How do I parse a two's complement string to a number

Question

I have binary form for -3 in two's complement form - 11111111111111111111111111111101, and use it with parseInt function:

parseInt('11111111111111111111111111111101', 2)

But it returns 4294967293, which is the integer that results if 11111111111111111111111111111101 is parsed as unsigned int. How can I parse integer as a signed one?

Answer 1

~~parseInt('11111111111111111111111111111101',2)// == -3

is what you are looking for.

Related answer ~~ vs-parseint

var x = ~~y; is a 'trick' (similar to var x = y << 0;) that (ab)uses the unary bitwise NOT operator to force the result to be in the range of a signed 32-bit integer, discarding any non-integer portion.

Answer 2

i had the same problem, you can use the following:

function parseInt2complement(bitstring,bitcount)
{
    value = parseInt(bitstring, 2);

    if ((value & (1<<(bitcount-1))) > 0) {
       value = value - (1<<(bitcount));
    }
    return value;
}

console.log(parseInt2complement('111111111111111111111111111101', 30))
console.log(parseInt2complement('1111111111111111111111111111101', 31))