Replace method returning 'Undefined' in form

const usernameInput = document.getElementById("username");
const passwordInput = document.getElementById("password");
const telephoneInput = document.getElementById("telephone");
const emailInput = document.getElementById("email");

/**
 * 
 * VALIDATORS
 *  
 */

// Can only contain letters a-z in lowercase
function isValidUsername(username) {
  return /^[a-z]+$/.test(username)
}

// Must contain a lowercase, uppercase letter and a number
function isValidPassword(password) {
  return /[a-z]+/.test(password) && /[A-Z]+/.test(password) && /\d+/.test(password)
  /*Using "lookaheads": /^(?=.*\d)(?=.*[a-z])(?=.*[A-Z]).*$/
    .*\d = zero or more characters and a digit
    .*[a-z] = zero or more characters and a lowercase letter
    .*[A-Z] = zero or more characters and an uppercase letter
  */ 
}

// The telephone number must be in the format of (555) 555-5555
function isValidTelephone(telephone) {
  return /^\D*\d{3}\D*\d{3}\D*\d{4}\D*$/.test(telephone)
}

// Must be a valid email address
function isValidEmail(email) {
  return /\[^@]+@[^@.]+\.[a-z]+/i.test(email)
}

/**
 * 
 * FORMATTING FUNCTIONS
 * 
 */

function formatTelephone(text) {
  const regex = /^\D*(\d{3})\D*(\d{3})\D*(\d{4})\D*$/;
  text.replace(regex, '($1) $2-$3');
}

/**
 * 
 * SET UP EVENTS
 * 
 */

function showOrHideTip(show, element) {
  // show element when show is true, hide when false
  if (show) {
    element.style.display = "inherit";
  } else {
    element.style.display = "none";
  }
}

function createListener(validator) {
  return e => {
    const text = e.target.value;
    const valid = validator(text);
    const showTip = text !== "" && !valid;
    const tooltip = e.target.nextElementSibling;
    showOrHideTip(showTip, tooltip);
  };
}

usernameInput.addEventListener("input", createListener(isValidUsername));

passwordInput.addEventListener("input", createListener(isValidPassword));

telephoneInput.addEventListener("input", createListener(isValidTelephone));

telephoneInput.addEventListener("blur", e => {
  e.target.value = formatTelephone(e.target.value);
});

emailInput.addEventListener("input", createListener(isValidEmail));