Question

I’m trying to convert a large number into an 8 byte array in javascript.

Here is an IMEI that I am passing in: 45035997012373300

var bytes = new Array(7);
for(var k=0;k<8;k++) {
  bytes[k] = value & (255);
  value = value / 256;
}

This ends up giving the byte array: 48,47,7,44,0,0,160,0. Converted back to a long, the value is 45035997012373296, which is 4 less than the correct value.

Any idea why this is and how I can fix it to serialize into the correct bytes?

Answer 1

Your value and the largest JavaScript integer compared:

45035997012373300  // Yours
 9007199254740992  // JavaScript's biggest integer

JavaScript cannot represent your original value exactly as an integer; that’s why your script breaking it down gives you an inexact representation.

Related:

var diff = 45035997012373300 - 45035997012373298;
// 0 (not 2)

Edit: If you can express your number as a hexadecimal string:

function bytesFromHex(str,pad){
  if (str.length%2) str="0"+str;
  var bytes = str.match(/../g).map(function(s){
    return parseInt(s,16);
  });
  if (pad) for (var i=bytes.length;i<pad;++i) bytes.unshift(0);
  return bytes;
}

var imei = "a000002c072f34";
var bytes = bytesFromHex(imei,8);
// [0,160,0,0,44,7,47,52]

If you need the bytes ordered from least-to-most significant, throw a .reverse() on the result.