MongoDB Node findone how to handle no results?

Not finding any records isn’t an error condition, so what you want to look for is the lack of a value in result. Since any matching documents will always be “truthy”, you can simply use a simple if (result) check. E.g.,

collection.findOne({query}, function(err, result) {
    if (err) { /* handle err */ }

    if (result) {
        // we have a result
    } else {
        // we don't
    }
}