Update isbn-verifier.spec.js

[pyko]

Updating X is only valid as a check digit test so if X in the middle of ISBN is treated as 10 the test will fail.

Changing check digit to 9 means checksum('3-598-2X507-9') mod 11 == 0, so solutions which simply replace X with 10 (without checking location) will treat 3-598-2X507-9 as a valid ISBN.

[SleeplessByte]

@petertseng just tagging you because I want to be certain this can actually be applied and applied everywhere. In that case its a bugfix and we can lift it to problem-spec.

[petertseng]

This PR is the correct thing to do; an implementation that treats X as 10 in all positions would still reject 3-598-2X507-0, therefore 3-598-2X507-0 is not a good test case. Such an implementation would declare 3-598-2X507-9 a valid ISBN, so 3-598-2X507-9 is the correct test case. Please see exercism/problem-specifications#993.

[petertseng]

Please see the following:

0 * 1 (0)
7 * 2 (14)
0 * 3 (0)
5 * 4 (20)
X * 5 (50)
2 * 6 (12)
8 * 7 (56)
9 * 8 (72)
5 * 9 (45)
3 * 10 (30)
total: 299 (divisible by 11? false)\

9 * 1 (9)
7 * 2 (14)
0 * 3 (0)
5 * 4 (20)
X * 5 (50)
2 * 6 (12)
8 * 7 (56)
9 * 8 (72)
5 * 9 (45)
3 * 10 (30)
total: 308 (divisible by 11? true)

[SleeplessByte]

Sweet. Thank you for that elaboration @petertseng ❤️

[tejasbubane]

Thanks 👍