Container Code Check Digit Calculation - ISO 6346

checkdigit.js

// let input = 'ECMU465749'; // 6
let input = 'GVTU300038'; // 9

function getCheckDigit(input) {
    const char_map = {
        'A': 10, 'B': 12, 'C': 13, 'D': 14, 'E': 15, 'F': 16, 'G': 17,
        'H': 18, 'I': 19, 'J': 20, 'K': 21, 'L': 23, 'M': 24, 'N': 25,
        'O': 26, 'P': 27, 'Q': 28, 'R': 29, 'S': 30, 'T': 31, 'U': 32,
        'V': 34, 'W': 35, 'X': 36, 'Y': 37, 'Z': 38
    };
    
    // split per character
    let parts = input.split('');
    
    // replace first for letter value with number
    parts[0] = char_map[parts[0]];
    parts[1] = char_map[parts[1]];
    parts[2] = char_map[parts[2]];
    parts[3] = char_map[parts[3]];
    
    // convert all to numbers
    parts = parts.map(x => parseInt(x, 10));
    
    // is multiplied by 2position, where position is the exponent to basis 2
    parts = parts.map((x, i) =>  x * Math.pow(2, i));
    
    // sum all
    const a = parts.reduce(function (a, b) {
        return a + b;
    }, 0);
    
    // divide by 11
    const b = a / 11;
    
    // remove decimal part
    const c = Math.floor(b);
    
    // multiply by 11
    const d = c * 11;
    
    // subtract from a
    return a - d;
}

let check_digit = getCheckDigit(input);

console.log(input, check_digit);