Credit Card Masking #### with RegEx in javascript

creditCardMaskify.js

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

/** _Tests__ **/

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"), "############0694");
  });

  it('should not mask the digits of short credit cards', () => {
    assert.equal(maskify("54321"), "54321");
  });

  it('should return non digit characters without masking', () => {
    assert.equal(maskify("Skippy"), "Skippy");
  });
  
  it('should handle the empty string input correctly', () => {
    assert.equal(maskify(""), "");
  });

});