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]));

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
}