unary + or - operator just converts it operand to a number. Empty array converted to number is 0. So you got the same result for both [+[]] and [-[]].

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Unary_plus

https://tc39.es/ecma262/multipage/ecmascript-language-expressions.html#sec-unary-plus-operator