Алгоритм вычитания очень больших чисел на JavaScript

bigint.js

#!/usr/bin/env node

function sub(a, b) {
  let prefix = '0'.repeat(Math.abs(a.length - b.length));
  let flip = false;

  if (prefix.length) {
    if (a.length > b.length) {
      b = `${prefix}${b}`;
    } else if (b.length > a.length) {
      a = `${prefix}${a}`;

      // flipped
      a = [b, b = a][0];
      flip = true;
    }
  }

  const r_buffer = [];

  for (let i = a.length - 1; i >= 0; i--) {
    r_buffer[i] = parseInt(a[i]) - parseInt(b[i]);
  }

  for (let i = r_buffer.length - 1; i > 0; i--) {
    if (0 > r_buffer[i]) {
      r_buffer[i] = 10 + r_buffer[i];
      r_buffer[i - 1] = r_buffer[i - 1] - 1;
    }
  }

  while (r_buffer[0] === 0) {
    r_buffer.shift();
  }

  let c = r_buffer.length ? r_buffer.join('') : '0';

  return flip ? -c : c;
}

const a = '80';
const b = '90';

console.log(sub(a, b));