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

function isbnCheck(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 sbnToIsbn(sbn) {

  if(isbnCheck(sbn.toString())) {
    console.error("Invalid SBN!");
    process.exit(1);
  }
  
  // Prepend 0 and remove old check digit
  const str = '0'+sbn.toString().slice(0,-1);

  const checkDigit = isbnCheck(str);

  var isbn10 = str;
  if(checkDigit == 10) {
    isbn10 += 'X';
  } else {
    isbn10 += checkDigit;
  }

  if(isbnCheck(isbn10)) {
    console.error("Conversion failed");
    process.exit(1);
  }

  console.log("ISBN-10:", isbn10);
}

if(process.argv.length != 3) {
  console.log("Usage: ./sbn_to_isbn <sbn>");
  process.exit(1)
}

sbnToIsbn(process.argv[2]);