Great solution!

Take a look at this link from MDN... Math.sign().... https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Math/sign
Negative zero IS a valid usage! and the kata description does not say don't return a negative zero!

the solution is ok, but the last test of a single-element array (a [0] one) returns -0, which is a mistake. You have to make teh code don`t put this minus befor this zero.

Yeah, if and else statements should have curly brackets (you can remove it if the following code is a single line).

You put the return true and return false in the same if statement, meaning it would always return true and never return false. The improvement I suggested would leave the return false outside the if statement and into the main body, leading to there always being a return statement.

Thanks. It is due to the fact that if and else statements should each have curly brackets, right?

You put the curly bracket in the wrong place.

Remove the curly brace after return false and put it before instead.