图片上传即时显示缩略图解决方法（出自高手“风雪之隅” ）

</>code
<script language="javascript" type="text/javascript">
 var allowExt = ['jpg', 'gif', 'bmp', 'png', 'jpeg'];
 var preivew = function(file, container){
 try{
var pic =  new Picture(file, container);
 }catch(e){
 alert(e);
 }
 }
 //缩略图类定义
     var Picture  = function(file, container){
 var height
= 0, 
 widht 
= 0, 
 ext
= '',
 size
= 0,
 name   = '',
 path  
=  '';
 var self 
= this;
 if(file){ 
     name = file.value;
 if (window.navigator.userAgent.indexOf("MSIE")>=1){ 
 file.select(); 
 path = document.selection.createRange().text; 
 }else if(window.navigator.userAgent.indexOf("Firefox")>=1){ 
 if(file.files){ 
 path =  file.files.item(0).getAsDataURL(); 
 }else{
 path = file.value;
 }
 } 
 }else{
 throw "bad file";
 } 


 ext = name.substr(name.lastIndexOf("."), name.length);

 if(container.tagName.toLowerCase() != 'img'){
 throw "container is not a valid img label";
 container.visibility = 'hidden';
 }
 container.src = path;
 container.alt = name;
 container.style.visibility = 'visible';
 height = container.height;
 widht  = container.widht;
 size   = container.fileSize;


 this.get = function(name){
 return self[name];
 }

 this.isValid = function(){
 if(allowExt.indexOf(self.ext) !== -1){
 throw 'the ext is not allowed to upload';
 return false;
 }
 }
 }
</script>

</>code
   <div class='previewDemo'>
<input id="file" type="file" onchange="preivew(this, document.getElementById('img'));">
<img id="img"  style="visibility:hidden" height="100px" width="100px">
</div>