AI solution for JavaScript Coding Question on BFE.dev
62. implement BigInt addition

One possible solution is to implement the addition algorithm we use in elementary school, where we add the numbers digit by digit, keeping track of any carry we have. Here's an implementation:

function add(a, b) {
  const maxLength = Math.max(a.length, b.length);
  const aPadded = a.padStart(maxLength, '0');
  const bPadded = b.padStart(maxLength, '0');
  
  let carry = 0;
  let result = '';
  
  for (let i = maxLength - 1; i >= 0; i--) {
    const digitSum = parseInt(aPadded[i]) + parseInt(bPadded[i]) + carry;
    const resultDigit = digitSum % 10;
    carry = (digitSum - resultDigit) / 10;
    result = resultDigit + result;
  }
  
  if (carry > 0) {
    result = carry + result;
  }
  
  return result;
}

This function first pads the inputs with leading zeros so that they have the same length, making it easier to loop through them digit by digit.

Then, it initializes a carry variable to 0 and a result variable to an empty string.

The function loops through the digits of the input strings from right to left, adding them together with the carry and computing the resulting digit and the new carry with some simple math.

Finally, if there is still a carry left over after the loop, it is added to the front of the result string.

Note that this function assumes that the inputs are non-negative and that they contain only digits. A more robust implementation would check for invalid inputs and throw an error if necessary.