GS1 checksum utilities

gs1-checksum.js

/*
 *
 * Wikipedia:
 *
 *    The final digit of a Universal Product Code is a check digit computed as follows:
 *
 *       1) Take digits up to but not including the check digit.
 *       2) Sum the digits in the odd-numbered positions (first, third, fifth, etc.) together and multiply by three.
 *       3) Add the sum of digits in the even-numbered positions (0th, 2nd, 4th, 6th, etc.) to the result.
 *       4) Take result modulo 10
 *       5) If result is 0, return 0. Otherwise return 1 - result.
 *
*/

function checksum(value) { // Generic GS1 checkdigit calculator ( ISBN, EAN, UPC, etc )

    let d =  value.split('')
        .map( Number )
        .map(   (x,idx) => idx % 2 ? x * 3 : x )
        .reduce( (accum, x ) => accum + x )
    d = d % 10;
    return d === 0 ? d : 10 - d;
}

function validate( code ) {
   return code && code.slice(-1) == checksum(code.slice(0,-1));
}

function isUPC( code ) {
   return validate(code) && code.length == 12;
}

module.exports = {
   isUPC: isUPC,
   validate: validate,
   checksum: checksum
}

if( module.parent === null ) {
   [
   //  1234567890123
      '9788131701713', // ISBN
      '0011985810416',
      '0038472368106',
      '0751937198628',
      '0068113106583',
      '012345678905'
   ].forEach(
      s => console.log( s, 'valid:', validate(s), 'len:', s.length )
   )
}