The 3x3 blocks aren't particularly hard. Meanwhile, I got caught learning that if you modify the result of the array slice method, you'll modify the array itself.

Well obviously this kata wouldn't work if there's five or less digits in the input. (Or okay, with five digits it would give the product of all digits in the string.)

I'll leave a mention in the description that the input string has always more than five digits, so that people won't overthink about if there's going to be mischiefs.

edit: at least -> more than

It's fixed now, sorry about that. That test didn't have the correct answer. My final solution is also updated now.

I've tried a better explanation... sorry, english isn't my mother tongue. I hope it is a bit better. I also think the failing error can be used to understand the process.