fixing-mutation.js

const maskSensitivityInfos = (card) => {
  // we use SPREAD OPERATOR to copy all properties from 'card' parameter
  // this will remove the reference binding.
  // In other words: NO MUTATION
  const cc = { ...card };

  cc.number = cc.number.replace(/.(?=.{4})/g, '#');
  
  return cc;
}

const displaySelectedPayment = (card) => {
  console.log(`Paying with ${card.type} ${card.number}`);
}


const processPayment = (card) => {
    console.log('Card details', card);
};

const creditCard = { type: 'MasterCard', number: '5431111111111111', cvv: '595' };


const maskedCreditCard = maskSensitivityInfos(creditCard);

displaySelectedPayment(maskedCreditCard);
// output: 'Paying with MasterCard ############1111'

processPayment(creditCard);
// output: 'Card details: { type: 'MasterCard', number: '5431111111111111', cvv: '595' }'
// no side effects! data still reliable and ready to process the transaction