Mask a card number

easypaymentsTypeScript

Support staff can see the last four digits of a card and nothing else.

Every character except the last four becomes an asterisk.
Anything four characters or shorter comes back untouched — there is nothing safe to hide.

maskCard(cardNo: string) → string

Where you start

function maskCard(cardNo: string): string {
  
}

Build the asterisks from the length, then glue on the tail.

Reference solution in TypeScript

function maskCard(cardNo: string): string {
  if (cardNo.length <= 4) return cardNo;
  return '*'.repeat(cardNo.length - 4) + cardNo.slice(-4);
}