index, string1)) ===
      String(string2)
    ) {
      insertion = item;
      return false;
    }
    return true;
  });
  return insertion;
}
function replace(s, t) {
  const string1 = s.split('');
  const string2 = t.split('');
  const diffs = differ(string1, string2);
  let replacer = null;
  diffs.every((item) => {
    const copyOfString1 = [...string1];
    copyOfString1.splice(item.index, 1, item.element);
    if (String(copyOfString1) === String(string2)) {
      replacer = item;
      replacer.toBeReplaced = string1[item.index];
      return false;
    }
    return true;
  });
  return replacer;
}
function solution(S, T) {
  const normalizedS = (S || '').toLowerCase();
  const normalizedT = (T || '').toLowerCase();
  const si = insert(normalizedS, normalizedT);
  if (si) {
    return `INSERT ${si.element}`;
  }
  const sr = replace(normalizedS, normalizedT);
  if (sr) {
    return `REPLACE ${sr.toBeReplaced} ${sr.element}`;
  }
}