multer不允许默认图像的问题

所以我和multer有个问题,我想不出来。当我添加带有图像的项目时,一切正常,但如果我尝试添加没有图像的项目,则会失败。我有一个if语句,它将定义一个默认映像if req。文件未定义,但失败,错误为req。文件文件名未定义。。。这是代码

Multer设置:

var storage = multer.diskStorage({
  destination: function(req, file, callback) {
    callback(null, "./public/uploads");
  },
  filename: function(req, file, callback) {
    callback(null, Date.now() + file.originalname);
  }
});
var upload = multer({ storage: storage }).single("file");

路线:

router.post("/item/add", middleware.isLoggedIn, (req, res) => {
  User.findById(req.user._id, (err, user) => {
    upload(req, res, err => {
      if (err) {
        req.flash("error", "failed to upload file");
        return res.redirect("/products");
      }
      var item = new Item();
      item.name = req.body.name;
      item.description = req.body.description;
      item.price = req.body.price;
      item.createdBy = { id: req.user._id, username: req.user.username };
      if (typeof req.file === undefined) {
        item.image = "/uploads/no-img.png";
      } else {
        item.image = "/uploads/" + req.file.filename;
      }
      item.save(function(err) {
        if (err) {
          res.send(err);
        }
        return res.redirect("/products");
      });
    });
  });
});

所以我想我的问题是。。。如果没有选择图像,我将如何设置它不会失败?