javascript bitwise operator question

Question

In Javascript when I do this

var num = 1;

~ num == -2

why does ~num not equal 0

in binary 1 is st

Answer 1

The reason for this is that using a bitwise NOT reverses all the bits of a value. If you are storing the value of 1 in a signed 8-bit integer, you're storing the binary value 00000001. If you apply a bitwise NOT, you get 11111110, which for a signed 8-bit integer is the binary value for -2.