frontend.js

var login = document.getElementById("login");
var modal = document.getElementById('myModal');
var btn = document.getElementById("signup");
var signUp = document.getElementById("signUpTwo");

const PORT = 5001;
const API_BASE_URL = `http://localhost${PORT ? ":" + PORT : ""}`;

window.onload = function(){
  const xhr = new XMLHttpRequest;
  xhr.withCredentials = true;
  const url = `${API_BASE_URL}/stuff`;
  // We needed to add a callback function to handle the response!
  xhr.onreadystatechange = function () {
    if (this.readyState == 4 && this.status == 200) {
      console.log(xhr.responseText);
    }
  };
  xhr.open("GET",url,true);
  xhr.send();
}

login.addEventListener("click",function(){
    var name = document.getElementById("username").value;
    var pass = document.getElementById("password").value;
    const xhr = new XMLHttpRequest;
    xhr.withCredentials = true;
    const url = `${API_BASE_URL}/check?username=${name}&password=${pass}`;
    xhr.onreadystatechange = function () {
      if (this.readyState == 4 && this.status == 200) {
        console.log(xhr.responseText);
      }
    };
    xhr.open("POST",url,true);
    xhr.send();
})

signUp.addEventListener("click",function(){
  var username = document.getElementById("signUpUsername").value;
  var email = document.getElementById("signUpEmail").value;
  var password = document.getElementById("signUpPassword").value;
  console.log(username + " " + password + " " + email);
  const xhr = new XMLHttpRequest;
  const url = `${API_BASE_URL}/signUp`;
  xhr.onreadystatechange = function () {
    if (this.readyState == 4 && this.status == 200) {
      console.log(xhr.responseText);
    }
  };
  xhr.open("POST",url,true);
  xhr.setRequestHeader("Content-Type", "application/x-www-form-urlencoded");
  xhr.send("username="+username+"&password="+password+"&email="+email);//"username="+username+"&password="+password+"&email="+email
})

btn.onclick = function() {
    modal.style.display = "block";
  }

window.onclick = function(event) {
    if (event.target == modal) {
      modal.style.display = "none";
    }
  }