Converting Javascript Integer To Byte Array And Back

Answer :

Using the hint provided by Susanoh13, here are the two functions that allow conversion of number from/to ByteArray:

longToByteArray = function(/*long*/long) {
    // we want to represent the input as a 8-bytes array
    var byteArray = [0, 0, 0, 0, 0, 0, 0, 0];

    for ( var index = 0; index < byteArray.length; index ++ ) {
        var byte = long & 0xff;
        byteArray [ index ] = byte;
        long = (long - byte) / 256 ;
    }

    return byteArray;
};

byteArrayToLong = function(/*byte[]*/byteArray) {
    var value = 0;
    for ( var i = byteArray.length - 1; i >= 0; i--) {
        value = (value * 256) + byteArray[i];
    }

    return value;
};

In JavaScript bit shifts (>>, <<) are always performed on signed, 32-bits integers. This leads to range overflow for large numbers.

try (** is power operator, << and >>> are bit-shift operators) - intFromBytes works only for arrays generated from positive integers

function getInt64Bytes(x) {
  let y= Math.floor(x/2**32);
  return [y,(y<<8),(y<<16),(y<<24), x,(x<<8),(x<<16),(x<<24)].map(z=> z>>>24)
}

function intFromBytes(byteArr) {
    return byteArr.reduce((a,c,i)=> a+c*2**(56-i*8),0)
}

function getInt64Bytes(x) {
  let y= Math.floor(x/2**32);
  return [y,(y<<8),(y<<16),(y<<24), x,(x<<8),(x<<16),(x<<24)].map(z=> z>>>24)
}

function intFromBytes(byteArr) {
    return byteArr.reduce((a,c,i)=> a+c*2**(56-i*8),0)
}


// TEST

let n = 40*2**40 + 245*2**32 + 194*2**24 + 143*2**16 + 92*2**8 + 40;
let b = getInt64Bytes(n);
let i = intFromBytes(b);

console.log(`number      : ${n}`);
console.log(`int to bytes: [${b}]`);
console.log(`bytes to int: ${i}`);