validateForm.js

var header = document.getElementById("header")
var button = document.getElementById("login-button")

button.addEventListener("click", function() {
  validateForm()
})


function validateForm() {
  if(checkPassword() == true && checkUsername() == true) {
    header.innerHTML = "<h1>You're now logged in!</h1>"
  } else {
    alert("Incorrect!")
  }
}

function checkPassword() {
  var password = document.getElementById("password").value

  if (password == "12345678") {
    return true
  } else {
    return false
  }
}

function checkUsername() {
  var username = document.getElementById("username").value

  var characters = username.split("")

  var presence = characters.map(function(element) {
    return Number.isInteger(parseInt(element))
  })

  if (presence.indexOf(true) !== -1) {
    return true
  } else {
    return false
  }
}