js实现图片预览

//下面用于图片上传预览功能
      function setImagePreviewUpdate(avalue)
       {
           var docObj=document.getElementById("uploadImg_update");
          
           var imgObjPreview=document.getElementById("img_preview_update");
           if(docObj.files &&docObj.files[0])
           {
               //张宇乐自定义             
               var localImgId = document.getElementById("localImg_update");
               localImgId.style.display = 'block';
               var uploadDiv = document.getElementById("uploadDiv_update");
               //uploadDiv.style.display = 'none';
               //
               //火狐下，直接设img属性
               imgObjPreview.style.display = 'block';
               //docObj.style.display='none';
               imgObjPreview.style.width = '144px';
               imgObjPreview.style.height = '169px';
              
               //imgObjPreview.src = docObj.files[0].getAsDataURL();
               //$('#uploadImg').css('display','none');
               //火狐7以上版本不能用上面的getAsDataURL()方式获取，需要一下方式
               imgObjPreview.src = window.URL.createObjectURL(docObj.files[0]);
           }
           else
           {
                //IE下，使用滤镜
                docObj.select();
                var imgSrc = document.selection.createRange().text;
                var localImgId = document.getElementById("localImg_update");
                //必须设置初始大小
                localImgId.style.width = "144px";
                localImgId.style.height = "169px";
                //图片异常的捕捉，防止用户修改后缀来伪造图片
                try
                {
                    localImagId.style.filter="progid:DXImageTransform.Microsoft.AlphaImageLoader(sizingMethod=scale)";
                    localImagId.filters.item("DXImageTransform.Microsoft.AlphaImageLoader").src = imgSrc;
                }
                catch(e)
                {
                     alert("您上传的图片格式不正确，请重新选择!");
                     return false;
                 }
                 imgObjPreview.style.display = 'none';
                 document.selection.empty();
            }
            return true;
      }