JavaScript function for computing the checksum of a given number
Your approach is far too complicated.

How about just

function computeChecksum(num) {
  if (typeof num !== 'number' || isNaN(num)) {
    throw new Error('Number, which is not NaN, expected as Parameter.');
  }

  num = Math.floor(Math.abs(num));

  const sum = num.toString().split('').reduce(
                                  (acc, val) =&gt; acc + parseInt(val),
                                  0);
  return sum;
}


Or instead of converting between string and number you can use modulo:

function computeChecksum(num) {
  if (typeof num !== 'number' || isNaN(num)) {
    throw new Error('Number, which is not NaN, expected as Parameter.');
  }

  num = Math.floor(Math.abs(num));

  let checksum = 0;

  while (num &gt; 0) {
    checksum += num % 10;
    num = Math.floor(num / 10);
  }

  return checksum;
}