index.js

window.onscroll = function() {scrollFunction()};
    
function scrollFunction() {
  if (document.body.scrollTop > 80 || document.documentElement.scrollTop > 80) {
    document.getElementById("nav").style.padding = "15px 10px";
  } else {
    document.getElementById("nav").style.padding = "80px 10px";
  }
}

let slideIndex = 1;
Slides(slideIndex);

function plusSlides(n) {
Slides(slideIndex += n);
}

function currentSlide(n) {
Slides(slideIndex = n);
}

function Slides(n) {
let i;
let slides = document.getElementsByClassName("slides");
if (n > slides.length) {slideIndex = 1}    
if (n < 1) {slideIndex = slides.length}
for (i = 0; i < slides.length; i++) {
slides[i].style.display = "none";  
}
slides[slideIndex-1].style.display = "block";  
dots[slideIndex-1].className += " active";
}

function contactAdd() {
  var name = document.getElementById("name").value;
  var email = document.getElementById("email").value;
  var phone = document.getElementById("phone").value;
  var message = document.getElementById("message").value;


  var xhr = new XMLHttpRequest();
  xhr.open("POST", "contact.php", true);
  xhr.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
  xhr.onreadystatechange = function() {
    if (this.readyState == 4 && this.status == 200) {
      alert(this.responseText);
    }
  };
  xhr.send("name=" + name + "&email=" + email + "&phone=" + phone + "&message=" + message);

}