Increment and decrement Rethinkdb

RethinkDB Increment and Decrement Single Value

Need some help, I'm learning Nodejs and Rethinkdb.

Currently i'm using this Rethinkdb code to perform the Update operation.

r.table('items').get(id).update(item).run(self.connection, function(err, result) {
    if(result && result.replaced === 1) {
        res.send(item);
    }
    else if(err) {
        debug("[ERROR] updateItem %s:%s\n%s", err.name, err.msg, err.message);
        res.send({error: 'An error occurred when updating the item with id: ' + id});
    }
    else {
        debug("[ERROR] updateItem (%s): %j", id, result);
        res.send({error: 'An error occurred when updating the item with id: ' + id});
    }
});

Now the question is: How can i update a value in a row by Incrementing its value? For example lets say its about the stock of a car. so in a row i'll have:

    {
    "code":  "001" ,
    "brand":  "Ferrari" ,
    "stock":  "2"
    }

Now lets say i make a form to control how many cars will be added or removed from the stock, So for example lets say i want to add 3 new ferrari's. how can i update the value stock, incrementing the stock by 3 for a total of 5 and not replacing 2 for 3. also lets say i sold 2 ferraris, how can i decrease the value so the stock value will be 0.

and Please excuse my poor english. not my natal language.

via RocketFuel