exercism-ts/luhn/luhn.ts

export function valid(digitString: string): boolean {
    digitString = digitString.replace(/ /g, '');
    if (digitString.length < 2) {
        return false;
    }
    if (digitString.match(/\D/)) {
        return false;
    }
    const len = digitString.length;
    const zero = 0x30; // ASCII '0'
    let sum = 0;
    for (let i = len - 1; i >= 0; i--) {
        // We know digit is in [0,9] because we filtered against \D above.
        let digit = (digitString.codePointAt(i) ?? zero) - zero
        if ((len - i) % 2 == 0) {
            digit *= 2;
            if (digit > 9) {
                digit -= 9;
            }
        }
        sum += digit;
        sum %= 10;
    }
    return sum % 10 === 0;
}