Luhn validation algorithm

luhn.js

/**
 * 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]));

@ShirtlessKirk What does the array at the end do?

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

@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.

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

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;
}

https://gist.github.com/carlosvega20/8ec8b472de22626a70a65f5893de82e9?permalink_comment_id=4748205#gistcomment-4748205

@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
}