AI solution for JavaScript Coding Question on BFE.dev
77. implement BigInt subtraction with sign

One possible solution:

function subtract(str1, str2) {
  const isNegative = determineSign(str1, str2);
  
  str1 = removeSign(str1);
  str2 = removeSign(str2);
  
  const [longer, shorter] = padWithZeros(str1, str2);
  
  let carry = 0;
  let result = '';
  
  for (let i = longer.length - 1; i >= 0; i--) {
    let digit1 = Number(longer[i]);
    let digit2 = Number(shorter[i]);
    let difference = digit1 - digit2 - carry;
    
    if (difference < 0) {
      difference += 10;
      carry = 1;
    } else {
      carry = 0;
    }
    
    result = difference.toString() + result;
  }
  
  result = removeLeadingZeros(result);
  
  if (isNegative) {
    result = '-' + result;
  }
  
  return result;
}

function determineSign(str1, str2) {
  const hasMinus = str1[0] === '-' || str2[0] === '-';
  const hasPlus = str1[0] === '+' || str2[0] === '+';
  
  if (hasMinus && hasPlus) {
    throw new Error('Invalid input');
  } else if (hasMinus) {
    return true;
  } else if (hasPlus || str1[0] === '+' || str2[0] === '+') {
    return false;
  } else {
    return determineSign('+' + str1, '+' + str2);
  }
}

function removeSign(str) {
  if (str[0] === '-' || str[0] === '+') {
    return str.slice(1);
  } else {
    return str;
  }
}

function padWithZeros(str1, str2) {
  let maxLength = Math.max(str1.length, str2.length);
  
  str1 = str1.padStart(maxLength, '0');
  str2 = str2.padStart(maxLength, '0');
  
  return [str1, str2];
}

function removeLeadingZeros(str) {
  let i = 0;
  
  while (str[i] === '0' && i < str.length) {
    i++;
  }
  
  return str.slice(i) || '0';
}

The subtract function takes two strings, str1 and str2, representing integers, and returns their difference as a string. The signs of the input strings are handled by the determineSign, removeSign, and padWithZeros helper functions. The actual subtraction is performed in a loop over the digits of the longer input string, with borrows and carries handled as needed. Finally, leading zeros are removed from the result, and a negative sign is added if needed.

Note that the function assumes that the input strings are valid integers and does not perform any input validation. Also, the function does not handle inputs that result in numbers whose magnitude is greater than Number.MAX_SAFE_INTEGER.