Slideshow buttons only working on second click when coming to the last slide

I am making a slideshow that uses JSON data to populate it's contents. When I come to the last slide I have to click 2-3 times before the slideshow starts on the first slide again. I want to make sure this doesn't happen and one click is enough to start on the first slide again. A similar bug is when I first load the page and click on the back button for the first time i have to click twice, after that it works as it should. I am sure there is a problem in one of my if statements, but I can't figure out whats wrong.

index.html

<div class="slideshow-container">
    <div class="mySlides fade">
        <img id="image" style="width:100%">
        <div id="description"></div>
    </div>
    <a id="prev">&#10094;</a>
    <a id="next">&#10095;</a>
</div>

<script>
    window.onload = startup;
    var xmlhttp;
    function startup () {
        xmlhttp = new XMLHttpRequest();
        xmlhttp.onreadystatechange = statechange;
        xmlhttp.open("GET", "slideshow.json", true);
        xmlhttp.send();
    }
    function statechange() {
        if (xmlhttp.readyState === 4 && xmlhttp.status === 200) {
            var printout = "";
            var doc = JSON.parse(xmlhttp.responseText);
            var amountOfElements = doc.length;

            var current = 0;

            var imagePath = "assets/stop2/";
            document.getElementById("image").src = imagePath + doc[current].image;
            document.getElementById("description").innerHTML = doc[current].text;

            console.log(current);

            prev.onclick = function() {
                if(current !== 0){
                    current = current - 1;

                    document.getElementById("image").src = imagePath + doc[current].image;
                    document.getElementById("description").innerHTML = doc[current].text;

                    if(current === 0) {
                        current = amountOfElements;
                    }
                }
                else {
                    current = amountOfElements;
                }
                console.log(current);
            };

            next.onclick = function() {
                if(current < amountOfElements) {
                    current = current + 1;

                    document.getElementById("image").src = imagePath + doc[current].image;
                    document.getElementById("description").innerHTML = doc[current].text;
                }
                else {
                    current = -1;
                }
                console.log(current);
            };

        }
        else if (xmlhttp.readyState === 4 && xmlhttp.status === 404) {
            alert("Something went wrong");
        }
    }
</script> 

slideshow.json

  {
    "name": "First image",
    "text": "blablabla",
    "image": "img1.png",
    "audio": "audio1.mp3"
  },
  {
    "name": "Second image",
    "text": "Lorem ipsum..",
    "image": "img2.png",
    "audio": "audio2.mp3"
  },
  {
    "name": "Third image",
    "text": "blablabla",
    "image": "img3.png",
    "audio": "audio3.mp3"
  },
  {
    "name": "Last image",
    "text": "blablablaaaaaa",
    "image": "img4.png",
    "audio": "audio4.mp3"
  }
] ```