Credit Card maskify in Javascript

creditCardMaskify.js

function maskify(creditCard) {
	if (creditCard.length < 6) return creditCard;
	const last4Characters = creditCard.substr(-4);
	const firstCharacter = creditCard.substr(0, 1);
	const maskingCharacters = creditCard.substr(1, creditCard.length - 5).replace(/\d/g, '#');
	return `${firstCharacter}${maskingCharacters}${last4Characters}`;
}

// let assert = require('chai').assert
// require('mocha').describe;
// require('mocha').it;

// describe('Challenge - Maskify', () => {

//   it('should return a string ouput', () => {
//     assert.typeOf(maskify(""), 'string');
//   });
  
//   it('should mask the digits of standard credit cards', () => {
//     assert.equal(maskify("5512103073210694"), "5###########0694");
//   });

//   it('should not mask the digits of short credit cards', () => {
//     assert.equal(maskify("54321"), "54321");
//   });
  
//   it('should not mask the - or special characters between digits', () => {
//     assert.equal(maskify("5432-1312-1231/4567"), "5###-####-####/4567");
//   });
  
//   it('should return non digit characters without masking', () => {
//     assert.equal(maskify("Skippy"), "Skippy");
//   });
  
//   it('should handle the empty string input correctly', () => {
//     assert.equal(maskify(""), "");
//   });

// });

You can easily use Regular expression

function maskify(input) {
  return input.replace(/.(?=.{4})/g, "#");
}

thanks for the suggestion @AlaeddineMessadi