main.js

var slideIndex = 1;
showSlides(slideIndex);

// Next/previous controls
function plusSlides(n) {
  showSlides(slideIndex += n);
}

// Thumbnail image controls
function currentSlide(n) {
  showSlides(slideIndex = n);
}

var slideIndex = 0;
showSlides();

function showSlides() {
  var i;
  var slides = document.getElementsByClassName("mySlides");
  for (i = 0; i < slides.length; i++) {
    slides[i].style.display = "none";
  }
  slideIndex++;
  if (slideIndex > slides.length) {slideIndex = 1}
  slides[slideIndex-1].style.display = "block";
  setTimeout(showSlides, 6000); // Change image every 6 seconds
}

//Get the button:
mybutton = document.getElementById("myBtn");

// When the user scrolls down 20px from the top of the document, show the button
window.onscroll = function() {scrollFunction()};

function scrollFunction() {
  if (document.body.scrollTop > 20 || document.documentElement.scrollTop > 20) {
    mybutton.style.display = "block";
  } else {
    mybutton.style.display = "none";
  }
}

// When the user clicks on the button, scroll to the top of the document
function topFunction() {
  document.body.scrollTop = 0; // For Safari
  document.documentElement.scrollTop = 0; // For Chrome, Firefox, IE and Opera
}