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                <main id="wrapper-outer">

    <div id="outer" class="outer">

        <div class="image"><img src="" alt="add an alt"/></div>
        <div id="wrapper-percent" class="image"><img src="" alt="add an alt"/></div>
      <div id="night" class="image"><img src="" alt="add an alt"/></div>

      <div class="image caption">A winter and summer wonderland</div>




                /* reset */
* {
  margin: 0;
  box-sizing: border-box;

body {
  box-sizing: border-box;
  margin: 0;

main {
  width: 100vw;
  min-height: 100vh;
  display: flex;
  justify-content: center;
  align-items: center;

.outer {
  width: 1010px;
  position: relative;
  height: 750px;

.image {
  position: absolute;
  overflow: hidden;
  height: 800px;

.caption {
  position: absolute;
  bottom: 0;
  height: 40px;
  width: 100%;
  background-color: #1b2b34;
  color: white;
  line-height: 1.2em;
  padding: .5em;
  font-family: "Cabin", Arial, Geneva, sans-serif;
  font-size: 1.2em;

.image:nth-child(2) {
  width: 50%;

.image:nth-child(2) img {

#night {
  opacity: 0;
  transition: opacity 2s;



                const wrapperOuter  =  document.getElementById("wrapper-outer");
const wrapperPercent  =  document.getElementById("wrapper-percent");
const outer = document.getElementById("outer");
const night = document.getElementById("night");

let width = document.body.clientWidth;
let imageWrapperPercent;

wrapperOuter.addEventListener('mousemove', mouseMove );

function mouseMove(e) {
  var x = e.clientX;
  var outerTracker = outer.getBoundingClientRect();
  let mousePointerPos;
  mousePointerPos = x - outerTracker.left;
  // = x + "px"; // wrapper width using clientX = mousePointerPos + "px"; // wrapper width using mouse postion in the box
  // convert to a % 
  //imageWrapper =  mousePointerPos/10;
  // = imageWrapper + "%";

  // Night Image Show and Hide
  /*if(mousePointerPos < 0) { = "100"
  if(mousePointerPos > 0) { = "0"