Javascript - convert byte to sbyte

Question

How can I convert byte value (0.255) to (-128 - +127) ? eg 240 -> -16

The method should be fast (thinking about bitwise operators maybe).

Answer 1

the classic would be

var sbyte = (byte & 127) - ((byte & 128) ? 128 : 0);

Or

var sbyte = (byte & 127) - (byte & 128);