zig Tryit Editor

<!DOCTYPE html>
<html>
<head>
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<style>
* {box-sizing: border-box;}

.img-comp-container {
  position: relative;
  height: 200px; /*should be the same height as the images*/
}

.img-comp-img {
  position: absolute;
  width: auto;
  height: auto;
  overflow:hidden;
}

.img-comp-img img {
  display:block;
  vertical-align:middle;
}

.img-comp-slider {
  position: absolute;
  z-index:9;
  cursor: ew-resize;
  /*set the appearance of the slider:*/
  width: 40px;
  height: 40px;
  background-color: #2196F3;
  opacity: 0.7;
  border-radius: 50%;
}
</style>
<script>
  function initComparisons() {
    var x, i;
    /*find all elements with an "overlay" class:*/
    x = document.getElementsByClassName("img-comp-overlay");
    for (i = 0; i < x.length; i++) {
      /*once for each "overlay" element:
      pass the "overlay" element as a parameter when executing the compareImages function:*/
      compareImages(x[i]);
    }
    function compareImages(img) {
      var slider, img, clicked = 0, w, h;
      /*get the width and height of the img element*/
      w = img.offsetWidth;
      h = img.offsetHeight;
      /*set the width of the img element to 50%:*/
      img.style.width = (w / 2) + "px";
      /*create slider:*/
      slider = document.createElement("DIV");
      slider.setAttribute("class", "img-comp-slider");
      /*insert slider*/
      img.parentElement.insertBefore(slider, img);
      /*position the slider in the middle:*/
      slider.style.top = (h / 2) - (slider.offsetHeight / 2) + "px";
      slider.style.left = (w / 2) - (slider.offsetWidth / 2) + "px";
      /*execute a function when the mouse button is pressed:*/
      slider.addEventListener("mousedown", slideReady);
      /*and another function when the mouse button is released:*/
      window.addEventListener("mouseup", slideFinish);
      /*or touched (for touch screens:*/
      slider.addEventListener("touchstart", slideReady);
      /*and released (for touch screens:*/
      window.addEventListener("touchend", slideFinish);
      function slideReady(e) {
        /*prevent any other actions that may occur when moving over the image:*/
        e.preventDefault();
        /*the slider is now clicked and ready to move:*/
        clicked = 1;
        /*execute a function when the slider is moved:*/
        window.addEventListener("mousemove", slideMove);
        window.addEventListener("touchmove", slideMove);
      }
      function slideFinish() {
        /*the slider is no longer clicked:*/
        clicked = 0;
      }
      function slideMove(e) {
        var pos;
        /*if the slider is no longer clicked, exit this function:*/
        if (clicked == 0) return false;
        /*get the cursor's x position:*/
        pos = getCursorPos(e)
        /*prevent the slider from being positioned outside the image:*/
        if (pos < 0) pos = 0;
        if (pos > w) pos = w;
        /*execute a function that will resize the overlay image according to the cursor:*/
        slide(pos);
      }
      function getCursorPos(e) {
        var a, x = 0;
        e = (e.changedTouches) ? e.changedTouches[0] : e;
        /*get the x positions of the image:*/
        a = img.getBoundingClientRect();
        /*calculate the cursor's x coordinate, relative to the image:*/
        x = e.pageX - a.left;
        /*consider any page scrolling:*/
        x = x - window.pageXOffset;
        return x;
      }
      function slide(x) {
        /*resize the image:*/
        img.style.width = x + "px";
        /*position the slider:*/
        slider.style.left = img.offsetWidth - (slider.offsetWidth / 2) + "px";
      }
    }
  }
</script>
</head>
<body>

<h1>Compare Two Images</h1>

<p>Click and slide the blue slider to compare two images:</p>

</body>

</html>