ok, I got it.
+d //converting a string to number
|| 0 //if there was no number inside that string, it gives us 0, instead of NaN.

Can you please explain why do we use AND here and what is happening in this part? Thank you!
n &&

upd: found a solution, so here if n is falsey we just return false before executing the second part.

thanks a lot! It doesn't allow me switch the flag now :(

I have got an error - 9 characters is too long.
But the description says that the maximum length is 43 chars.

Thanks! That's the best answer!