I need to add or remove object from array depends on condition based on assignUser

await healthPlanCardsCollection.findOneAndUpdate({ _id: cardId }, {
    $cond: {
        if: assignUser,
        then: { $push: { assignedUsers: { userId: new ObjectId(user._id), ...user } } },
        else: { $pull: { assignedUsers: { userId: new ObjectId(user._id) } } },
    }
})

But in such case I've error: "MongoServerError: Unknown modifier: $cond. Expected a valid update modifier or pipeline-style update specified as an array"

"mongodb": "^4.2.0"

How could I rewrite it in more elegant way and do not have an error?