JSFiddle - React, Tailwind, and code Playground

by TheDiamondDoge

JavaScript

function maskify(cc) {
  if (cc.length <= 4) {
    return cc;
  }
  
  return [...new Array(cc.length - 4).fill("#"), ...cc.slice(cc.length - 4)].join("");
}

console.log(maskify('4556364607935616'));