GS1 GTIN-14 check bit calculation in JS, RFC

snorblorb.js

    function checksum(gtin)
    {
        const len = gtin.length;
        
        // Split the code into an array of characters
        // then map that into an array of numbers:
        const chs = gtin.split('').map( (e) => parseInt(e) );
        
        // Since this is just a GTIN-14 example:
        if( 14 === len )
        {
            // 12345678901234
            // 00842650000272
            // The GS1 AI (01) is stripped (obv).
            // Padding that was removed (leading 00) is added back
            // Technically the evens (odd array positions) are multiplied by 1,
            // but according to my Grade 1 maths class... we good.
            // This could be put in a loop, but this is clear and not too long imo:
            let sum = 0;
            sum += chs[0] * 3;
            sum += chs[1];
            sum += chs[2] * 3;
            sum += chs[3];
            sum += chs[4] * 3;
            sum += chs[5];
            sum += chs[6] * 3;
            sum += chs[7];
            sum += chs[8] * 3;
            sum += chs[9];
            sum += chs[10] * 3;
            sum += chs[11];
            sum += chs[12] * 3;
            
            // Per GS1 spec: "Subtract sum from nearest equal or higher multiple of ten"
            // But we can modulo and subtract that for the right answer:
            const chk = 10 - (sum % 10);
            return chs[13] === chk;
        }
        
        /* Here you'd also want to check GTIN-13, GTIN-12, and GTIN-8.
         *
         * For GTIN-12 and GTIN-8 you can just pad the left with zeros
         * to "make" them a GTIN-14, but not for GTIN-13 as the check
         * value would change, since it works on opposite fields to
         * the rest.
         *
         * If in doubt, just follow 7.9.1 of the spec.
         */

        return false;
    }

    // smoller-boi:
    function checksum(gtin)
    {
        const chs = gtin.split('').map( (e) => parseInt(e) );

        let sum = 0;
        for( let a=0; a < chs.length-1; a++ )
            sum += ( a % 2 === 1 ? chs[a] : chs[a] * 3 );

        const chk = 10 - (sum % 10);
        return chs[chs.length-1] === chk;
    }