js判断上传文件大小

var isIE = /msie/i.test(navigator.userAgent) && !window.opera;
    var fileSize = 0;
    if (isIE && !target.files) {        
         var filePath = target.value;  
         var fileSystem = new ActiveXObject("Scripting.FileSystemObject");  
         var file = fileSystem.GetFile(filePath);  
        fileSize = file.Size;  alert(fileSize);               
    }else{
          fileSize = target.files[0].size;   
    }       
    var ext=target.value.substring(target.value.lastIndexOf("."),target.value.length).toUpperCase();
    if(ext!=".BMP"&&ext!=".PNG"&&ext!=".GIF"&&ext!=".JPG"&&ext!=".JPEG"){
        alert("图片限于bmp,png,gif,jpeg,jpg格式");
    }else{
        var size = fileSize / 1024;     
        if(size>30||size==0){   
             alert("附件大小不能大于30k或为0k！");   
            var file=document.getElementById(imgId);  
            file.outerHTML=file.outerHTML;
            $("#"+imgId).attr("src","img/bac_add.jpg");  
        }else{
            var oldImgSrc = $("#"+imgId+"Bac").val();
            $("#imgForm").ajaxForm({
                url:"newspaper/saveShowImg.html?oldImgSrc="+oldImgSrc+"&imgId="+imgId,
                type:"post",
                success:function(data){
                    $("#"+imgId).attr("src",data);
                    $("#"+imgId+"Bac").val(data);
                },error:function(data){
             
                }
            });
            $("#imgForm").submit();
        }   
    }     

<input type="file" id="focusPic" name="focusPic" class="fileupload-hidden" onchange="showCurrentImg(this,'focusImg')">
<img src="img/bac_add.jpg" id="focusImg" class="newsImgList">

 

function fileChange(target,id) {   
        var fileSize = 0;        
        if (isIE && !target.files) {    
              var filePath = target.value;   
              var fileSystem = new ActiveXObject("Scripting.FileSystemObject");
              
              if(!fileSystem.FileExists(filePath)){
                 alert("附件不存在，请重新输入！");
                 var file=document.getElementById(id); 
                    file.outerHTML=file.outerHTML;
                    return;
              }
              var file = fileSystem.GetFile (filePath);
              fileSize = file.Size;   
        } else {   
               fileSize = target.files[0].size; 
        }  
        
        var size = fileSize / 1024;   
        if(size>100000){ 
                alert("附件大小不能大于100M！"); 
                var file=document.getElementById(id); 
                file.outerHTML=file.outerHTML
        }  
        if(size<=0){
            alert("附件大小不能为0M！"); 
            var file=document.getElementById(id); 
                file.outerHTML=file.outerHTML
        } 
     }