I have a signed value given as a hex number, by example `0xffeb`

and want convert it into `-21`

as a "normal" Javascript integer.

I have written some code so far:

```
function toBinary(a) { //: String
var r = '';
var binCounter = 0;
while (a > 0) {
r = a%2 + r;
a = Math.floor(a/2);
}
return r;
}
function twoscompl(a) { //: int
var l = toBinaryFill(a).length;
var msb = a >>> (l-1);
if (msb == 0) {
return a;
}
a = a-1;
var str = toBinary(a);
var nstr = '';
for (var i = 0; i < str.length; i++) {
nstr += str.charAt(i) == '1' ? '0' : '1';
}
return (-1)*parseInt(nstr);
}
```

The problem is, that my function returns 1 as MSB for both numbers because only at the MSB of the binary representation "string" is looked. And for this case both numbers are 1:

```
-21 => 0xffeb => 1111 1111 1110 1011
21 => 0x15 => 1 0101
```

Have you any idea to implement this more efficient and nicer?

Greetings, mythbu