Bitcoin difficulty target to compact bits conversion

compactBits.js

var MAX_PRECISION = 28;

function _findExp (n, base, exp = 1) {
  var pow = Math.pow(base, exp);
  var div = Math.floor(n / pow);
  if (exp > MAX_PRECISION) {
    return div;
  }
  return div + _findExp(n, base, exp + 1);
}

// Convert an integer, `n`, to a specified numeric
// base, `base`
function _toBase(n, base) {
  var rem = n % base;
  return _findExp(n, base) + ' ' + rem;
}

// Convert a integer string, `s`, into hexadecimal value
// between 00 and ff. The leftmost value is padded
// with a 0 if needed.
function _toHex(s) {
  var hex = parseInt(s).toString(16);
  return (hex.length === 1) ? '0' + hex : hex;
}

function _toBits(target) {
  var arr = _toBase(target, 256).split(' ');

  // If the first digit is greater than 0x7f, prepend an 0x00
  if (arr[0] > 0x7f) {
    arr.unshift(0);
  }

  // Prepend the length of the hex string
  arr.unshift(arr.length);

  // If there are less than 4 bytes in total,
  // right-pad with 0x00s
  var delta = 4 - arr.length;
  while (delta > 0) {
    arr.push(0);
    delta--;
  }

  // Only keep 2 bytes of precision
  return '0x' + arr.slice(0, 4).map(_toHex).join('');
}