father son.js

function sc(s){
  var upperCaseLetters = [];
  var lowerCaseLetters = [];
  var letters = s.split('');
  letters.forEach(function(letter, index) {
    if(letter === letter.toUpperCase()) {
      upperCaseLetters.push(letter);
      return;
    }
    lowerCaseLetters.push(letter);
  });
  console.log(lowerCaseLetters, upperCaseLetters);
  var x = letters.filter(function(letter) {
    if(letter === letter.toUpperCase()) {
      oppositeBatch = lowerCaseLetters;
      letter = letter.toLowerCase();
    } else {
      oppositeBatch = upperCaseLetters;
      letter = letter.toUpperCase();
    }
    var z = oppositeBatch.indexOf(letter);
    if(z >= 0) {
      // opposite matched
      oppositeBatch.splice(z, 1);
      return true;
    }
    return false;
  }).join('');
  console.log('###########', x);
  return x;
}