Luhn's algorithm to validate credit card numbers

luhn.js

function validate(n){
  return n.toString().split('').map(Number).reduceRight(function (prev, digit, index, arr) {
    if ((arr.length - index) % 2 === 0) {
      return prev + (digit * 2).toString().split('').map(Number).reduce((a, b) => a + b)
    }
    else {
      return prev + digit
    }
  }, 0) % 10 === 0
}