Keys are coming out to be undefined [duplicate]

I have a loop where fields in a mongoDB document is updated using the req sent by the user.
I am using mongoose to manage the DB.

Here is the code

router.put("/edititem", async (req, res) => {
  try {
    const newMenuItem = await menuItems.findById(req.body._id);
    for (key in req.body) {
      if (key === "_id") continue;
      console.log(newMenuItem.key);
      req.body.key ? (newMenuItem.key = req.body.key) : null;
    }
    newMenuItem.save();
    res.status(200).json(newMenuItem);
  } catch (error) {
    res.status(500).json(error);
  }
});

What it should do:

• Change the specified keys in the specified document.

What it is doing:

• The value of keys are undefined inside the loop.