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
}
}