Command-line node.js script to calculate an ISBN from an SBN

sbn_to_isbn.js

#!/usr/bin/env node

// Command-line node.js script to calculate an ISBN from an SBN
// Note that this re-calculates the check digit (the last digit)
// which may not be the correct thing to do.
// Wikipedia claims that it should _not_ be recalculated but cites no sources
//
// This script can also re-calculate a bad check digit in an SBN or ISBN
// using ./sbn_to_isbn -f <sbn_or_isbn>

var tryFix = false

function calculateCheckDigit(isbn) {

  const str = isbn.toString();

  var sum = 0;
  var i, number;
  for(i=0; i < str.length; i++) {
    number = str[i].toLowerCase();
    if(number == 'x') number = 10;
    sum += (i+1) * parseInt(str[i]);
  }

  return sum % 11;
}

function fixCheckDigit(sbnOrIsbn, convertSbnToIsbn) {

  // Remove old check digit
  var digits = sbnOrIsbn.toString().slice(0,-1);

  // Prepend 0 if we're converting
  if(convertSbnToIsbn) {
    digits = '0'+digits;
  }

  checkDigit = calculateCheckDigit(digits);

  if(checkDigit == 10) {
    digits += 'X';
  } else {
    digits += checkDigit;
  }

  return digits;
}

function sbnToIsbn(sbnOrIsbn) {
  
  if(calculateCheckDigit(sbnOrIsbn.toString())) {
    console.error("Invalid SBN!");
    if(!tryFix) {
      process.exit(1);
    }
    sbnOrIsbn = fixCheckDigit(sbnOrIsbn);
    if(sbnOrIsbn.length > 9) {
      console.log("Attempted fixed ISBN-10:", sbnOrIsbn);
      process.exit(0);
    } else {
      console.log("Attempted fixed SBN:", sbnOrIsbn);
    }
  }

  const isbn10 = fixCheckDigit(sbnOrIsbn, true);
  
  console.log("ISBN-10:", isbn10);
}

if(process.argv.length < 3) {
  console.log("Usage: ./sbn_to_isbn [-f] <sbn>");
  console.log('');
  console.log("  -f: Attempt to fix by re-calculating SBN check digit");
  console.log('');
  process.exit(1)
}

var sbnOrIsbn = process.argv[2];
if(process.argv.length > 3 && process.argv[2] == '-f') {
  tryFix = true;
  sbnOrIsbn = process.argv[3]
}

sbnToIsbn(sbnOrIsbn);