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;

}