Write a JavaScript program to find the shortest possible string which can create a string to make it a palindrome by adding characters to the end of it

function build_Palindrome(new_str) {
  var flag;
  for (var i = new_str.length;; i++) {
    flag = true;
    for (var j = 0; j < i - j - 1; j++) {
      if (i - j - 1 < new_str.length && new_str[j] != new_str[i - j - 1]) {
        flag = false;
        break;
      }
    }
    if (flag) {
      for (var j = new_str.length; j < i; j++) {
        new_str += new_str[i - j - 1];
      }
      return new_str;
    }
  }
}

console.log(build_Palindrome("abcddc"))
console.log(build_Palindrome("122"))