Menampilkan Preview Gambar Sebelum Upload

<script type="text/javascript">
function tampilkanPreview(userfile,idpreview)
{
  var gb = userfile.files;
  for (var i = 0; i < gb.length; i++)
  {
    var gbPreview = gb[i];
    var imageType = /image.*/;
    var preview=document.getElementById(idpreview);
    var reader = new FileReader();
    if (gbPreview.type.match(imageType))
    {
      //jika tipe data sesuai
      preview.file = gbPreview;
      reader.onload = (function(element)
      {
        return function(e)
        {
          element.src = e.target.result;
        };
      })(preview);
      //membaca data URL gambar
      reader.readAsDataURL(gbPreview);
    }
      else
      {
        //jika tipe data tidak sesuai
        alert("Tipe file tidak sesuai. Gambar harus bertipe .png, .gif atau .jpg.");
      }
  }
}
</script>

Gambar:
<input type="file" name="userfile" id="userfile" onchange="tampilkanPreview(this,'preview')"/>
<br><b>Preview Gambar</b><br>
<img id="preview" width="350px"/>