NodeJS + Mongo: Insert if not exists, otherwise - update

I have an object in my mongodb collection. Its schema is:

{
    "instruments": ["A", "B", "C"],
    "_id": {
        "$oid": "508510cd6461cc5f61000001"
    }
}

My collection may have such object, but may not. I need to check if object with key "instruments" exists (please, notе, I don't know what value "instrument" is at this time, it may contain any value or an array), and if exists - perform update, otherwise – insert a new value. How can I do this?

collection.find( {  "instruments" : { $exists : true } }, function(err, object){
    if (object) {
        //update
    } else {
        //insert
    }
});

doesn't work ((