How to convert a Javascript number to a Uint8Array?

Question

I have a Javascript integer (whose precision can go up to 2^53 - 1), and I am trying to send it over the wire using an ArrayBuffer. I suppose I could use BigInt64Array, but the browser support still seems relatively new with that.

I cannot use Int32Array (which was my original solution), because the precision for that is up to 2^32 - 1, whereas a Javascript integer can safely go up to 2^53 - 1. This is my problem.

Is there an easy way to simply turn any Javascript integer into a Uint8Array of length 8?

For example, I am looking for a function like this:

function numToUint8Array(num) {
  let arr = new Uint8Array(8);

  // fill arr values with that of num

  return arr;
}

let foo = numToUint8Array(9458239048);
let bar = uint8ArrayToNum(foo); // 9458239048

Does something like this exist in the standard library already? If not, is there a way to write something like this?

Answer 1

@Bergi, is something like this what you had in mind?

function numToUint8Array(num) {
  let arr = new Uint8Array(8);

  for (let i = 0; i < 8; i++) {
    arr[i] = num % 256;
    num = Math.floor(num / 256);
  }

  return arr;
}

function uint8ArrayToNumV1(arr) {
  let num = 0;

  for (let i = 0; i < 8; i++) {
    num += Math.pow(256, i) * arr[i];
  }

  return num;
}

function uint8ArrayToNumV2(arr) {
  let num = 0;

  for (let i = 7; i >= 0; i--) {
    num = num * 256 + arr[i];
  }

  return num;
}

let foo = numToUint8Array(9458239048);
let bar = uint8ArrayToNumV1(foo); // 9458239048
let baz = uint8ArrayToNumV2(foo); // 9458239048

console.log(foo, bar, baz);