EAN to ISBN

ean2isbn.ts

// ref: https://tritonstore.com.au/ean-vs-isbn/

function checkDigitISSN (value: string): number | 'X' {
    let sum = 0;
    const len = value.length;
    
    for (let i = 0; i < value.length; i++) {
        sum += (parseInt(value.charAt(i)) * ((len - i) + 1));
    }

    let checkdigit: number | 'X' = 11 - (sum % 11);

    if (checkdigit === 10) {
        checkdigit = 'X';
    }

    return checkdigit;
}

function ean2isbn10 (ean: string, separator = '='): string | undefined {
    if (!(ean.startsWith('978') || ean.startsWith('979')) || ean.length !== 13) {
        throw new Error(`EAN ${ean} does not represent an ISBN`);
    }


    const value = ean.substring(3, ean.length - 1);
    const checkDigit = checkDigitISSN(value);

    return `${value.charAt(0)}${separator}${value.substring(1, 4)}${separator}${value.substring(4, 10)}${separator}${checkDigit}`;
}

function ean2isbn13 (ean: string): string | undefined {
    if (!(ean.startsWith('978') || ean.startsWith('979')) || ean.length !== 13) {
        throw new Error(`EAN ${ean} does not represent an ISBN`);
    }

    return `${ean.substring(0, 3)}-${ean.substring(4)}`;
}
                                     
export { ean2isbn10, ean2isbn13 };