Image upload before image preview with javascript

<script type="text/javascript">
 function chkform(form)
{
  // image preview
   function readURL(input)
   {
    if (input.files && input.files[0])
{
        var reader = new FileReader();
        reader.onload = function (e) {
            document.getElementById('preview').src=e.target.result;
        }
        reader.readAsDataURL(input.files[0]);
    }
  }
    </script>
   
     <form name="f2" method="post" enctype="multipart/form-data" style=" margin-top: -100px;"  onsubmit="return chkform(this);">
    <<input type="file" name="file" id="imgfile" onchange="readURL(this)";/>
            <img id="preview" src="#" alt="your image" />
      </form>