var str = "Hello6 9World 2, Nic8e D7ay!";

function NumberSearch(str) {
  var chars = str.replace(/[^a-zA-z]/g, ''),
      nums = str.replace(/[^0-9]+/g, ''),
      total = 0;

  for (var i = nums.length - 1; i >= 0; i--) {
    total += Number(nums[i]);
  }

  return Math.round(total / chars.length);
}

NumberSearch(str);

var str = "Hello6 9World 2, Nic8e D7ay!";

function NumberSearch(str) {
  var chars = str.replace(/[^a-zA-z]/g, ''),
      nums = str.replace(/[^0-9]+/g, ''),
      total = 0, total_chars = chars.length, total_nums = nums.length;

  for (var i = total_nums - 1; i >= 0; i--) {
    total += parseInt(nums[i]);
  }

  return Math.round(total / total_chars );
}

NumberSearch(str);