Vanilla Javascript Slider Issue

Question

I am trying to create simple vanilla Javascript slider but i am having some issues, hopefully someone can help me.

html code :

<div class="box" id="boxid">
        <div class="inside_box" id="inside_box_id">
            <ul class="main_ul" id="main_ul_id">
                <li class="one" id="oneid"><a href = "#"></a></li>
                <li class="one" id="twoid"><a href = "#"></a></li>
                <li class="one" id="threeid"><a href = "#"></a></li>
                <li class="one" id="fourid"><a href = "#"></a></li>
                <li class="one" id="fiveid"><a href = "#"></a></li>
            </ul>
        </div> <!-- end of inside_box -->
    </div><!-- end of box -->

    <button id="previd" class="btn">Prev</button>
    <button id="nextid" class="btn">Next</button>

css code:

body {
    background:grey;
    font-family: sans-serif;
}

.box {
    width:800px;
    height:300px;
    border:1px solid black;
    margin: 0 auto;
}

.inside_box {
    width:800px;
    height:300px;
    padding:0px;
    margin:0px;
    overflow: hidden;
}

.main_ul {
    width:4000px;
    padding:0px;
    margin:0px;
}

.main_ul li {
    list-style: none;
    float:left;
}

.one {
    width:800px;
    height:300px;
}

#oneid {background:blue;}
#twoid {background:orange;}
#threeid {background:brown;}
#fourid {background:green;}
#fiveid {background:purple;}


.btn {
    padding:10px;
    width:100px;
    margin-left:50%;
    cursor: pointer;
    margin-top:10px;
}

JS code

var prevBtn = document.getElementById("previd");
var nextBtn = document.getElementById("nextid");

nextBtn.addEventListener("click", function(){
    var slideLeft = document.getElementById("main_ul_id");
    slideLeft.style.marginLeft = "-800px";
}, false);

At the moment when i click the Next button it takes me to the second slide but when i click it again it doesnt do anything. How can i make it so when i click it to margin-left 800px each time when it is clicked?

Thanks.

Answer 1

You need to get current value and subtract -800px from it .

When you click Next button 1st time, it will set margin-left as -800px and you see the result. But for next press, it will again set margin-left as -800px .

Something like below:

nextBtn.addEventListener("click", function() {
  var slideLeft = document.getElementById("main_ul_id");
  var style = slideLeft.style;
  var totalImages = 5;
  var margin = 800;

  style.marginLeft = (parseInt(style.marginLeft || "0") - margin)%(totalImages*margin) + "px"; 
}, false);

Taking remainder with (totalImages*margin) will bring the first slide after hitting next from the last slide. Not the optimal logic but just to give you an idea.

Here's the working JSFiddle: https://jsfiddle.net/dsa3du93/7/