function correct_SFID(id) {
  if (id.length != 18) {
    throw 'BAD SF ID - invalid length, must be 18 chars. current length = '+id.length;
  }
  var corrected_id = '';
  var corrected_csum = '';
  const CSUM_IDX = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ012345';
  for (var i = 0; i < 3; i++) {
    var csum_ch = id.charAt(15+i).toUpperCase();
    corrected_csum += csum_ch;
    flags = CSUM_IDX.indexOf(csum_ch)
    if (flags < 0) { throw 'BAD SF ID - invalid checksum character: "'+csum_ch+'"!'; }
    var rchunk = '';
    for (var j = 4; j >= 0; j--) {
      id_ch = id.charAt(i*5 + j)
      if (flags >= (1<<j)) {
        // According to checksum, this should be an uppercase char
        flags -= (1<<j);
        if ((id_ch >= 'a') && (id_ch <= 'z')) {
          rchunk += id_ch.toUpperCase();
        } else if ((id_ch >= 'A') && (id_ch <= 'Z')) {
          rchunk += id_ch;
        } else {
          throw 'BAD SF ID - invalid digit where alpha is expected: "' + id_ch + '"!';
        }
      } else if ((id_ch >= 'A') && (id_ch <= 'Z')) {
        // According to checksum, this should be a lowercase char or digit
        rchunk += id_ch.toLowerCase();
      } else {
        // Unfortunately, we can't tell whether this char should be a
        // lowercase letter or a digit, but it's probably fine...
        rchunk += id_ch;
      }
    }
    // Add this chunk to the reconstructed ID in reverse order
    for (var j = rchunk.length; j >= 0; j--) { corrected_id += rchunk.charAt(j); }
  }
  // Add the corrected case checksum
  corrected_id += corrected_csum;
  return corrected_id;
}

var id = process.argv[2];
console.log('Checking  SFID['+id+']');
console.log('Corrected SFID['+correct_SFID(id)+']');

% node correct_sfid.js 0031i00000dzeh6qal
Checking  SFID[0031i00000dzeh6qal]
Corrected SFID[0031I00000DZeH6QAL]