Coding Challenge: The Luhny Bin http://corner.squareup.com/2011/11/luhny-bin.html

maskCreditCardNumber.js

function maskCreditCardNumber(value) {
    function multiply(x, y) {
        return x * y;
    }

    function identyOf(number) {
        return multiply(number, 1);
    }

    function toArrayOfIndividualDigits(value) {
        return value
            .split('')
            .map(identyOf);
    }

    function reduce(combine, base, array) {
        array.forEach(function (element) {
            base = combine(base, element);
        });

        return base;
    }

    function add(x, y) {
        return x + y;
    }

    function sum(numbers, total) {
        return reduce(add, total, numbers);
    }

    function sumOf(func, array) {
        return reduce(function (total, element) {
            return sum(func(element), total);
        }, 0, array);
    }

    function individualDigits(number) {
        var result =  [];
        if (number > 9) {
            result.push(1);
        }
        result.push(number % 10);

        return result;
    }

    function takeEvery(func, array) {
        return array.filter(func);
    }

    function oddIndex(element, index, array) {
        return index % 2 !== 0;
    }

    function evenIndex(element, index, array) {
        return index % 2 === 0;
    }

    function timesTwo(number) {
        return multiply(number, 2);
    }

    function containsValidCharacters(value) {
        return (/^[\d\s\-]+$/).test(value);
    }

    function isValidLength(value) {
        return value.length >= 14 && value.length <= 16;
    }

    function reverseArrayOfIndividualDigits(digits) {
        var arrayOfDigits = toArrayOfIndividualDigits(digits);

        return arrayOfDigits.reverse();
    }

    function calculateLuhn(digits) {
        var fromDigits = reverseArrayOfIndividualDigits(digits);

        return sum(takeEvery(evenIndex, fromDigits),
            sumOf(individualDigits,
                (takeEvery(oddIndex, fromDigits).map(timesTwo))));
    }

    function scrub(value) {
        return value.split(/\D/).join('');
    }

    function isLuhn(value) {
        var digits = scrub(value);

        return containsValidCharacters(value) &&
            isValidLength(digits) &&
            calculateLuhn(digits) % 10 === 0;
    }

    function mask(value) {
        return value.replace(/\d/g, "X");
    }

    return isLuhn(value) ? mask(value) : value;
}