前端判断上传文件大小

var maxsize = 20*1024*1024;//20M
var errMsg = "上传的文件不能超过20M！！！";
var tipMsg = "您的浏览器暂不支持计算上传文件的大小，确保上传文件不要超过20M，建议使用360、FireFox、Chrome浏览器。";
var pwdMsg = "请输入文件密码";
var  browserCfg = {};
var ua = window.navigator.userAgent;

if (ua.indexOf("MSIE")>=1){
    browserCfg.ie = true;
}else if(ua.indexOf("Firefox")>=1){
    browserCfg.firefox = true;
}else if(ua.indexOf("Chrome")>=1){
    browserCfg.chrome = true;
}
function checkfile(){

}
$("#sub_btn").click(function(){
    try{
        var obj_file = document.getElementById("upload_file");
        if(obj_file.value==""){
            $("#upload_error_text").text("请先选择上传文件");
            $("#upload_error").show();
            return;
        }
        var filesize = 0;
        if(browserCfg.firefox || browserCfg.chrome ){
            filesize = obj_file.files[0].size;
        }else if(browserCfg.ie){
            var obj_img = document.getElementById('tempimg');
            obj_img.dynsrc=obj_file.value;
            filesize = obj_img.fileSize;
        }else{
            $("#upload_error_text").text(tipMsg);
            $("#upload_error").show();
            return;
        }
        if(filesize==-1){
            $("#upload_error_text").text(tipMsg);
            $("#upload_error").show();
            return;
        }else if(filesize>maxsize){
            $("#upload_error_text").text(errMsg);
            $("#upload_error").show();

            return;
        }else{
            if($("#file_pwd").val()==''){
                $("#upload_error_text").text(pwdMsg);
                $("#upload_error").show();
                return;
            }

            alert("您的文件密码很重要，请牢记！！！");
            $("#form_").submit();
        }
    }catch(e){
        console.log(e);
        return false;
    }
});