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