This bug was reported via nashorn-dev. See also http://mail.openjdk.java.net/pipermail/nashorn-dev/2015-December/005783.html

The test cut-pasted here for convenience. 

Example of a Java function I would like to get working in Nashorn / jss javascript below:

  public static BigInteger fibonacci(int n) {
    BigInteger prev = new BigInteger("0");
    if (n == 0) {
      return prev;
    }
    BigInteger next = new BigInteger("1");
    if (n == 1) {
      return next;
    }
    BigInteger fib = null;
    int i;
    for (i = 1; i < n; i++) {
      fib = prev.add(next);
      prev = next;
      next = fib;
    }
    return fib;
  }

We can test with these values:

n=77: 5527939700884757
n=78: 8944394323791464
n=79: 14472334024676221

So far so good. Now I try the - what I think is - equivalent function in
javascript:

function fibonacci(n) {
  var BigInteger = Java.type("java.math.BigInteger");
  prev = new BigInteger("0");
  if (n == 0) return prev;

  next = new BigInteger("1");
  if (n == 1) return next;

  var i, fib = null;
  for (i = 1; i < n; i++) {
    fib = prev.add(next);
    prev = next;
    next = fib;
  }
  return fib;
}

However, now we get different results:

n=77: 5527939700884757
n=78: 8944394323791464
n=79: 14472334024676220

Note that the value for n=79 is off by one.