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# ShirtlessKirk/luhn.js

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Luhn validation algorithm
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 /** * Luhn algorithm in JavaScript: validate credit card number supplied as string of numbers * @author ShirtlessKirk. Copyright (c) 2012. * @license WTFPL (http://www.wtfpl.net/txt/copying) */ var luhnChk = (function (arr) { return function (ccNum) { var len = ccNum.length, bit = 1, sum = 0, val; while (len) { val = parseInt(ccNum.charAt(--len), 10); sum += (bit ^= 1) ? arr[val] : val; } return sum && sum % 10 === 0; }; }([0, 2, 4, 6, 8, 1, 3, 5, 7, 9]));

### jefelewis commented Oct 1, 2021

@ShirtlessKirk What does the array at the end do?

``````}([0, 2, 4, 6, 8, 1, 3, 5, 7, 9]));
``````

### ShirtlessKirk commented Oct 6, 2021

@jefelewis it's a lookup table. Basically, instead of calculating the summed result of the numbers that are doubled (every second digit from right) each time, the code uses the precalculated value stored in the array at the index of the digit in question.

For example, if the digit is 6, the summed double is 3 (6 * 2 = 12, 1 + 2 = 3). In the array, the value at index 6 (the original digit used as a reference) is set to 3.

### bdr193 commented May 10, 2022

@ShirtlessKirk Can this be implemented in Shopify or Woocommerce? How can you install it without having access to the payment iframe

### carlosvega20 commented Nov 3, 2023

Declarative/functional approach:

``````const checkLuhn = cardNumber => {
const sum = [...cardNumber].reduceRight((prev, curr, i, arr) =>
prev+= (i%2)?Number(arr[Number(curr)]):Number(curr)
,0);
return sum && sum % 10 === 0;
}
``````

@carlosvega20 I'm a bit confused by `Number(arr[Number(curr)])` expression and not sure if this works correctly for check

Can suggest instead smth like this:

``````const isLuhn = (cardNumber) => {
const checkSum = [...cardNumber].reduceRight((prev, curr, i, arr) => {
if(i % 2) {
return prev += Number(curr);
} else {
const d  = Number(curr) * 2
return prev += d > 9 ? d - 9 : d
}
}, 0);

return checkSum && checkSum % 10 === 0
}
``````