imag.html

<!DOCTYPE html>
<html>
<head>
    <meta charset="utf-8">
    <title></title>
</head>
<body>
    <!-- <input type="file" id="file" onchange="FileValidation(event)" >
   Uploaded Image<img id="output">
</body> -->
<input type="file" id="filetag">
<img src="" id="preview">
<script>
//     FileValidation = (event) => {
//         var image = document.getElementById('output');
//     image.src = URL.createObjectURL(event.target.files[0]);
//         const fi = document.getElementById("file");
//         if (fi.files.length > 0) {
//             for (const i = 0; i <= fi.files.length - 1; i++) {
//                 const fsize = fi.files.item(i).size;
//                 const file = Math.round((fsize / 1024));
// }
// }
// }
var fileTag = document.getElementById("filetag"),
    preview = document.getElementById("preview");
    
fileTag.addEventListener("change", function() {
  changeImage(this);
});

function changeImage(input) {
  var reader;

  if (input.files && input.files[0]) {
    reader = new FileReader();

    reader.onload = function(e) {
      preview.setAttribute('src', e.target.result);
    }

    reader.readAsDataURL(input.files[0]);
  }
}
</script>
</body>
</html>