Implementation of Luhn algorithm

Compact Luhn validator:

var luhn_validate = function(imei){
    return !/^\d+$/.test(imei) || (imei.split('').reduce(function(sum, d, n){ 
            return sum + parseInt(((n + imei.length) %2)? d: [0,2,4,6,8,1,3,5,7,9][d]);
        }, 0)) % 10 == 0;
};

Works fine for both CC and IMEI numbers. Fiddle: http://jsfiddle.net/8VqpN/

the array [0,1,2,3,4,-4,-3,-2,-1,0] is used as a look up array for finding the difference between a number in 0-9 and the digit sum of 2 times its value. For example, for number 8, the difference between 8 and (2*8) = 16 -> 1+6 = 7 is 7-8 = -1.

Here is graphical presentation, where {n} stand for sum of digit of n

[{0*2}-0, {1*2}-1, {2*2}-2, {3*2}-3, {4*2}-4, {5*2}-5, {6*2}-6, {7*2}-7....]
   |       |        |         |        |        |       |         |  
[  0  ,    1    ,   2    ,    3  ,     4   ,   -4  ,   -3   ,    -2  ....] 

The algorithm you listed just sum over all the digit and for each even spot digit, look up the the difference using the array, and apply it to the total sum.

Unfortunately none of the codes above worked for me. But I found on GitHub a working solution

// takes the form field 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.
    var nCheck = 0, nDigit = 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) {
            if ((nDigit *= 2) > 9) nDigit -= 9;
        }

        nCheck += nDigit;
        bEven = !bEven;
    }

    return (nCheck % 10) == 0;
}