function findCommonChar(a, b) {
  for (let i = 0; i < b.length; i++) {
    if (a.indexOf(b.charAt(i)) >= 0) {
      return b.charAt(i);
    }
  }
  return '';
}
@param {string} a
@param {string} b
@return {string} any common character between two strings a and b.