javascript实现上传图片前的预览效果

< s cript >
function yulan()
{
var fileext = document.form1.UpFile.value.substring(document.form1.UpFile.value.lastIndexOf( " . " ),document.form1.UpFile.value.length)
fileext = fileext.toLowerCase()
if  ((fileext != ' .jpg ' ) && (fileext != ' .gif ' ) && (fileext != ' .jpeg ' ) && (fileext != ' .png ' ) && (fileext != ' .bmp ' ))
{
alert( " 对不起，系统仅支持标准格式的照片，请您调整格式后重新上传，谢谢 ! " );
document.form1.UpFile.focus();
}
else
{
// alert(''+document.form1.UpFile.value) // 把这里改成预览图片的语句
document.getElementById( " preview " ).innerHTML = " <img src=' " + document.form1.UpFile.value + " ' width=120 style='border:6px double #ccc'> "
}
}
</ s cript >
< form name = " form1 "  method = " POST "  enctype = " multipart/form-data " >
< input type = " file "  name = " UpFile "  size = " 46 "  onchange = " yulan() " >
< div id = " preview " ></ div >
</ form >