Luhn algorithm in Javascript. Check valid credit card numbers

validate_credit_card.js

// Takes a credit card string value and returns true on valid number
function valid_credit_card(value) {
  // Accept only digits, dashes or spaces
	if (/[^0-9-\s]+/.test(value)) return false;

	// The Luhn Algorithm. It's so pretty.
	let nCheck = 0, bEven = false;
	value = value.replace(/\D/g, "");

	for (var n = value.length - 1; n >= 0; n--) {
		var cDigit = value.charAt(n),
			  nDigit = parseInt(cDigit, 10);

		if (bEven && (nDigit *= 2) > 9) nDigit -= 9;

		nCheck += nDigit;
		bEven = !bEven;
	}

	return (nCheck % 10) == 0;
}

The code can be rewritten as:

function valid_credit_card(value) {
  // Accept only digits, dashes or spaces
	if (/[^0-9-\s]+/.test(value)) return false;

	// The Luhn Algorithm. It's so pretty.
	let nCheck = 0;
	value = value.replace(/\D/g, "");

	for (let n = 0; n < value.length; n++) {
		let nDigit = parseInt(value[n], 10);

		if (!((n + value.length) % 2) && (nDigit *= 2) > 9) nDigit -= 9;

		nCheck += nDigit;
	}

	return (nCheck % 10) === 0;
}

Or even cleaner

export const luhn = (value) => {
    let nCheck = 0;
    if (value && /[0-9-\s]+/.test(value)) {
        value = value.replace(/\D/g, '');

        value.split('').forEach((v, n) => {
            let nDigit = parseInt(v, 10);

            if (!((value.length + n) % 2) && (nDigit *= 2) > 9) {
                nDigit -= 9;
            }

            nCheck += nDigit;
        });
    }

    return (nCheck % 10) === 0;
};

A shitty one-liner:

const luhn = numbers => numbers.split('').map((value, index) => index % 2 === 0 ? Number(value) * 2 <= 9 ? Number(value) * 2 : Number(Number(`${Number(value) * 2}`.split('')[0]) + Number(`${Number(value) * 2}`.split('')[1])) : Number(value)).reduce((a, b) => a + b).toString().split('')[1] === '0';

Great Luhn implementation 👍

One small edge case: an empty string or whitespace input may return true after stripping non-digits. You might want to add a guard like:

if (!value) return false;

This helps avoid false positives.