What I would recommend is making use of a setInterval() function such as the following:
setInterval(function() {
slideIndex++;
showSlides(slideIndex);
}, 3000);
The above function increases the current index, then triggers the showSlides() function, passing the index as a parameter. The 3000 denotes the speed at which the automation should occur, with the time specified in millseconds. Note that setInterval() should be used rather than setTimeout(), because setInteravl() fires more than once.
This gives the following result:
var slideIndex = 1;
showSlides(slideIndex);
function plusSlides(n) {
showSlides(slideIndex += n);
}
function currentSlide(n) {
showSlides(slideIndex = n);
}
function showSlides(n) {
var i;
var slides = document.getElementsByClassName("mySlides");
var dots = document.getElementsByClassName("dot");
if (n > slides.length) {
slideIndex = 1
}
if (n < 1) {
slideIndex = slides.length
}
for (i = 0; i < slides.length; i++) {
slides[i].style.display = "none";
}
for (i = 0; i < dots.length; i++) {
dots[i].className = dots[i].className.replace(" active", "");
}
slides[slideIndex - 1].style.display = "block";
dots[slideIndex - 1].className += " active";
}
setInterval(function() {
slideIndex++;
showSlides(slideIndex);
}, 3000);
* {
box-sizing: border-box
}
body {
font-family: Verdana, sans-serif;
margin: 0
}
.mySlides {
display: none
}
/* Slideshow container */
.slideshow-container {
max-width: 1000px;
position: relative;
margin: auto;
}
/* Next & previous buttons */
.prev,
.next {
cursor: pointer;
position: absolute;
top: 50%;
width: auto;
padding: 16px;
margin-top: -22px;
color: white;
font-weight: bold;
font-size: 18px;
transition: 0.6s ease;
border-radius: 0 3px 3px 0;
}
/* Position the "next button" to the right */
.next {
right: 0;
border-radius: 3px 0 0 3px;
}
/* On hover, add a black background color with a little bit see-through */
.prev:hover,
.next:hover {
background-color: rgba(0, 0, 0, 0.8);
}
/* Caption text */
.text {
color: #f2f2f2;
font-size: 15px;
padding: 8px 12px;
position: absolute;
bottom: 8px;
width: 100%;
text-align: center;
}
/* Number text (1/3 etc) */
.numbertext {
color: #f2f2f2;
font-size: 12px;
padding: 8px 12px;
position: absolute;
top: 0;
}
/* The dots/bullets/indicators */
.dot {
cursor: pointer;
height: 13px;
width: 13px;
margin: 0 2px;
background-color: #bbb;
border-radius: 50%;
display: inline-block;
transition: background-color 0.6s ease;
}
.active,
.dot:hover {
background-color: #717171;
}
/* Fading animation */
.fade {
-webkit-animation-name: fade;
-webkit-animation-duration: 1.5s;
animation-name: fade;
animation-duration: 1.5s;
}
@-webkit-keyframes fade {
from {
opacity: .4
}
to {
opacity: 1
}
}
@keyframes fade {
from {
opacity: .4
}
to {
opacity: 1
}
}
/* On smaller screens, decrease text size */
@media only screen and (max-width: 300px) {
.prev,
.next,
.text {
font-size: 11px
}
}
<!DOCTYPE html>
<html>
<head>
<meta name="viewport" content="width=device-width, initial-scale=1">
</head>
<body>
<div class="slideshow-container">
<div class="mySlides fade">
<div class="numbertext">1 / 3</div>
<img src="https://www.w3schools.com/howto/img_nature_wide.jpg" style="width:100%">
<div class="text">Caption Text</div>
</div>
<div class="mySlides fade">
<div class="numbertext">2 / 3</div>
<img src="https://www.w3schools.com/howto/img_fjords_wide.jpg" style="width:100%">
<div class="text">Caption Two</div>
</div>
<div class="mySlides fade">
<div class="numbertext">3 / 3</div>
<img src="https://www.w3schools.com/howto/img_mountains_wide.jpg" style="width:100%">
<div class="text">Caption Three</div>
</div>
<a class="prev" onclick="plusSlides(-1)">❮</a>
<a class="next" onclick="plusSlides(1)">❯</a>
</div>
<br>
<div style="text-align:center">
<span class="dot" onclick="currentSlide(1)"></span>
<span class="dot" onclick="currentSlide(2)"></span>
<span class="dot" onclick="currentSlide(3)"></span>
</div>
</body>
</html>
Hope this helps! :)