luhn.js

let validCard = (card = '') => {
  card = card.replace(/\s/g, '').split('').reverse().join('');
  let total = Number(card[0]);

  for(let i = 1; i < card.length; i++) {  
    let digit = card[i];
    if(i % 2 == 1) {
      digit = digit * 2;
      if(digit > 9) {
        let values = (digit + "").split('');
        digit = Number(values[0]) + Number(values[1]);
      } 
    }
    total += Number(digit);  
  }

  return (total % 10) == 0;
}