W3Schools Tryit Editor

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

.img-zoom-container {
  position: relative;
}

.img-zoom-lens {
  position: absolute;
  border: 1px solid #d4d4d4;
  /*set the size of the lens:*/
  width: 40px;
  height: 40px;
}

.img-zoom-result {
  border: 1px solid #d4d4d4;
  /*set the size of the result div:*/
  width: 300px;
  height: 300px;
}
</style>
<script>
function imageZoom(imgID, resultID) {
  var img, lens, result, cx, cy;
  img = document.getElementById(imgID);
  result = document.getElementById(resultID);
  /*create lens:*/
  lens = document.createElement("DIV");
  lens.setAttribute("class", "img-zoom-lens");
  /*insert lens:*/
  img.parentElement.insertBefore(lens, img);
  /*calculate the ratio between result DIV and lens:*/
  cx = result.offsetWidth / lens.offsetWidth;
  cy = result.offsetHeight / lens.offsetHeight;
  /*set background properties for the result DIV:*/
  result.style.backgroundImage = "url('" + img.src + "')";
  result.style.backgroundSize = (img.width * cx) + "px " + (img.height * cy) + "px";
  /*execute a function when someone moves the cursor over the image, or the lens:*/
  lens.addEventListener("mousemove", moveLens);
  img.addEventListener("mousemove", moveLens);
  /*and also for touch screens:*/
  lens.addEventListener("touchmove", moveLens);
  img.addEventListener("touchmove", moveLens);
  function moveLens(e) {
    var pos, x, y;
    /*prevent any other actions that may occur when moving over the image:*/
    e.preventDefault();
    /*get the cursor's x and y positions:*/
    pos = getCursorPos(e);
    /*calculate the position of the lens:*/
    x = pos.x - (lens.offsetWidth / 2);
    y = pos.y - (lens.offsetHeight / 2);
    /*prevent the lens from being positioned outside the image:*/
    if (x > img.width - lens.offsetWidth) {x = img.width - lens.offsetWidth;}
    if (x < 0) {x = 0;}
    if (y > img.height - lens.offsetHeight) {y = img.height - lens.offsetHeight;}
    if (y < 0) {y = 0;}
    /*set the position of the lens:*/
    lens.style.left = x + "px";
    lens.style.top = y + "px";
    /*display what the lens "sees":*/
    result.style.backgroundPosition = "-" + (x * cx) + "px -" + (y * cy) + "px";
  }
  function getCursorPos(e) {
    var a, x = 0, y = 0;
    e = e || window.event;
    /*get the x and y positions of the image:*/
    a = img.getBoundingClientRect();
    /*calculate the cursor's x and y coordinates, relative to the image:*/
    x = e.pageX - a.left;
    y = e.pageY - a.top;
    /*consider any page scrolling:*/
    x = x - window.pageXOffset;
    y = y - window.pageYOffset;
    return {x : x, y : y};
  }
}
</script>
</head>
<body>

<h1>Image Zoom</h1>

<p>Mouse over the image:</p>

<p>The image must be placed inside a container with relative positioning.</p>
<p>The result can be put anywhere on the page, but must have the class name "img-zoom-result".</p>
<p>Make sure both the image and the result have IDs. These IDs are used when a javaScript initiates the zoom effect.</p>

</body>
</html>