js实现FileUpload选择图片后预览功能

当asp.net的FileUpload选择一个图片后不需要上传就能显示出图片的预览功能，
代码：
<%@ Page Language="C#" AutoEventWireup="true" CodeFile="Default2.aspx.cs" Inherits="Default2" %>

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
    <title></title>
          <script type="text/javascript">
                                function selected(obj) {
                                    var str = "carpoolpic";
                                    str = str + obj.value;
                                    var imgSrc = document.getElementById(str).value;
                                    if (imgSrc == "" || imgSrc == null) {
                                        alert("此项没有图片");
                                        obj.checked = false;
                                    }
                                    changesrc(str);
                                }

                                var flag = true;
                                function changesrc(sender) {
                                    if (isFirefox = navigator.userAgent.indexOf("Firefox") > 0) {
                                        var imgSrc = document.getElementById(sender).value;
                                        var t = document.getElementById(sender);
                                        if (imgSrc == "") {
                                            flag = false;
                                            return false;
                                        }
                                        if (/(.*?).jpg$/.test(imgSrc.toLowerCase()) == false) {
                                            document.getElementById(sender).value = "";
                                            alert("只能选择jpg格式!");
                                            flag = false;
                                            return false; 
                                        } else {
                                            var imgs = document.createElement("img");
                                            imgs.src = imgSrc;

                                            if (imgs.fileSize > 50 * 1024) {
                                                alert("图片大小不能超过 50 KB!");
                                                flag = false;
                                                return false; 
                                            }
                                            flag = true;
                                        }
                                        document.getElementById("upImg").src = window.URL.createObjectURL(t.files[0]);
                                    } else {
                                        document.getElementById(sender).select();
                                        var imgSrc = document.selection.createRange().text;
                                        if (imgSrc == "") {
                                            flag = false;
                                            return false;
                                        }
                                        if (/(.*?).jpg$/.test(imgSrc.toLowerCase()) == false) {
                                            document.getElementById(sender).value = "";
                                            alert("只能选择jpg格式!");
                                            flag = false;
                                            return false; 
                                        } else {
                                            var imgs = document.createElement("img");
                                            imgs.src = imgSrc;
                                            flag = true;
                                        }
                                        document.getElementById("upImg").src = imgSrc;
                                    }
                                }

                                function checkpic() {
                                    var raFlag = false;
                                    var obj = document.getElementsByName("select");
                                    for (var i = 0; i < obj.length; i++) {
                                        if (obj[i].checked) {
                                            raFlag = true;
                                            break;
                                        } else {
                                            raFlag = false;
                                        }
                                    }
                                    if (raFlag == false && flag == true) {
                                        alert("请选择最新的照片");
                                    }
                                    var subFlag = false;
                                    subFlage = flag && raFlag;
                                    flag = false;
                                    return subFlage;
                                }
                            </script> 
</head>
<body>
    <form id="form1" runat="server">
    <div>  

<asp:FileUpload ID="fupweb_image" runat="server" class="chenck_upload left" onchange="changesrc(this.id)" /></span>
   <img id="upImg" runat="server" src=" "   />


    </div>
    </form>
</body>
</html>