Validating credit card numbers

A Typescript implementation of the Luhn algorithm in validating credit card numbers.

/**
 * Returns `true` if and only if the given credit card number is valid.
 */
function isLuhnValid2(ccNum: string): boolean {
    const chars = [...ccNum];
    const sumOfAltDoubles: number = chars.reduce((sumSoFar: number, value: string, index: number) => {
        const multiplier = (index % 2 === 0) ? 2 : 1;
        let tempValue: number = parseInt(value) * multiplier;
        if (tempValue >= 10) {
            tempValue = tempValue - 9;
        }

        return sumSoFar += tempValue;
    }, 0);

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

Source of algorithm: https://www.groundlabs.com/blog/anatomy-of-a-credit-card/