The js not operator used twice comes up now and then in many code examples in the wild. In some cases it might seem unnecessary, but it can come in handy when feature testing and I want to return a boolean value rather than another value.
The reason why this works is because calling the js not operator once will convert a value to a boolean, but it will also invert the value of that boolean value. So calling the js not operator once more will then invert it back to its true logical value.
The number zero will work out to a false boolean value, so using the js not operator will result in a true boolean value. The opposite of this will work out to false by using the js not operator with the number one for example.