JavaScript实现将UPC转换成ISBN的方法

if (indexisbn.indexOf("978") == 0) {
  isbn = isbn.substr(3,9);
  var xsum = 0;
  var add = 0;
  var i = 0;
  for (i = 0; i < 9; i++) {
    add = isbn.substr(i,1);
    xsum += (10 - i) * add;
  }
  xsum %= 11;
  xsum = 11 - xsum;
  if (xsum == 10) { xsum = "X"; }
  if (xsum == 11) { xsum = "0"; }
  isbn += xsum;
}