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?
~~parseInt('11111111111111111111111111111101',2)// == -3
is what you are looking for.
Related answer ~~ vs-parseint
var x = ~~y;is a 'trick' (similar tovar 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.
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))
For 32-bit numbers, it's enough to cast it to a 32-bit integer (no need for parseInt here), which can be done by applying any bitwise operation to it (and indicating that it's in binary by prefixing with 0b).
function parse(num){
return ('0b'+num) | 0
}
console.log(parse('11111111111111111111111111111101'))For a non-32-bit number, however, you'll need some tricks:
function parse(num, length = num.length){
if(num[num.length-length] !== '1')
return +('0b'+num)
let inverse = ''
for(const digit of num.slice(-length))
inverse += +!+digit
return -('0b' + inverse) - 1
}
console.log(parse('1101', 4)) //-3
console.log(parse('1111111111111101', 16)) //-3
console.log(parse('1111111111111111111111111111111111111111111111111111111111111101', 64)) //-3
console.log(parse('1111111111111111111111111111111111111111111111111111111111111101')) //-3, bit length inferred from the length of the string