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

Look up Two's complement for signed binary numbers

Lets assume that a javascript Number is 8 bits wide (which its not):

then

1 = 0000 0001b

and

~1 = 1111 1110b

Which is the binary representation of -2

0000 0010b =  2
0000 0001b =  1
0000 0000b =  0
1111 1111b = -1
1111 1110b = -2