Use spoiler flag next time please.

No problem, I raised an issue about the error message being confusing above, because, it is in that case.

Oh right - that makes sense sorry I didn't think of that

Thank you for your help :) :)

In javascript (at least), create the string of the input value in the error message before the user can change it. When they mutate the input, the error message is misleading otherwise.

There is no such test, I'm guessing you're mutating the input value. Use console.log(board) as the first line in your code to see the input as it was before your code mutated it.

Thanks for your response could you explain please where I am going wrong with this one

Expected 'true' on input of

8 7 1 7 6 4 1 4 2

7 6 4 1 5 8 4 9 7

1 4 2 7 9 7 8 1 3

7 1 4 6 5 9 4 8 7

6 5 9 5 7 3 9 3 9

4 8 7 9 3 9 1 3 8

1 4 8 4 9 1 2 7 3

4 9 1 8 3 3 7 9 8

2 7 3 7 9 8 3 8 4

All javascript tests are ok. And if they weren't, you have to provide proof, like the input value, the expected value, the value you're returning and think it is the correct one.

Lots of the testcases are not correct in javascript!

What is the best practice for Unicode strings then?

Suprised so many have upvoted this. Concatination is slow and it does not work for non ASCII strings.