hitAndBlow.js

const getHitAndBlow = (entry, mirror, hit, blow) => {
  const first = Object.keys(entry)[0];

  if (first === undefined) return { hit, blow };

  const { [first]: __, ...newEntry } = entry;

  if (mirror[first]) {
    if (mirror[first] === entry[first]) {
      const { [first]: ___, ...newMirror } = mirror;
      
      return getHitAndBlow(newEntry, newMirror, hit + 1, blow);
    }

    return getHitAndBlow(newEntry, mirror, hit, blow + 1)
  }
  
  return getHitAndBlow(newEntry, mirror, hit, blow)
};

const entry  = '98756'.split('');
const mirror = '57231'.split('');


const stringToMap = (entry) => {
  return entry.reduce((acc, element, index) => {
    return {...acc, [element]: index.toString()}
  }, {});
};

console.log(getHitAndBlow(stringToMap(entry), stringToMap(mirror), 0, 0));