@natlibfi/issn-verify/src/issnVerify.js

Verify ISSN (International Standard Serial Number)

export function issn(input) {
    if (input.length < 8) {
        input = input.padStart(8, '0');
    }

    const reg = /^\d{4}\-?\d{3}[\dX]$/;
    if (!input.match(reg)) {
        return false;
    }

    const num = input.substr(0, (input.length - 1))
        .replace(/\-/, '')
        .split('')
        .reverse()
        .reduce(function (prv, cur, idx) {
            let i = idx + 2;
            return prv + (cur * i);
        }, 0)
        % 11;

    const check = num === 0 ? 0 : 11 - num;

    if (check === 10) {
        return "X" === input.substr(-1);
    }

    return check === parseInt(input.substr(-1));
}