How to insert auto-play option in my custom slider JavaScript code

I have created a slider code but the issue is I have to make it autoplay. This code actually compatible with my custom slider and I just want it to autoplay. I have tried some code shown below:

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("contents-2");
    var slides2 = document.getElementsByClassName("contents-1");
    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].className = slides[i].className.replace(" cont-1", "");
    }
    for (i = 0; i < slides2.length; i++) {
        slides2[i].className = slides2[i].className.replace(" cont-2", "");
    }
    for (i = 0; i < dots.length; i++) {
        dots[i].className = dots[i].className.replace(" active-page", "");
    }
    dots[slideIndex - 1].className += " active-page";
    slides[slideIndex - 1].className += " cont-1";
    slides2[slideIndex - 1].className += " cont-2";
    setTimeout(currentSlide, 2000);
}

Please help me with this code, the code above does not autoplay, thank you.