test.js

const f = (char) => {
  const inside = (char) => {
    if(char) {
      output += char;
      console.log(output);
      return true;
    } else {
      output = output + 'o';
      return inside;
    }
  }
  let output = 'f';
  if(char){
    console.log(output + char);
  } else {
    inside();
  }
  return inside;
};

const g = (letter) => {
  console.log('f');
  if (letter) console.log(letter);

  const _f = (_letter = 'o') => {
    if (_letter === 'l') console.log('o');

    console.log(_letter);
    return _f;
  };

  return _f;
};

function f (char, myString) {
    let string = myString
    if (typeof (string) === 'undefined') {
        string = 'f';
    }
    
   if (typeof (char) !== 'undefined'){
        string = string + char;
    } else {
        return function (char) {
            if (typeof (char) !== 'undefined'){
                return string = string + 'o' + char;
            }
            else {
                string = string + 'o';
                return f(char, string);
            }
        }
    }
    return string;
}